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Surge in insect resistance to transgenic crops and prospects for sustainability
Abstract
Transgenic crops have revolutionized insect pest control, but their effectiveness has been reduced by evolution of resistance in pests. We analyzed global monitoring data reported during the first two decades of transgenic crops, with each case representing the responses of one pest species in one country to one insecticidal protein from Bacillus thuringiensis (Bt). The cases of pest resistance to Bt crystalline (Cry) proteins produced by transgenic crops increased from 3 in 2005 to 16 in 2016. By contrast, in 17 other cases there was no decrease in pest susceptibility to Bt crops, including the recently introduced transgenic corn that produces a Bt vegetative insecticidal protein (Vip). Recessive inheritance of pest resistance has favored sustained susceptibility, but even when inheritance is not recessive, abundant refuges of non-Bt host plants have substantially delayed resistance. These insights may inform resistance management strategies to increase the durability of current and future transgenic crops.
... There are many reasons behind insect resistance https://doi.org/10.55627/agribiol.003.01.1043 against transgenic cotton: one is that insects have evolved resistance and another is that the level of resistance in plants is insufficient to control the pest. Insect resistance against transgenic cotton Firstly, it was reported that insect pests had developed resistance against transgenic crops (Tabashnik and Carrière, 2017) as presented in Table 1. The binding of Bt toxins to the insect target site (midgut apical membrane) is important for the death of pest larvae as shown in figure 2 (Adang et al., 2014;Fabrick et al., 2023), but the reduction of this binding causes the resistance of pests to toxins (Tabashnik, 2015;Zhang et al., 2012). ...
... More use of refuge aids to increase the susceptible populations from non-Bt crops which assist in dilution of the resistant population surviving from Bt cotton. Because random mating of recessive resistant (RR) individual (from Bt crop) and susceptible (SS) individual (from non-Bt crop) results in the production of heterozygous (RS) progeny (Crowder and Carrière, 2009;Huang et al., 2010;Sisterson et al., 2005;Tabashnik and Carrière, 2017). Its heterozygous progeny was killed when feeding on the Bt crop (Tabashnik and Carrière, 2017). ...
... Because random mating of recessive resistant (RR) individual (from Bt crop) and susceptible (SS) individual (from non-Bt crop) results in the production of heterozygous (RS) progeny (Crowder and Carrière, 2009;Huang et al., 2010;Sisterson et al., 2005;Tabashnik and Carrière, 2017). Its heterozygous progeny was killed when feeding on the Bt crop (Tabashnik and Carrière, 2017). However, refuge delayed resistance against Bt crops in bollworm (Jin et al., 2015;Tabashnik et al., 2008) (Jin et al., 2015;Tabashnik et al., 2008) and helped Bt crops to sustain for a long period (Carrière et al., 2016). ...
... 5,6 Over time, this was followed by reports of Cry1Ab-resistant B. fusca populations throughout the maize production region and the conclusion that it evolved practical resistance to Cry1Ab maize. [7][8][9][10][11] To mitigate the problem with Cry1Ab resistance in South Africa, MON 89034 maize, which contains two Bt proteins (Cry1A.105 and Cry2Ab2) was commercialized in 2011. ...
... The evolution of resistance to Bt proteins by target pests threatens the continued efficacy of Bt crops like MON 89034 maize. 9,11 Practical resistance to a Bt crop represents field-evolved resistance that reduces the efficacy of the Bt crop and has practical consequences for pest control. 9,11 Nine Lepidoptera species, including B. fusca, have practical resistance to certain Cry proteins and early signs of resistance have also been reported for several other species. ...
... 9,11 Practical resistance to a Bt crop represents field-evolved resistance that reduces the efficacy of the Bt crop and has practical consequences for pest control. 9,11 Nine Lepidoptera species, including B. fusca, have practical resistance to certain Cry proteins and early signs of resistance have also been reported for several other species. 11 In this article, we report on assessments of the susceptibility of B. fusca populations sampled in South Africa to MON 810 and MON 89034 maize, using artificial diet and plant-based assays, including three populations sampled from fields with reported greater than expected damage. ...
BACKGROUND
Resistance of Busseola fusca (Lepidoptera: Noctuidae) to Cry1Ab was documented in 2006, 7 years after the first cultivation of MON 810 in South Africa. This was mitigated by introducing a second‐generation Bacillus thuringiensis (Bt) maize (MON 89034), which contains the Cry1A.105 and Cry2Ab2 proteins. The first reports of B. fusca infestations of MON 89034 maize came in the KwaZulu‐Natal province (2017–2018 cropping season), followed by reports in the Mpumalanga province (2022–2023 season). Here we report results of artificial diet‐ and plant‐based laboratory assays to assess the susceptibility of B. fusca populations to the Bt proteins in MON 89034.
RESULTS
Larvae were sampled from nine locations which included three where greater than expected injury due to B. fusca had been reported to MON 89034. Larval mortality in assays with diet‐incorporated Cry2Ab2 protein was 100% for all except the three problem populations, showing that the Cry2Ab2 protein in MON 89034 is highly efficacious against B. fusca. In contrast, assays with Cry1A.105 did not cause significant mortality in any of the B. fusca populations including larvae from a susceptible reference. Larval survival on leaf tissue of MON 89034 maize after 7 days ranged between 75% and 91% for the three problematic B. fusca populations, compared to 0.4% to 9.6% for the five other populations.
CONCLUSION
Therefore, MON 89034 is effectively a single‐mode‐of‐action technology against B. fusca and carries an inherent high risk for the evolution of resistance. This study shows that the three B. fusca populations collected from locations with greater than expected damage to MON 89034 have resistance to the Cry2Ab2 protein and therefore to MON 89034 maize. This research emphasizes the importance of resistance monitoring and implementation of effective insect resistance management tactics. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.
... They have been widely adopted for insect management around the globe, and at present, 85% of corn and 89% of cotton planted in the United States express Bt traits [24]. Bt crops are, therefore, a major selective force in agroecosystems [25,26]. ...
... Under these circumstances, if resistance is recessive and resistance alleles incurred a fitness cost in the absence of these toxic proteins, the emergence and spread of resistance may be preventable [32]. Although there have been cases of successful resistance management in some target pests of Bt crops, practical resistance has emerged in several lepidopteran and coleopteran species, including the polyphagous pest, Helicoverpa zea [26]. ...
Strong and shifting selective pressures of the Anthropocene are rapidly shaping phenomes and genomes of organisms worldwide. Crops expressing pesticidal proteins from Bacillus thuringiensis (Bt) represent one major selective force on insect genomes. Here we characterize a rapid response to selection by Bt crops in a major crop pest, Helicoverpa zea. We reveal the polygenic architecture of Bt resistance evolution in H. zea and identify multiple genomic regions underlying this trait. In the genomic region of largest effect, we identified a gene amplification event, where resistant individuals showed variation in copy number for multiple genes. Signals of this amplification increased over time, consistent with the history of field-evolved Bt resistance evolution. Modern wild populations from disparate geographical regions are positive for this variant at high, but not fixed, allele frequencies. We also detected selection against single copy variants at this locus in wild H. zea collected from Bt expressing plants, further supporting its role in resistance. Multiple genes were annotated in this genomic region, and all appeared to be significantly upregulated in Bt resistant H. zea. We functionally characterized genes within the copy number variant (CNV), providing insight into their potential roles in resistance evolution. Our findings reveal the nature of rapid genome evolution in a major crop pest following anthropogenic selection and highlight the role that CNVs can have in rapid evolutionary responses.
... This transgenic potato has been enhanced by using the CryIIIA gene against the Colorado potato beetle (Halterman et al. 2016). However, the global adoption of Bt methods and the use of overdoses result in increased insect resistance (reviewed in Tabashnik and Carrière 2017). Thus, new techniques might be needed to manage insect pests (Didoné et al. 2021). ...
... However, GM crops have the potential to reduce or eliminate the use of synthetic insecticides by engineering and expressing certain insecticidal proteins. However, the continued use of Bt crops has resulted in the development of resistance in insect pests against these crops (Tabashnik and Carrière 2017). The researchers are finding alternate ways to combat losses from insect pests. ...
The tomato leaf miner Tuta absoluta Meyrick is the most serious pest of potato worldwide. In the present study, we explored an alternate approach to induce resistance in potatoes against T. absoluta by engineering Jaburetox-2Ec, an insecticidal peptide derived from an isoform of urease from Jack bean (Canavalia ensiformis L.). The construct was developed by cloning a PCR-amplified fragment of Jaburetox-2Ec (270 bp) in a pBIN61 vector under the control of a 35S promoter and was named pBIN-JBTX. Furthermore, the recombinant plasmid expressing Jaburetox-2Ec was transformed into potato cv. Lady Olympia using Agrobacterium tumefaciens strain EHA105. The results showed the proper integration of the T-DNA region in primary transformants. Quantitative PCR assays showed the independent transgenic lines had enhanced transcript levels of Jaburetox. The leaf biotoxicity assays were performed by releasing pre-starved 2nd, 3rd, and 4th instar larvae of T. absoluta on transgenic potato plants. As a result, depending on the larval stage, the mortality rate of larvae feeding on transgenic plants ranged from 11 to 33%. Additionally, it was found that larvae feeding on control plants gained more weight than those feeding on transgenic plants. Besides that, deformities in larvae feeding on transgenic plants were also recorded. Overall, the results exhibited the insecticidal activity of Jaburetox 2-Ec against T. absoluta. The transgenic lines can be used as a source of germplasm for an efficient potato breeding programme.
... Despite efforts to delay the development of resistance, the number of field-evolved insect resistance populations has continued to increase in parallel with the increased planting of Bt crops around the world [3,11,12]. As of 2016, the number of reported fieldevolved insect resistance populations has increased from 3 in 2008 to 16 in 2016 [11]. ...
... Despite efforts to delay the development of resistance, the number of field-evolved insect resistance populations has continued to increase in parallel with the increased planting of Bt crops around the world [3,11,12]. As of 2016, the number of reported fieldevolved insect resistance populations has increased from 3 in 2008 to 16 in 2016 [11]. The mechanisms of Bt resistance in these field-evolved insect resistance populations often involve changes to protein binding to their target receptors within the larval gut [13]. ...
Insect control traits are a key component of improving the efficacy of insect pest management and maximizing crop yields for growers. Insect traits based on proteins expressed by the bacteria Bacillus thuringiensis (Bt) have proven to be very effective tools in achieving this goal. Unfortunately, the adaptability of insects has led to resistance to certain proteins in current commercial products. Therefore, new insecticidal traits representing a different mode of action (MoA) than those currently in use are needed. Cry1Ja has good insecticidal activity against various lepidopteran species, and it provides robust protection against insect feeding with in planta expression. For Bt proteins, different MoAs are determined by their binding sites in the insect midgut. In this study, competitive binding assays are performed using brush border membrane vesicles (BBMVs) from Helicoverpa zea, Spodoptera frugiperda, and Chrysodeixis includens to evaluate the MoA of Cry1Ja relative to representatives of the various Bt proteins that are expressed in current commercial products for lepidopteran insect protection. This study highlights differences in the shared Cry protein binding sites in three insect species, Cry1Ja bioactivity against Cry1Fa resistant FAW, and in planta efficacy against target pests. These data illustrate the potential of Cry1Ja for new insect trait development.
... 47,[49][50][51] The goal is to alter the frequency of resistant individuals in the population, allowing susceptible phenotypes a greater chance of survival. 47,52 In this study, we demonstrated that after six generations, when susceptible individuals were introduced into the system under laboratory conditions, a reduction in the resistance ratios of up to 13-fold was observed. This reduction was reflected in a 45% decrease in survival rate, even when the initial population consisted of 80% of resistant individuals. ...
BACKGROUND
Pyrethroid insecticides have been a primary strategy for managing Dalbulus maidis (Hemiptera: Cicadellidae) in Brazil. Howeve, failures in the control of D. maidis with pyrethroids have been reported. In this study, we selected a bifenthrin‐resistant strain of D. maidis under laboratory cage conditions to investigate the inheritance pattern of resistance, cross‐resistance to other insecticides, and resistance stability.
RESULTS
The estimated LC50 of the Bif‐R was 2,055.72 μg a.i. mL⁻¹, while that of the susceptible (Sus) strain was 0.64 μg a.i. mL⁻¹, resulting in a 3,170‐fold resistance ratio (RR). Reciprocal crosses (H1: Bif‐R ♀ × Sus ♂ and H2: Bif‐R ♂ × Sus ♀) and backcrosses between heterozygous H1 and H2 with the Sus strain indicated autosomal, incompletely dominant, and polygenic resistance. Potential cross or multiple‐resistance was observed between Bif‐R and lambda‐cyhalothrin, imidacloprid, and acetamiprid, with resistance ratios varying from 300‐ to 2,000‐fold. No cross‐resistance was detected between Bif‐R and methomyl, carbosulfan or acephate. Cage studies with different proportions of Sus and Bif‐R strains revealed that resistance of D. maidis to bifenthrin is unstable. A decrease in the LC50 of the field‐collected population from 113.61 to 10.73 μg bifenthrin mL⁻¹ was detected in the absence of selection pressure.
CONCLUSIONS
Our findings provide insights into the evolution of resistance of D. maidis to bifenthrin. This study is the first comprehensive analysis of pyrethroid resistance in D. maidis and will contribute to insect resistance management (IRM) strategies to preserve the efficacy of bifenthrin and other insecticides. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.
... With the development of transgenic plants, researchers have introduced the cry genes from Bt, which exhibit high insecticidal activity against pests, into crops to achieve pest resistance and ensure crop yields. However, with the promotion and large-scale cultivation of transgenic crops, some pests have developed resistance to transgenic Bt crops due to selective pressure, posing a serious threat to the sustainable application of Bt products [41]. To address this issue, there is an urgent need to discover new insecticidal genes, modify target genes, and construct transgenic crops with stacked genes. ...
The larvae of Hyphantria cunea feed on plant leaves, causing significant losses to forestry and agricultural production. At present, cry1 genes such as cry1Ac and cry1Ah are mainly used to control H. cunea. To delay the problem of pest resistance induced by a single insecticidal gene, it is crucial to discover and develop new insecticidal genes or gene combinations. This study found cry9Aa3 and cry9Aa4 showed insecticidal activity against H. cunea. The toxicity of 14 mutants of Cry9Aa3 was analyzed and the LC50 of the triple-amino-acid substitution mutant 316LRG318AAA was 3.69 μg/g, which represents a 1.49-fold increase in insecticidal activity compared to Cry9Aa3. Additionally, enhanced stability of this mutant was detected in the midgut juice of H. cunea. Cry9Aa3 and 316LRG318AAA, in combination with Cry1Ah, demonstrated synergistic effects against H. cunea, with synergistic factors of 4.76 and 8.33, respectively. This study has identified the mutant 316LRG318AAA and its combination with Cry1Ah as exhibiting high toxicity against H. cunea, providing valuable genetic resources for the development of transgenic poplars and holding significant importance for delaying resistance in this pest.
... Bollworm management initially relied on the cultivation of transgenic cotton without the use of chemical pesticides. However, due to the widespread cultivation of Bt cotton without the implementation of proper refuge strategies, bollworms developed resistance against Bt cotton (Gould 1998;Banerjee et al., 2017;Tabashnik & Carrière 2017). Another contributing factor to bollworm resistance is the variability in the expression of Cry1Ac (Jamil et al., 2021). ...
Transgenic cotton has brought about a significant transformation in the management of bollworms. Nevertheless, a considerable challenge has emerged in the form of Pectinophora gossypiellafor transgenic cotton. In current study, the detection and quantification of Cry1Ac and Cry2Ab protein in pot samples of different transgenic cotton cultivars encompassing single (Cry1Ac) and double gene (Cry2Ab) cultivars was conducted, employing bt strips and ELISA methods at three distinct time points, namely 40 days, 80 days, and 120 days after sowing, coinciding with different growth phases (i.e., leaves, squares, bolls, and seeds). The outcomes revealed that the highest toxin levels of Cry1Ac protein were present across all cultivars, whereas elevated levels of Cry2Ab protein were exclusively observed in MNH-1045 cultivar ranged from 0.05 ug/g to 1.46 ug/g. The findings highlighted visible differences in the concentrations of Cry1Ac and Cry2Ab proteins among all cultivars. The Cry1Ac concentrations ranged from 0.12 ug/g to 0.81 ug/g, 0.18 ug/g to 0.69 ug/g, 0.19 ug/g to 0.38 ug/g after 40 days, 80 days, and 120 days respectively, and ranged from 0.22 ug/g to 0.59 ug/g, 0.20 ug/g to 0.36 ug/g, 0.00 ug/g to 0.05 ug/g and 0.22 ug/g to 0.30 ug/g, in leaf, square, boll, and seed stage respectively. The concentration of Cry2Ab protein ranged from 0.04 to 1.46, 0.02 to 1.128, 0.01 to 0.28ug/g after 40 days, 80 days, and 120 days respectively, and ranged from 0.4 ug/g to 1.45 ug/g, 0.01 ug/g to 0.63 ug/g, 0.00 ug/g to 0.05 ug/g and 0.00 ug/g to 0.62 ug/g, in leaf, square, boll, and seed stage respectively. It was detected that the expression of toxins is depending upon the developmental stage of the crop and the duration of growth. This study's findings will aid entomologists and plant breeders in developing high-toxin cotton cultivars (especially expressing high at the boll stage) and strategies for global cotton production sustainability, such as refuge maintenance or hybrid development, to protect transgenic cotton from pink bollworm infestations.
... Pest insect resistance development is a major concern of IR transgenic crops (Zhao et al. 2011;Tabashnik and Carriere 2017). Pyramiding different insecticidal proteins with Fig. 4 Leaf disc insect bioassay of five majoy soybean lepidopteran pests. ...
Key message
Transgenic soybean event CAL-16 expressing fusion Bt protein Cry1Ab-Vip3A was developed for conferring broad-spectrum resistance to lepidopteran pests
Abstract
Lepidopteran insect species are important soybean pests causing significant yield loss and quality degradation worldwide. Transgenic soybeans expressing crystal (Cry) insecticidal proteins have been utilized for insect pest management. However, the efficacy of currently adopted insect-resistant soybean is challenged by insect resistance evolution. Vegetative insecticidal proteins (Vips) are highly active against a broad spectrum of lepidopteran insects. They differ from Cry in modes of action, and show great potential for lepidopteran pest management. Here, we report the creation and characterization of a transgenic soybean event CAL-16 which expresses a fusion protein of Cry1Ab and Vip3A. CAL-16 is a single copy T-DNA insertion transgenic event highly resistant to a broad-spectrum of lepidopteran insects. Insect bioassays demonstrated that CAL-16 caused 100% mortality to neonates of Helicoverpa armigera, Spodoptera litura, Agrotis ipsilon, Spodoptera exigua and Spodoptera frugiperda. Field trial also demonstrated its excellent resistance to Leguminivora glycinivorella, a severe pest feeding on soybean seeds. The expression of the fusion protein was found to be constitutively high in CAL-16 throughout developmental stages, and highly stable over 12 generations. Moreover, there was no statistical difference in agronomic traits between CAL-16 and its non-transgenic recipient control plants in field trial. In conclusion, CAL-16 is an elite soybean event with high efficacy toward major lepidopteran pests. It is expected to be released for commercial cultivation in the near future in China as it has been deregulated in China in 2023.
... Bacterial insecticides were first developed in the 1950s and, due to their ease of mass production, became the first microbial pesticides to be commercialized. For over 40 years, these products have been in use and currently account for 90% of biopesticides marketed globally (Nathan et al. 2014;Osman et al. 2015;Tabashnik and Carrière 2017;Villareal et al. 2017;Kumar et al. 2021). While these insecticides offer advantages such as high effectiveness and specificity, they also face challenges, including low stability, high production costs, and inconsistent results in field trials (Cooping and Menn 2000; Gullino et al. 2005;Hynes and Boyetchko 2006;Gupta et al. 2010;Xu et al. 2011;Chandler et al. 2011;Leng et al. 2011;Glare et al. 2012;Gasic and Tanovic 2013;Bhattacharjee and Dey 2014;Siever et al. 2014;Ritika and Utpal 2014;Mishra et al. 2015;Mnif et al. 2015;Kumar and Singh 2015;Tijjani et al. 2016;Aneja et al. 2016;Shiberu et al. 2016;Kachhawa 2017;Lengai et al. 2018;Samada and Tambunan 2020;Hakim et al. 2020;Kumar et al. 2021) (Fig. 2). ...
Bacterial insecticides have emerged as a biotechnological strategy for controlling insect pests in crops, primarily due to the ineffectiveness of synthetic pesticides, which have led to resistance in insect populations due to their overuse. For many years, Bacillus thuringiensis has been recognized as the most promising microorganism for this purpose, while other entomopathogenic bacteria, which have demonstrated efficacy against various insect orders, have been largely overlooked. These entomopathogenic bacteria employ diverse mechanisms to control insect pests, including damage via contact or ingestion, disruption of intestinal cell integrity, interference with the insect's central nervous system, and alterations to reproductive processes. Among the insecticidal compounds produced by these bacteria are Cry homologous toxins, non-homologous toxins, biosurfactant-type compounds, macrocyclic lactones, lipopeptides, chitinases, and other metabolites that remain underexplored. This paper provides a comprehensive overview of the current state of lesser-known entomopathogenic bacteria utilized in the development of these biopesticides. It discusses the advantages and disadvantages of these biotechnological products, the mechanisms of action of entomopathogenic bacteria, genetic engineering strategies aimed at enhancing these biopesticides, and the application of these bacteria in commercial production. Additionally, the paper reviews patents related to this field and examines the role of insecticidal bacteria as inducers of systemic resistance (ISR) in plants, highlighting their potential for the development of commercial bioproducts.
... Bt crops appear harmless to human consumers (Koch et al. 2015), enable IPM by supporting biological control (Romeis et al. 2019), and have revolutionised some modern farming systems (Qaim and Zilberman 2003;Wilson, Whitehouse, and Herron 2018). However, in response to the widespread adoption of Bt crops, there has been a surge in the level of Bt-resistant pests (Tabashnik and Carrière 2017;Tabashnik, Fabrick, and Carrière 2023). ...
Pigeonpea is one of the world's most important grain legume crops. Mostly grown and consumed in India, where it is a staple food, pigeonpea production also occurs elsewhere in Asia, Africa, Latin America and Australia. Despite widespread cultivation and staple food status, pigeonpea yields have barely increased over the last half century. The prevalence and severity of insect pests present major constraints to increasing pigeonpea yields. Two of the most significant pests of pigeonpea are the lepidopteran 'pod-borers'-Helicoverpa armigera and Maruca vitrata. The pod fly (Melanagromyza obtusa) and several species of pod-feeding Hemiptera are also regular pests, and numerous other minor or sporadic pests have been recorded throughout the cultivated distribution of the crop. Current pigeonpea pest management practices rely heavily on the application of synthetic insecticides. Most research has fo-cused on the management of H. armigera, M. vitrata and M. obtusa due to their damaging feeding behaviour, and the propensity of H. armigera to evolve resistance to synthetic insecticides. Not surprisingly, pest management in pigeonpea is largely based around these three major pests, particularly the lepidopteran pod-borers which appear to be more damaging to modern short-duration cul-tivars than to older cultivars. A large amount of research has attempted to develop pigeonpea cultivars with conventional host-plant resistance to pod-borers and pod fly, but with limited success. Future pigeonpea pest management research should take a more integrated approach, exploring underexamined areas such as: understanding how modern pigeonpea varieties and traditional lan-draces respond to pest herbivory, identifying what cultural control methods are available to smallholder farmers, and investigating how biological control can be incorporated into management practices. Future research has the potential to develop IPM strategies in pigeonpea and provide farmers with an alternative to an unsustainable dependence on synthetic insecticides.
... T ransgenic, or genetically engineered, crop technologies targeting key insect pests have revolutionized global food production, helping feed a growing population with minimal environmental harm (1,2). However, the onset of pest resistance inevitably follows increased adoption of transgenic varieties, rendering the technology less effective over time (3,4). In 2003, transgenic maize hybrids producing rootworm-active insecticidal proteins derived from the bacterium Bacillus thuringiensis (Bt) were introduced. ...
Widespread use of genetically engineered maize targeting the corn rootworm complex ( Diabrotica species) has raised concerns about insect resistance. Twelve years of university field trial and farm survey data from 10 US Corn Belt states indicate that maize hybrids expressing toxins derived from the bacterium Bacillus thuringiensis (Bt maize) exhibited declining protection from rootworm feeding with increased planting while pest pressures simultaneously decreased. The analysis revealed a tendency to overplant Bt maize, leading to substantial economic losses; this was particularly striking in eastern Corn Belt states. Our findings highlight the need to go beyond the “tragedy of the commons” perspective to protect sustainable use of Bt and other crop biotechnology resources. We propose moving toward a more diversified and transparent seed supply.
... The frequency of susceptible insects in the refuge depends on several factors, including pest bionomics, genetic mode, stability of resistance, and the relative performance of susceptible and resistant insects. However, in Argentina, the failure to comply with refuge requirements, as showed by the limited area of non-Bt soybean planted (Sistema de Información Simplificado Agrícola (SISA), 2021; Paredes et al., 2022), may be a key factor contributing to the buildup of pest resistance to Bt crops (Arends et al., 2021;Tabashnik and Carrière, 2017). ...
Rachiplusia nu (Guenée) (Lepidoptera: Noctuidae) is a significant agricultural pest in South America infesting several crops, including soybeans. Bacillus thuringiensis (Berliner) (Bt) soybean, expressing Cry1Ac protein, is widely planted as a control method for numerous lepidopteran pests. However, insect resistance to Bt proteins poses a threat to its sustainability. Recent field reports from Argentina, Uruguay, and Brazil have documented unexpected injury to Bt soybean caused by R. nu populations, which may indicate the development of resistance to Cry1Ac in this species. This study aimed to evaluate the biological performance, reproductive parameters, and reproductive compatibility of two R. nu strains, one susceptible (SS) and the other exhibiting reduced susceptibility to Bt toxin (RR), when reared on Bt and non-Bt soybean. Reproductive compatibility between strains was investigated through parental and hybrid crosses, evaluating fecundity, fertility, and mating success. SS larvae fed with Bt soybean failed to complete their life cycle, whereas RR larvae exhibited higher survival rates. Egg and larval stages of RR larvae were longer when reared on Bt soybean. Pupal mass was lower for Bt-fed resistant strain, although this did not reflect on fecundity and longevity. Results on parental crosses revealed that Bt-fed RR strain displayed reduced mating success, fecundity, and fertility, compared to the non-Bt treatment. Hybrid crosses showed evidence for prezygotic and postzygotic incompatibility. These results suggest a shift in susceptibility of R. nu to Cry1Ac protein and highlight the importance of implementing robust insect resistance management strategies to maintain the effectiveness of Bt crops.
... In India, Srivastava et al. (1966) emphasized that P. gossypiella destroyed 25 % of the locules and 10-15 % of the produced seeds. The dynamic character of bollworm infestations has been highlighted in recent research, especially in light of the emergence of resistance to traditional control strategies, such as genetically modified Bt cotton Carrière, 2017 andWan et al., 2019). Cotton bolls are directly damaged by bollworm infestations, which lower the amount and caliber of cotton fibers produced. ...
... Pest management is increasingly challenging due to evolving resistance to Bt toxins and pesticides (Tabashnik and Carriè re, 2017) and the rising risk of invasive species (Early et al., 2016). Climate change further alters disease distribution and severity. ...
... These transgenic cotton varieties exhibit enhanced protection against targeted pests, contributing to more resilient and productive agricultural systems. Another innovative approach, RNA interference (RNAi), leverages double-stranded RNA molecules to silence specific target genes essential for pest survival or reproduction, offering a precise and environmentally friendly method for pest management (Tabashnik et al., 2017). ...
... This is because of the possibility of LPBs developing resistance to the Cry1Ab protein over time [43]. In other crops, pest resistance development has been managed through pyramiding genes expressing different insecticidal proteins [44][45][46]. This option to improve the durability of Songotra T is already in progress, and transgenic cowpeas containing the cry2Ab gene, which provides protection against LPB, have been tested successfully in confined field trials in Nigeria, Ghana, and Burkina Faso. ...
Cowpea (Vigna unguiculata) is a vital crop in sub-Saharan Africa, but the legume pod borer (LPB), Maruca vitrata, can cause over 80% yield losses. Natural resistance to this lepidopteran pest is absent in cowpea germplasm, and insecticides are ineffective due to the pest’s cryptic behavior. To address this, a genetically modified (GM) cowpea expressing the cry1Ab protein from Bacillus thuringiensis (Bt) was developed, providing complete LPB resistance. This Bt cowpea, commercialized as Sampea 20-T in Nigeria, was recently approved in Ghana as Songotra T. To evaluate its performance and the financial returns of its cultivation, field trials were conducted across multiple locations in northern Ghana to compare it to the non-transgenic Songotra control and two commercial cultivars, Kirkhouse-Benga and Wang-Kae. Songotra T exhibited protection against LPB infestations and damage, achieving a grain yield of 2534 kg/ha compared to 1414–1757 kg/ha for the other entries. As expected, non-LPB pest infestations and damage were similar across all entries. Economic analysis revealed that Songotra T had the highest return on investment (464%), outperforming the other tested cultivars (214%). These results demonstrate the potential of GM crops to enhance yields and profitability for resource-poor farmers, underscoring the value of biotechnology for addressing critical agricultural challenges.
... As an alternative pest control measure have genetically modified crops, such as Bt (with a toxin from Bacillus thuringiensis) cotton, shown a good control effect of H. armigera over a period. But, as for synthetic pesticides, resistant populations have developed also for Bt crops, making well deliberated resistance management strategies necessary (Jin et al., 2015;Downes et al., 2016;Tabashnik and Carrière, 2017). Today, IPM strategies based on forecast, monitoring and decision support systems combined with biological, chemical, and physical control measures must be developed and used for successful control of H. armigera (Downes et al., 2016;Jones et al., 2019;Riaz et al., 2021). ...
Invasive pests and plant pathogens pose a significant threat to ecosystems and economies worldwide, prompting the need of anticipatory strategies. Preventing their introduction by detection at the ports of entry has been proven extremely difficult. This review explores the potential of biogenic volatile detection as a reliable preventive solution. It underscores the importance of early detection and rapid response as integral components of effective invasive pest management, and it discusses the limitations of current control measures and the increasing globalization that facilitates the spread of pests and pathogens. Through a synthesis of existing literature, this review analyzes the Volatile Organic Compound (VOC) emissions in five invasive model species: three insects, Halyomorpha halys , Spodoptera frugiperda , Helicoverpa armigera , a nematode, Bursaphelenchus xylophilus , and an oomycete, Phytophthora ramorum . The review focuses on the specific volatiles, released by both the invasive organisms and the infested host plants. If available, the volatiles emitted from similar species were considered for comparison. Ultimately, this review highlights specific pest volatile and shared Herbivore Induced Plant Volatiles (HIPVs) as a reliable and innovative solution in pest detection. If possible, candidate compounds are provided, whilst the lack of some emphasizes the urge of expanding the information available.
... One such method involves spraying crops with toxins generated from Bacillus thuringiensis (BT), which has been employed for over six decades (Bravo et al. 2011;Heckel 2020). Nevertheless, numerous significant insect pest species exhibit resistance to conventional management approaches, and certain previously vulnerable species have acquired resistance to treatments such as BT toxins (Tabashnik et al. 2013;Tabashnik and Carrière 2017). The demand for novel and sustainable crop protection measures is being stimulated by factors such as a growing population, the extension of pest ranges due to climate change, community and regulatory requirements, and the development of pest resistance to conventional agricultural pesticides. ...
RNA interference (RNAi) is a powerful technique used to regulate gene expression and silence specific genes. RNAi has been widely adopted in agriculture to enhance plant resistance against biotic stresses and improve crop traits. The mechanism of RNAi involves the introduction of small RNA molecules into cells, which then bind to complementary mRNA molecules and prevent their translation into proteins. The small RNA molecules responsible for gene silencing include small interfering RNAs (siRNAs) and microRNAs (miRNAs). siRNAs can be introduced into host cells and combine with the RNA-induced silencing complex (RISC). The siRNA binds to the target mRNA molecule and leads to its degradation, preventing protein synthesis. miRNAs can also incorporated into the RISC and bind to their target mRNAs resulting in translational repression. To initiate targeted gene silencing using RNAi, siRNAs or artificial miRNAs are then introduced into plant cells using various methods. In the context of increasing plant resistance against biotic stresses, RNAi has been used to silence genes involved in plant-pathogen interactions. By targeting virulence or host susceptibility genes, RNAi can disrupt the infection process and enhance plant resistance. Also, RNAi could be employed as an efficient tool to improve crop traits, grain yield, and fruit quality characteristics. Generally, by silencing genes that negatively regulate plant defense responses or enhance resistance, researchers could be able to develop crops with enhanced resistance to pathogenes and diseases with enhanced attributes.
... Smith), have developed practical resistance to Bt crops, greatly reducing the control efficacy and utilization life of Bt crops. 20,21,23,[44][45][46] Therefore, managing the resistance of the ACB is necessary for the commercial planting of Bt-Cry1Ab maize. Based on 30 years of experience in the cultivation of Bt crops, the high dose/refuge strategy and multi-gene transformation strategy (MTS) are the basic strategies used for target pest resistance management. ...
BACKGROUND
The Asian corn borer (ACB), Ostrinia furnacalis (Guenée), is a major pest restricting maize production in Asia. The Chinese government has approved the commercial planting of Bt‐Cry1Ab maize (event DBN9936), but its control potential against the ACB in southern regions remains unclear. This study evaluated the sensitivity of ACB to Cry1Ab protein expressed in Bt‐Cry1Ab maize and determined the control efficacy of Bt‐Cry1Ab maize against the ACB in Sichuan Province, a major maize‐producing region in southern China, based on pilot planting in the field, and larval feeding bioassays in the field and laboratory.
RESULT
The Cry1Ab protein contents in different tissues of Bt‐Cry1Ab maize ranged from 31.20–88.27 μg g⁻¹. The range of median lethal concentrations (LC50) and median growth inhibitory concentration (GIC50) values of Cry1Ab protein expressed in Bt‐Cry1Ab maize against ACB larvae were 0.036–0.109 μg mL⁻¹ and 0.002–0.018 μg mL⁻¹, respectively. The first and fourth instar ACB larvae were unable survive feeding on different tissues of Bt‐Cry1Ab maize plants. Field experiments conducted from 2023 to 2024 indicated that the number of ACB larvae per 100 plants, plant damage rate, leaf damage rate, male ear damage rate, female ear damage rate, and stalk damage rate in the Bt‐Cry1Ab maize fields were reduced by 95.36% ± 2.17%, 83.98% ± 1.73%, 89.45% ± 1.50%, 100.00% ± 0.00%, 69.79% ± 4.88% and 100.00% ± 0.00%, respectively, compared to conventional maize fields.
CONCLUSION
The ACB population in Sichuan Province, China is sensitive to Cry1Ab expressed in Bt‐Cry1Ab maize (event DBN993). Planting Bt‐Cry1Ab maize efficiently reduces the population of ACB larvae and the percentage of damaged maize plants, and has great application potential in the integrated pest management of the ACB in Sichuan Province, China. © 2024 Society of Chemical Industry.
... The active toxin interacts with insect midgut epithelium, leading to the destruction of midgut tissue cells, which in turn affects the integrity of the midgut membrane, causes inflammation and other diseases and finally leads to insect death [16][17][18]. It is precisely because of the toxic activity of the Cry protein of Bt to insect larvae that these toxins can be used for biological control of pests by spraying formula and transgenic crops, including Cry protein or some active fragments [19][20][21]. It is reported that Bt is active against pests such as Lepidoptera [10], Diptera [12], Coleoptera [22,23] and Hymenoptera [24,25], but it is safe for humans, aquatic organisms, bees and other organisms to use at appropriate concentrations [11]. ...
Dendrolimus kikuchii Matsumura (D. kikuchii) is a serious pest of coniferous trees. Bacillus thuringiensis (Bt) has been widely studied and applied as a biological control agent for a variety of pests. Here, we found that the mortality rate of D. kikuchii larvae after being fed Bt reached 95.33% at 24 h; the midgut membrane tissue was ulcerated and liquefied, the MDA content in the midgut tissue decreased and the SOD, CAT and GPx enzyme activities increased, indicating that Bt has toxic effects on D. kikuchii larvae. In addition, transmission electron microscopy showed that Bt infection caused severe deformation of the nucleus of the midgut tissue of D. kikuchii larvae, vacuoles in the nucleolus, swelling and shedding of microvilli, severe degradation of mitochondria and endoplasmic reticulum and decreased number. Surprisingly, metabolomics and transcriptome association analysis revealed that four metabolic-related signaling pathways, Nicotinate and nicotinamide metabolism, Longevity regulating pathway—worm, Vitamin digestion and absorption and Lysine degradation, were co-annotated in larvae. More surprisingly, Niacinamide was a common differential metabolite in the first three signaling pathways, and both Niacinamide and L-2-Aminoadipic acid were reduced. The differentially expressed genes involved in the four signaling pathways, including NNT, ALDH, PNLIP, SETMAR, GST and RNASEK, were significantly down-regulated, but only SLC23A1 gene expression was up-regulated. Our results illustrate the effects of Bt on the 5th instar larvae of D. kikuchii at the tissue, cell and molecular levels, and provide theoretical support for the study of Bt as a new biological control agent for D. kikuchii.
... Since 2002, China has commercially promoted transgenic poplar trees expressing Cry1A toxin to control the fall webworm; this is the earliest example of forest insect-resistant genetic engineering [2]. The cultivation of insect-resistant transgenic plants can effectively control pests, although the resistance of target pests to Bt toxin is the biggest challenge for the long-term and safe cultivation of transgenic Bt crops [3]. ...
Transgenic poplars have been used to control quarantine pests worldwide, such as the fall webworm (Hyphantria cunea, FW). However, the studies on the resistance mechanism of FW to Cry toxins are limited. This study obtained an FW strain with 45-fold resistance to Cry1Ab toxin by continuous screening in the laboratory. The resistance to Cry1Ab was autosomally completely recessive, and it had approximately 40-fold cross-resistance to Cry1Ac but no cross-resistance to Cry2Ab toxin. Analysis of indoor population life tables for the resistant and susceptible strains found no significant difference in the net reproduction rate (R0) between the two strains. The resistant strain had an insignificantly longer larval development duration and a comparable pupation rate, pupal weight, emergence rate, eggs laidper female, and egg hatching rate compared to the susceptible strain. This indicated that there was no fitness cost for Cry1Ab resistance. Our study helps in evaluating the risk of resistance to Cry1Ab toxin and the mechanisms of resistance to Cry1Ab toxin in FW.
... Transgenic breeding and genome editing can manipulate functional genes for trait improvement (Figure 1). At present, transgenic breeding is mostly used to improve resistance to pests and herbicides (Tabashnik and Carrière, 2017;Dong et al., 2021). Genome editing can knock in desirable alleles or knock out unfavorable genes in natural or breeding materials, and is expected to play an important role Empirical breeding usually includes phenotypic selection, and breeding based on general genetic theories, it is a process of accumulating data. ...
The past decade has witnessed rapid developments in gene discovery, biological big data (BBD), artificial intelligence (AI)‐aided technologies, and molecular breeding. These advancements are expected to accelerate crop breeding under the pressure of increasing demands for food. Here, we first summarize current breeding methods and discuss the need for new ways to support breeding efforts. Then, we review how to combine BBD and AI technologies for genetic dissection, exploring functional genes, predicting regulatory elements and functional domains, and phenotypic prediction. Finally, we propose the concept of intelligent precision design breeding (IPDB) driven by AI technology and offer ideas about how to implement IPDB. We hope that IPDB will enhance the predictability, efficiency, and cost of crop breeding compared with current technologies. As an example of IPDB, we explore the possibilities offered by CropGPT, which combines biological techniques, bioinformatics, and breeding art from breeders, and presents an open, shareable, and cooperative breeding system. IPDB provides integrated services and communication platforms for biologists, bioinformatics experts, germplasm resource specialists, breeders, dealers, and farmers, and should be well suited for future breeding.
... Transgenic cotton provides us with high cotton yield and sustainability [15]. But, with time the success ratio of transgenic cotton started decreasing when the pests developed a resistance mechanism against the Cry toxins produced by the Bt crops [16]. To obtain maximum yield of cotton, there is a great need to control pests and for this, it is important that the plant must produce toxins at a specific level, which is lethal to the targeted arthropods, and it should not survive above this target level. ...
Cotton is one of the world’s largest fiber-producing crops. It is also known as the important cash crop of Pakistan. Cotton crop products contribute to economy as cottonseed oil, animal feed and the fiber support the textile industries, thus, playing a significant role in uplifting the economy of a country. But over time, bollworm attacks had escalated, severely reducing the yield of cotton and damaging cotton production. Bt (Bacillus thuringiensis) cotton is being widely used nowadays due to its enhanced resistance against cotton bollworms which are responsible for reducing crop production leading to high economic loss. It greatly affects the economy by increasing the cost of production and protection. Using resistant strains of Bt cotton not only makes it easier to combat bollworm attacks but also reduces the substantial expense issues that farmers face. Bt has many toxic proteins among which the two crystal proteins, Cry1Ac and Cry2Ab, are used to develop resistance against bollworms in cotton. This study aimed to optimize the developing transgenic callus containing Cry1Ac and Cry2Ab genes in cotton. The dual gene construct (Cry1Ac + Cry2Ab) provides more resistance to the cotton crop as compared to the single gene construct against whom the most bollworms have developed resistance. The vector was inoculated into the hypocotyl segments and the transformation process was preceded by shifting the hypocotyls at different mediums Agrobacterium tumefaciens strain LBA4404 was used for cotton transformation. The construct-containing vector was introduced into a standard cotton line. A molecular confirmation test was carried out utilizing PCR and gene-specific primers after the formation of the transgenic callus. As a result of the research an indigenous cotton line that expresses double Bt genes for insect resistance was developed.
... Traditionally, strategies to manage resistance to synthetic pesticides and genetically modified crops involve weakening selection for resistance, for example by minimising pesticide application, employing crop refuges, or rotating use of products with different modes of action [15][16][17] . An additional approach is Negatively Correlated Cross-Resistance (NCCR), which aims to exploit the fact that resistance to one product sometimes trades-off with resistance to another 18 . ...
Pathogens often exert strong selection, yet host populations maintain considerable genetic variation for resistance, possibly due to environmental heterogeneity causing fitness trade-offs. These trade-offs could help explain genetic variation for pathogen defence, and also constrain resistance evolution to microbial pesticides (an underappreciated risk). However, the presence and strength of such trade-offs remain unclear. We investigate whether pathogen identity or host diet has a stronger effect on the fitness of resistance alleles. We quantify genetic variation and covariation for pathogen resistance in an insect pest across distinct pathogen and plant diet combinations. We demonstrate substantial heritability, indicating considerable risks of biopesticide resistance. Contrary to conventional thinking in host-pathogen biology, we found no strong genetic trade-offs for resisting two fungal pathogen species, but changes in plant diet dramatically altered selection for resistance, revealing diet-mediated genetic trade-offs affecting pest survival. Our data suggest that trade-offs not strictly related to immunocompetence could nevertheless maintain genetic variation for pathogen defence in natural and agricultural landscapes.
... The FDA has deemed Bt-expressing crops safe for consumption, and these crops are used in everyday products such as corn syrup and cotton-based textiles. While Bt crops have been effective in controlling pests like the corn borer, there is a growing need to extend this technology to other pest systems, such as plant-parasitic nematodes (Peferoen, 1997;Tabashnik and Carriè re, 2017). One of the main challenges has been the nematodes' sizeexclusion mechanisms, which were thought to prevent them from ingesting larger proteins such as Cry. ...
... However, resistance evolution in FAW populations significantly undermines the efficacy of these chemicals, necessitating higher doses or alternative compounds, which raises environmental and health concerns [9]. Resistance evolution has been shown to occur even against several events available in genetically modified (GM) crops expressing Bacillus thuringiensis (Bt) toxins, rapidly reducing the lifespan of this technology [10]. The use of biological control strategies with parasitoids, predators, and entomopathogens is often attempted but with very little efficacy due to the need for specific environmental conditions and the slow action of biocontrol agents [11]. ...
The Streptomyces genus has long been recognized as a prolific and valuable source of diverse secondary metabolites. These metabolites contribute significantly to the intricate chemical diversity exhibited by Streptomyces, making them an indispensable reservoir for drug discovery, agricultural applications, and industrial processes. Exploiting the potential of these natural compounds holds the promise of ushering in a new era in insect pest management, reducing reliance on synthetic chemicals and fostering ecologically sustainable solutions. This study dives into the realm of chemo diversity within isolates of Streptomyces nojiriensis and Streptomyces novaecaesareae, with a specific focus on the production of insecticidal compounds. We explored chromatographic techniques for the identification and isolation of insecticidal compounds, and two bioactive compounds were identified in extracts of S. novaecaesareae. Valinomycin was identified from hexanic extracts of strain Asp59, while naphthomycin from ethyl acetate extracts of strain Asp58. These compounds showed insecticidal activity against first instars of Spodoptera frugiperda (Asp59: LC50 = 10.82 µg/µL, LC90 = 26.25 µg/µL; Asp58: LC50 = 15.05 µg/µL, LC90 = 38.84 µg/µL). Notably, this is the first report of naphthomycin as an insecticidal compound. The present study suggests that valinomycin and naphthomycin may be a novel biological source for the control of Spodoptera frugiperda in early stages.
... This diversity in toxin mechanisms helps enhance the overall efficacy of Bt cotton in managing pest populations and reducing crop damage (Chen et al. 2017). Some countries swiftly adopted double and triple gene technologies in the cultivation plan, while Pakistan continues to rely solely on the initially introduced single gene (Cry1Ac) Bt cotton, which result in the development of resistance in the field (Tabashnik et al. 2013;Tabashnik and Carrière 2017). ...
Background: Bt technology has played significant role in controlling bollworms and increasing cotton yield in earlier days of its introduction, a subsequent decline in yield became apparent over time. This decline may be attributed to various environmental factors, pest dynamics, or combination of both. Therefore, the present biophysical survey and questionnaire were designed to evaluate the impact of Bt cotton on bollworms management and its effect on reducing spray costs, targeting farmers with varied landholdings and educational backgrounds. Additionally, data on farmers' cultivated varieties and the prevalence of bollworms and sucking insects in their fields were recorded. Subsequently, about eleven thousand cotton samples from farmer fields were tested for Cry1Ac, Cry2Ab and Vip3A genes by strip test.
Results: In this analysis, 83% of the farmers planting approved varieties believe that Bt technology control boll-worms, while 17% hold contradictory views. Similarly, among farmers cultivating unapproved varieties, 77% agree on effectiveness of Bt technology against bollworms, while 23% disagree. On the other hand, 67% of farmers planting approved varieties believe that Bt technology does not reduce spray costs, while 33% agree with the effectiveness. Similarly, 78% of farmers cultivating unapproved varieties express doubt regarding its role to reduce spray costs, while 22% are in favour of this notion. Differences in opinions on the effectiveness of Bt cotton in controlling boll-worms and reducing spray cost between farmers planting unapproved and approved varieties may stem from several factors. One major cause is the heavy infestation of sucking insects, which is probably due to the narrow genetic variation of the cultivated varieties. Additionally, the widespread cultivation of unapproved varieties (21.67%) is also an important factor to cause different opinions on the effectiveness of Bt cotton.
Conclusion: Based on our findings, we propose that the ineffective control of pests on cotton crop may be attributed to large scale cultivation of unapproved varieties and non-inclusion of double and triple transgene technologies in country's sowing plan. On the basis of our findings, we suggest cotton breeders, regulatory bodies and legislative bodies to discourage the cultivation of unapproved varieties and impure seed. Moreover, the adoption of double and triple Bt genes in cottons with a broad genetic variation could facilitate the revival of the cotton industry, and presenting a promising way forward.
... st. However, the effectiveness and long-term sustainability of these pyramids are compromised by cross-resistance and antagonism among Bt toxins, thus necessitating the exploration of alternative management strategies (Hackett & Bonsall. 2016). RNAi can be combine with Bt toxins to produce more effective and durable resistance against insect pests (Tabashnik & Carrière. 2017) (Fig. 4). RNAi employs small dsRNA to reduce target gene expression at a particular sequence. To accomplish safe and effective pest control with RNAi, the goal is to limit the expression of genes that encode proteins required by pests but not other organisms. Potential targets for RNAi include genes encoding proteins that manufacture or ...
Cotton plays a crucial role in shaping Indian economy and rural livelihoods. The cotton crop is prone
to numerous insect pests, necessitating insecticidal application, which increases production costs. The advent
of the expression of Bacillus thuringiensis (Bt) insecticidal protein in cotton has signifcantly reduced the burden
of pest without compromising environmental or human health. After the introduction of transgenic cotton,
the cultivated area expanded to 22 million hectares, with a 64% increase in adoption by farmers worldwide.
Currently, Bt cotton accounts for 93% of the cultivated cotton area in India. However, extensive use of Bt cotton
has accelerated resistance development in pests like the pink bollworm. Furthermore, the overreliance on Bt cotton
has reduced the use of broad-spectrum pesticides, favouring the emergence of secondary pests with signifcant
challenges. This emphasizes the urgent necessity for developing novel pest management strategies. The high-dose
and refuge strategy was initially efective for managing pest resistance in Bt cotton, but its implementation in India
faced challenges due to misunderstandings about the use of non-Bt refuge crops. Although gene pyramiding
was introduced as a solution, combining mono toxin also led to instances of cross-resistance. Therefore, there
is a need for further exploration of biotechnological approaches to manage insect resistance in Bt cotton. Advanced
biotechnological strategies, such as sterile insect release, RNA interference (RNAi)-mediated gene silencing, stacking
Bt with RNAi, and genome editing using clustered regularly interspaced short palindromic repeats/CRISPR-associated
protein (CRISPR-Cas), ofer promising tools for identifying and managing resistance genes in insects. Additionally,
CRISPR-mediated gene drives and the development of novel biopesticides present potential avenues for efective
pest management in cotton cultivation. These innovative approaches could signifcantly enhance the sustainability
and efcacy of pest resistance management in Bt cotton.
Helicoverpa zea is a major agricultural pest, particularly in cotton, and poses significant challenges due to its ability to develop resistance to chemical insecticides. This study evaluates the efficacy of the entomopathogenic nematode ( Steinernema adamsi ) and its mutualistic bacteria ( Xenorhabdus ) as biological control agents against H. zea larvae in both laboratory and field settings. In laboratory assays, mortality rates for 1 st to 4 th instars were high, ranging from 74.2% to 100%, while 5 th instars exhibited significantly lower susceptibility (<37% mortality). Pupae were completely resistant to nematode infection. The impact of UV radiation on nematode efficacy was assessed, with mortality decreasing from 100% in control conditions (0 hours of UV exposure) to 71.8% after 5 hours of UV exposure, highlighting the vulnerability of S. adamsi to UV degradation. In addition, Xenorhabdus caused 100% mortality in H. zea larvae when injected directly into the hemocoel, but oral toxicity was significantly lower, with 36% mortality in 7 days post-exposure. Field experiments demonstrated that the combination of S. adamsi with 0.05% sodium alginate (hygroscopic agent) and 0.02% Congo red (UV protectant) resulted in a significant increase in larval mortality. In field test A, where S. adamsi was applied in water, mortality averaged 56% with 82% EPN infection. In field test B, the combined treatment of sodium alginate and Congo red led to 98% larval mortality, although infection rates were lower and statistically non-significant. The addition of these protective agents likely enhanced the environmental stability and efficacy of the nematodes under field conditions. These findings suggest that S. adamsi can be an effective biological control agent for H. zea , particularly when combined with formulations that protect against UV radiation and desiccation. Future research should focus on optimizing nematode delivery systems to improve field efficacy under diverse environmental conditions.
Fall armyworm (FAW), Spodoptera frugiperda, invaded the south of China in December 2018 and has since posed a huge threat to crop production in China. However, transgenic Bacillus thuringiensis (Bt) corn can efficiently control the damage caused by FAWs. In fact, the Chinese government has issued biosafety certificates for several Bt corn hybrids expressing any one of four Bt proteins, Vip3A, Cry1F, Cry1Ab, and Cry2Ab, or combinations thereof, to control FAWs. These Bt corn events are soon to be commercialized in China. Therefore, it is necessary to monitor and evaluate whether the FAW has developed resistance to any of the Bt corn hybrids planted in fields in China. To address this issue, we collected 11 geographical populations of FAWs and determined the sensitivity of each to the aforementioned four purified Bt proteins as assessed by diet surface overlay bioassays. The ranges for the 50% lethal concentration (LC50) of the four Bt proteins to all FAW populations were as follows: 11.42–88.33 ng/cm² (for Vip3A), 111.21–517.33 (Cry1F), 135.76–1108.47 (Cry1Ab), and 994.42–5492.50 (Cry2Ab). The corresponding ranges for the 50% growth inhibition concentrations (GIC50) were 1.43–14.86, 2.35–138.97, 1.58–464.86, and 25.01–1266.07 ng/cm². The lethal effects and growth inhibition effects of the four Bt proteins on FAW were in the same order of Vip3A > Cry1F > Cry1Ab > Cry2Ab. A comparison with published LC50 values of Bt proteins towards sensitive FAW populations revealed that all 11 FAW populations in this study were sensitive to Vip3A, Cry1F, and Cry1Ab. This study provides foundational data for monitoring and controlling the resistance of Bt corn to FAW in China.
Transgenic crops that produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) are grown worldwide to manage insect pests. Western corn rootworm is a serious pest of maize in the United States and is managed with Bt maize. In the United States, the commercial cultivation of a Bt crop requires an accompanying resistance-management strategy to delay the evolution of Bt resistance. One of the primary resistance-management strategies consists of non-Bt refuges along with a Bt crop that produces two Bt toxins (i.e., a pyramid) that kill the same pest species. This approach delays resistance because individuals with resistance to one toxin are killed by the second. However, if a pest species is resistant to one toxin in a pyramid, the effectiveness of a pyramid to delay resistance is compromised, potentially leading to the evolution of resistance to both toxins. Here, we apply a meta-analysis to demonstrate the sequential evolution of resistance by western corn rootworm to Bt maize producing Cry3Bb1 followed by resistance to Gpp34/Tpp35Ab1 maize, with resistance to each Bt toxin increasing in a linear manner over time. Additionally, we show that Bt-resistant western corn rootworm imposed substantial feeding injury, in the field, to maize containing a pyramid of Gpp34/Tpp35Ab1 and Cry3Bb1. To minimize the risk of sequential evolution of resistance to multiple transgenic traits, an emphasis should be placed on developing transgenic pyramids not compromised by prior resistance, and in cases where resistance is already present, larger non-Bt refuges and more diversified pest-management approaches should be applied.
Debate over resistance management tactics for genetically engineered (GE) crops expressing insecticidal toxins is not new. For several decades, researchers, regulators, and agricultural industry scientists have developed strategies to limit the evolution of resistance in populations of lepidopteran and coleopteran pests. A key attribute of many of these events was insecticide resistance management (IRM) strategies designed around a presumed high-dose expression sufficient to kill 99.5% of exposed larvae for some of the main target pests in corn, Zea mays L. and cotton, Gossypium hirsutum L. In contrast, other target pests did not meet this high-dose criterion. Similarly, the recent release of ThryvOn cotton that expresses thysanopteran and hemipteran active Mpp51Aa2.834_16 toxin is not high dose, working on a combination of behavioral and sublethal effects to suppress populations. This unique mode of control has generated considerable uncertainty about what IRM strategies will be most effective to limit field-evolved resistance to this unique spectrum of pests. The goal of this manuscript is to present several knowledge gaps that exist in proposed Mpp51Aa2 IRM plans, focusing on its activity on thrips, Frankliniella spp. Addressing these gaps will be crucial to limit resistance and preserve the benefits that this technology may provide by alleviating reliance on conventional insecticides and seed treatments. Broadly, these considerations will be important for future GE events that are non-high dose but remain valuable components of a more holistic insect management programs that integrate multiple tactics to reduce conventional insecticide use for challenging pests.
In the search for effective strategies to control the Colorado Potato Beetle, RNA interference technology has emerged as a promising method due to its capacity to suppress genes selectively. Factors such as the target gene and double-stranded RNA (dsRNA) length are critical for optimizing gene silencing efficiency. In this study, we designed and synthesized in vitro dsRNAs of varying lengths targeting the ACE1 gene, which encodes the AChE1 isoform of acetylcholinesterase in the beetle. All tested dsRNA lengths (222 bp, 543 bp, 670 bp, and 870 bp) promoted transcript reduction. The 670 bp dsRNA was the most effective, reducing transcript levels by approximately 40% by day seven, followed by the 543 bp dsRNA. No significant differences were observed between the 222 bp and 870 bp dsRNAs. Furthermore, all of the dsRNA lengths resulted in reduced weight gain and increased mortality in larvae, with the 670 bp dsRNA showing the highest mortality rate, leaving only 63% larval survival, a trend that persisted through day nine. These findings emphasize that dsRNA length is a key factor in the silencing response, underscoring the importance of selecting the optimal length while considering the gene’s target, stability, and delivery methods. This study contributes to establishing design criteria for dsRNA, aiding in the development of more effective and sustainable pest management strategies.
BACKGROUND
Different approaches have been adopted to manage Spodoptera frugiperda resistance to Bt toxins. However, studying the synergism among these practices applied directly in crop fields is a major challenge. We used a computational model to investigate how the proportion of refuge strips [crop area occupied by non‐Bt corn (maize): 5%, 10%, 15%, or 20%] and the presence of naturally occurring parasitoid, affected or not by a pesticide with different selectivities applied in the field, could influence the dynamics of the resistance allele (R) in a S. frugiperda population.
RESULTS
Our model showed that proportions of 15% and 20% of non‐Bt corn, together with parasitism, led to a reduction in the initial frequency of the R allele. However, the efficiency of the parasitoid depended on the selectivity of the insecticide. Simulations with 5% non‐Bt corn and 50% selectivity, or without selectivity (0%), resulted in elimination of the parasitoid. In the absence of selection pressure, S. frugiperda larvae carrying at least one copy of the R allele showed higher susceptibility to parasitization.
CONCLUSION
Our findings suggest that parasitoids can potentially reduce the need for refuge zones, because they offset the reduction of non‐Bt crops. Nevertheless, preservation of these benefical insects hinges upon the implementation of appropriate management practices by producers. Further investigations are warranted to corroborate these results, including the study of additional natural enemies. © 2024 Society of Chemical Industry.
Soybean is a highly valuable commodity crop for Brazil’s economy. However, it faces significant threats from the attack of a complex of lepidopteran pests, particularly Chrysodeixis includens (Walker) and Spodoptera frugiperda (J. E. Smith). These pests have been managed primarily using transgenic Bt soybeans, but limited knowledge exists about the resistance levels of Bt and non-Bt cultivars adapted to novel soybean-growing areas in Brazil, such as the Minas Gerais state. This study evaluated the resistance levels of Bt and non-Bt soybean cultivars to C. includens and S. frugiperda, and whether the Bt cultivars can differentially affect these pests across larval stages. No-choice bioassays were conducted using Bt (NS6010 IPRO and P97R50 IPRO) and non-Bt soybeans (UFLA 6301 RR, P96R90 RR, and ANsc 80111 RR) at V4-stage in the laboratory with neonate (24 h) and third-instar larvae. Larvae were fed leaf discs in Petri dishes, recording the mortality, leaf consumption, and weight gain after 7 days. There was high mortality of C. includens neonates on the Bt cultivars, but this trend was not observed for older larvae. For S. frugiperda neonates, there was high mortality on the Bt cultivar NS 6010 IPRO and non-Bt cultivar UFLA 6301 RR, but only the former was effective for older larvae. Although the Bt cultivars did not kill the third instars, antinutritional effects were found, such that leaf tissue consumed was not converted to larval weight gain. These findings are important for defining regional strategies of integrated and resistance management of C. includens and S. frugiperda in expanding regions of soybean cultivation in Brazil.
Cotton, with a cultivated area of 31.92 × 10⁶ ha⁻¹ across 80 countries and an estimated annual turnover of USD 5.68 billion, is the world’s leading natural textile fiber. However, many cotton-producing countries have neglected to improve production practices, adversely affecting the environment and society. A systematic review of the sustainable cotton cultivation literature was performed for the first time to identify and suggest context-specific agricultural strategies that can be applied within different agroecosystems. The key aspects include (1) inoculation with arbuscular mycorrhizal species such as Gigaspora margarita, Funneliformis mosseae, and Acaulospora scrobiculata to enhance root exploration, biomass, and nutrient uptake; (2) using grass, legume, and brassica cover crops as a valid alternative to monoculture and fallow crop rotations to reduce resource depletion and increase the sustainability of cotton production; (3) adopting drip and mulched drip irrigation systems over traditional furrow and sprinkler systems for water conservation; (4) exploring the feasibility of prematurely terminating irrigation in humid subtropical and Mediterranean climates as an alternative to chemical defoliation without affecting cotton yield. This paper, which describes various farming practices adopted in different climates, provides farmers a guide for eco-friendly cotton agronomic management without sacrificing productivity.
Significance
Crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) kill some major pests and reduce use of insecticide sprays. However, evolution of pest resistance to Bt proteins decreases these benefits. We report a strategy for combating resistance by crossing transgenic Bt plants with conventional non-Bt plants and then sowing the second-generation seeds. This strategy yields a random mixture within fields of three-quarters of plants that produce Bt protein and one-quarter that does not. An 11-y field study in China shows this strategy countered resistance to Bt cotton of pink bollworm, one of the world’s most devastating pests. This outcome illustrates that non-Bt plants in a seed mixture can boost survival of susceptible insects and delay resistance.
The cotton bollworm Helicoverpa armigera (Hübner) is one of the most damaging cotton pests worldwide. In China, control of this pest has been dependent on transgenic cotton producing a single Bacillus thuringiensis (Bt) protein Cry1Ac since 1997. A small, but significant, increase in H. armigera resistance to Cry1Ac was detected in field populations from Northern China. Since Vip3Aa has a different structure and mode of action than Cry proteins, Bt cotton pyramids containing Vip3Aa are considered as ideal successors of Cry1Ac cotton in China. In this study, baseline susceptibility of H. armigera to Vip3Aa was evaluated in geographic field populations collected in 2014 from major cotton-producing areas of China. The LC 50 values of 12 field populations ranged from 0.05 to 1.311 µg/cm 2 , representing a 25-fold range of natural variation among populations. It is also demonstrated that four laboratory strains of H. armigera with high levels of resistance to Cry1Ac or Cry2Ab have no cross-resistance to Vip3Aa protein. The baseline susceptibility data established here will serve as a comparative reference for detection of field-evolved resistance to Vip3Aa in H. armigera after future deployment of Bt cotton pyramids in China.
Laboratory selection with Vip3Aa of a field-derived population of Heliothis virescens produced >2040-fold resistance in 12 generations of selection. The Vip-Sel resistant population showed little cross-resistance to Cry1Ab and no cross-resistance to Cry1Ac. Resistance was unstable after 15 generations without exposure to the toxin. F 1 reciprocal crosses between Vip-Unsel and Vip-Sel indicated a strong paternal influence on the inheritance of resistance. Resistance ranged from almost completely recessive (mean h = 0.04 if the resistant parental was female) to incompletely dominant (mean h = 0.53 if the resistant parental was male). Results from bioassays on the offspring from backcrosses of the F 1 progeny with Vip-Sel insects indicated that resistance was due to more than one locus. The results described in this paper provide the useful information for the insecticide resistance management strategies designed to overcome the evolution of resistance to Vip3Aa in the insect pests.
ImportanceHeliothis virescens is an important pest which has the ability to feed on many plant species. The extensive use of Bt crops or spray has already led to the evolution of insect resistance in the field for some species of Lepidoptera and Coleoptera. The development of resistance in insect pests is the main threat to of Bt crops. The effective resistance management strategies are very important to prolong the life of Bt plants. The lab selection is the key step to test the assumption and predictions of management strategies prior to the field evaluation. Resistant insects offer useful information to determine the inheritance of resistance and the frequency of resistance alleles, and to study the mechanism of resistance to insecticides.
Transgenic crops producing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) are extensively cultivated worldwide. To counter rapidly increasing pest resistance to crops that produce single Bt toxins, transgenic plant "pyramids" producing two or more Bt toxins that kill the same pest have been widely adopted. However, cross-resistance and antagonism between Bt toxins limits the sustainability of this approach. Here we describe development and testing of the first pyramids of cotton combining protection from a Bt toxin and RNA interference (RNAi). We developed two types of transgenic cotton plants producing double-stranded RNA (dsRNA) from the global lepidopteran pest Helicoverpa armigera designed to interfere with its metabolism of juvenile hormone (JH). We focused on suppression of JH acid methyltransferase (JHAMT), which is crucial for JH synthesis, and JH binding protein (JHBP), which transports JH to organs. In 2015 and 2016, we tested larvae from a Bt-resistant strain and a related susceptible strain of H. armigera on seven types of cotton: two controls, Bt cotton, two types of RNAi cotton (targeting JHAMT or JHBP), and two pyramids (Bt cotton plus each type of RNAi). Both types of RNAi cotton were effective against Bt-resistant insects. Bt cotton and RNAi acted independently against the susceptible strain. In computer simulations of conditions in northern China, where millions of farmers grow Bt cotton as well as abundant non-transgenic host plants of H. armigera, pyramided cotton combining a Bt toxin and RNAi substantially delayed resistance relative to using Bt cotton alone. This article is protected by copyright. All rights reserved.
Background:
Cases of western corn rootworm (WCR) field-evolved resistance to Cry3Bb1 and other corn rootworm (CRW) control traits have been reported. Pyramid products expressing multiple CRW traits can delay resistance compared to single trait products. We used field studies to assess the pyramid CRW corn products, SmartStax (expressing Cry3Bb1 and Cry34Ab1/Cry35Ab1) and SmartStax PRO (expressing Cry3Bb1, Cry34Ab1/Cry35Ab1 and DvSnf7), at locations with high WCR densities and possible Cry3Bb1 resistance, and to assess the reduction in adult emergence attributable to DvSnf7 and other traits. Insect resistance models were used to assess durability of SmartStax and SmartStax PRO to WCR resistance.
Results:
SmartStax significantly reduced root injury compared to non-CRW-trait controls at all but one location with measurable WCR pressure, while SmartStax PRO significantly reduced root injury at all locations, despite evidence of Cry3Bb1 resistance at some locations. The advantage of SmartStax PRO over SmartStax in reducing root damage was positively correlated with root damage on non-CRW-trait controls. DvSnf7 was estimated to reduce WCR emergence by approximately 80-95%, which modelling indicated will improve durability of Cry3Bb1 and Cry34Ab1/Cry35Ab1 compared to SmartStax.
Conclusion:
The addition of DvSnf7 in SmartStax PRO can reduce root damage under high WCR densities and prolong Cry3Bb1 and Cry34Ab1/Cry35Ab1 durability.
Background and Methodology
There is a continuing need to express new insect control compounds in transgenic maize against western corn rootworm, Diabrotica virgifera virgifera (LeConte) (WCR). In this study three experiments were conducted to determine cross-resistance between the new insecticidal DvSnf7 dsRNA, and Bacillus thuringiensis (Bt) Cry3Bb1; used to control WCR since 2003, with field-evolved resistance being reported. Laboratory susceptible and Cry3Bb1-resistant WCR were evaluated against DvSnf7 dsRNA in larval diet-incorporation bioassays. Additionally, the susceptibility of seven field and one field-derived WCR populations to DvSnf7 (and Cry3Bb1) was assessed in larval diet-overlay bioassays. Finally, beetle emergence of laboratory susceptible and Cry3Bb1-resistant WCR was evaluated with maize plants in the greenhouse expressing Cry3Bb1, Cry34Ab1/Cry35Ab1, or DvSnf7 dsRNA singly, or in combination.
Principal Findings and Conclusions
The Cry3Bb1-resistant colony had slight but significantly (2.7-fold; P<0.05) decreased susceptibility to DvSnf7 compared to the susceptible colony, but when repeated using a field-derived WCR population selected for reduced Cry3Bb1 susceptibility, there was no significant difference (P<0.05) in DvSnf7 susceptibility compared to that same susceptible population. Additionally, this 2.7-fold difference in susceptibility falls within the range of DvSnf7 susceptibility among the seven field populations tested. Additionally, there was no correlation between susceptibility to DvSnf7 and Cry3Bb1 for all populations evaluated. In greenhouse studies, there were no significant differences (P<0.05) between beetle emergence of susceptible and Cry3Bb1-resistant colonies on DvSnf7 and Cry34Ab1/Cry35Ab1, and between DvSnf7 and MON 87411 (DvSnf7 + Cry3Bb1) for the Cry3Bb1-resistant colony. These results demonstrate no cross-resistance between DvSnf7 and Cry3Bb1 against WCR. Therefore, pyramiding DvSnf7 with Bt proteins such as Cry3Bb1 and Cry34Ab1/Cry35Ab1 will provide a valuable IRM tool against WCR that will increase the durability of these Bt proteins. These results also illustrate the importance of using appropriate bioassay methods when characterizing field-evolved resistant WCR populations.
Background
Transgenic corn engineered with genes expressing insecticidal toxins from the bacterium Bacillus thuringiensis (Berliner) (Bt) are now a major tool in insect pest management. With its widespread use, insect resistance is a major threat to the sustainability of the Bt transgenic technology. For all Bt corn expressing Cry toxins, the high dose requirement for resistance management is not achieved for corn earworm, Helicoverpa zea (Boddie), which is more tolerant to the Bt toxins.
Methodology/Major Findings
We present field monitoring data using Cry1Ab (1996–2016) and Cry1A.105+Cry2Ab2 (2010–2016) expressing sweet corn hybrids as in-field screens to measure changes in field efficacy and Cry toxin susceptibility to H. zea. Larvae successfully damaged an increasing proportion of ears, consumed more kernel area, and reached later developmental stages (4th - 6th instars) in both types of Bt hybrids (Cry1Ab—event Bt11, and Cry1A.105+Cry2Ab2—event MON89034) since their commercial introduction. Yearly patterns of H. zea population abundance were unrelated to reductions in control efficacy. There was no evidence of field efficacy or tissue toxicity differences among different Cry1Ab hybrids that could contribute to the decline in control efficacy. Supportive data from laboratory bioassays demonstrate significant differences in weight gain and fitness characteristics between the Maryland H. zea strain and a susceptible strain. In bioassays with Cry1Ab expressing green leaf tissue, Maryland H. zea strain gained more weight than the susceptible strain at all concentrations tested. Fitness of the Maryland H. zea strain was significantly lower than that of the susceptible strain as indicated by lower hatch rate, longer time to adult eclosion, lower pupal weight, and reduced survival to adulthood.
Conclusions/Significance
After ruling out possible contributing factors, the rapid change in field efficacy in recent years and decreased susceptibility of H. zea to Bt sweet corn provide strong evidence of field-evolved resistance in H. zea populations to multiple Cry toxins. The high adoption rate of Bt field corn and cotton, along with the moderate dose expression of Cry1Ab and related Cry toxins in these crops, and decreasing refuge compliance probably contributed to the evolution of resistance. Our results have important implications for resistance monitoring, refuge requirements and other regulatory policies, cross-resistance issues, and the sustainability of the pyramided Bt technology.
Evolution of Helicoverpa armigera resistance to Bacillus thuringiensis (Bt) cotton producing Cry1Ac is progressing in northern China and replacement of Cry1Ac cotton by pyramided Bt cotton has been considered to counter such resistance. Here, we investigated four of the eight conditions underlying success of the refuge strategy for delaying resistance to Cry1Ac+Cry2Ab cotton, a pyramid that has been used extensively against H. armigera outside China. Laboratory bioassays of a Cry2Ab-selected strain (An2Ab) and a related unselected strain (An) reveal that resistance to Cry2Ab (130-fold) was nearly dominant, autosomally inherited, and controlled by more than 1 locus. Strong cross-resistance occurred between Cry2Ab and Cry2Aa (81-fold). Weaker cross-resistance (18- to 22-fold) between Cry2Ab and Cry1A toxins was also present and significantly increased survival of An2Ab relative to An on cotton cultivars producing the fusion protein Cry1Ac/Cry1Ab or Cry1Ac. Survival on Cry1Ac+Cry2Ab cotton was also significantly higher in An2Ab than An, showing that redundant killing on this pyramid was incomplete. Survival on non-Bt cotton did not differ significantly between An2Ab and An, indicating an absence of fitness costs affecting this trait. These results indicate that a switch to three-toxin pyramided cotton could be valuable for increasing durability of Bt cotton in China. This article is protected by copyright. All rights reserved.
The resistance of fall armyworm (FAW), Spodoptera frugiperda, has been characterized to some Cry and Vip3A proteins of Bacillus thuringiensis (Bt) expressed in transgenic maize in Brazil. Here we evaluated the effective dominance of resistance based on the survival of neonates from selected Bt-resistant, heterozygous, and susceptible (Sus) strains of FAW on different Bt maize and cotton varieties. High survival of strains resistant to the Cry1F (HX-R), Cry1A.105/Cry2Ab (VT-R) and Cry1A.105/Cry2Ab/Cry1F (PW-R) proteins was detected on Herculex, YieldGard VT PRO and PowerCore maize. Our Vip3A-resistant strain (Vip-R) exhibited high survival on Herculex, Agrisure Viptera and Agrisure Viptera 3 maize. However, the heterozygous from HX-R × Sus, VT-R × Sus, PW-R × Sus and Vip-R × Sus had complete mortality on YieldGard VT PRO, PowerCore, Agrisure Viptera, and Agrisure Viptera 3, whereas the HX-R × Sus and Vip-R × Sus strains survived on Herculex maize. On Bt cotton, the HX-R, VT-R and PW-R strains exhibited high survival on Bollgard II. All resistant strains survived on WideStrike, but only PW-R and Vip-R × Sus survived on TwinLink. Our study provides useful data to aid in the understanding of the effectiveness of the refuge strategy for Insect Resistance Management of Bt plants.
Soil microbes yield insecticidal peptide
The microbial peptide BT, derived from the bacterium Bacillus thuringiensis , is widely used to protect crops from insect pests. Schellenberger et al. identified another insecticidal peptide from a different soil-dwelling bacterium, Pseudomonas chlororaphis (see the Perspective by Tabashnik). Corn plants expressing the Pseudomonas peptide were protected from attack by western corn rootworm. Rootworms that were resistant to BT were susceptible to the Pseudomonas peptide. Addition of another insecticidal peptide diversifies the arsenal against insect pests, which may slow down the development of resistance.
Science , this issue p. 634 ; see also p. 552
Western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is a serious pest of corn in the United States, and recent management of western corn rootworm has included planting of Bt corn. Beginning in 2009, western corn rootworm populations with resistance to Cry3Bb1 corn and mCry3A corn were found in Iowa and elsewhere. To date, western corn rootworm populations have remained susceptible to corn producing Bt toxin Cry34/35Ab1. In this study, we used single-plant bioassays to test field populations of western corn rootworm for resistance to Cry34/35Ab1 corn, Cry3Bb1 corn, and mCry3A corn. Bioassays included nine rootworm populations collected from fields where severe injury to Bt corn had been observed and six control populations that had never been exposed to Bt corn. We found incomplete resistance to Cry34/35Ab1 corn among field populations collected from fields where severe injury to corn producing Cry34/35Ab1, either singly or as a pyramid, had been observed. Additionally, resistance to Cry3Bb1 corn and mCry3A corn was found among the majority of populations tested. These first cases of resistance to Cry34/35Ab1 corn, and the presence of resistance to multiple Bt toxins by western corn rootworm, highlight the potential vulnerability of Bt corn to the evolution of resistance by western corn rootworm. The use of more diversified management practices, in addition to insect resistance management, likely will be essential to sustain the viability of Bt corn for management of western corn rootworm.
The evolution of resistance and cross-resistance threaten the sustainability of genetically engineered crops that produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt). Western corn rootworm, Diabrotica virgifera virgifera LeConte, is a serious pest of maize and has been managed with Bt maize since 2003. We conducted laboratory bioassays with maize hybrids producing Bt toxins Cry3Bb1, mCry3A, eCry3.1Ab, and Cry34/35Ab1, which represent all commercialized Bt toxins for management of western corn rootworm. We tested populations from fields where severe injury to Cry3Bb1 maize was observed, and populations that had never been exposed to Bt maize. Consistent with past studies, bioassays indicated that field populations were resistant to Cry3Bb1 maize and mCry3A maize, and that cross-resistance was present between these two types of Bt maize. Additionally, bioassays revealed resistance to eCry3.1Ab maize and cross-resistance among Cry3Bb1, mCry3A and eCry3.1Ab. However, no resistance or cross-resistance was detected for Cry34/35Ab1 maize. This broad-spectrum resistance illustrates the potential for insect pests to develop resistance rapidly to multiple Bt toxins when structural similarities are present among toxins, and raises concerns about the long-term durability of Bt crops for management of some insect pests.
The risk of rapid pest adaptation to an insecticide is highly dependent on the initial frequency of resistance alleles in field populations. Because we have lacked empirical estimates of these frequencies, population–genetic models of resistance evolution have relied on a wide range of theoretical estimates. The recent commercialization of genetically engineered cotton that constitutively produces an insecticidal protein derived from the biocontrol agent, Bacillus thuringiensis (Bt) has raised concern that we lack data needed to quantify the risk of insect pests such as Heliothis virescens rapidly adapting to this ecologically valuable class of toxins. By individually mating over 2,000 male H. virescens moths collected in four states to females of a Bt toxin-resistant laboratory strain, and screening F 1 and F 2 offspring for tolerance of the toxic protein, we were able to directly estimate the field frequency of alleles for resistance as 1.5 10 3. This high initial frequency underscores the need for caution in deploying transgenic cotton to control insect pests. Our single-pair mating technique greatly increases the efficiency of detecting recessive resistance alleles. Because alleles that decrease target site sensitivity to Bt toxins and other insecticides are often recessive, this technique could be useful in estimating resistance allele frequencies in other insects exposed to trans-genic insecticidal crops or conventional insecticides.
RNA interference (RNAi) has become a widely used reverse genetic tool to study gene function in eukaryotic organisms and is being developed as a technology for insect pest management. The efficiency of RNAi varies among organisms. Insects from different orders also display differential efficiency of RNAi, ranging from highly efficient (coleopterans) to very low efficient (lepidopterans). We investigated the reasons for varying RNAi efficiency between lepidopteran and coleopteran cell lines and also between the Colorado potato beetle, Leptinotarsa decemlineata and tobacco budworm, Heliothis virescens. The dsRNA either injected or fed was degraded faster in H. virescens than in L. decemlineata. Both lepidopteran and coleopteran cell lines and tissues efficiently took up the dsRNA. Interestingly, the dsRNA administered to coleopteran cell lines and tissues was taken up and processed to siRNA whereas the dsRNA was taken up by lepidopteran cell lines and tissues but no siRNA was detected in the total RNA isolated from these cell lines and tissues. The data included in this paper showed that the degradation and intracellular transport of dsRNA are the major factors responsible for reduced RNAi efficiency in lepidopteran insects.
The majority of Bt maize production in the European Union (EU) is concentrated in northeast Spain, which is Europe’s only hotspot where resistance might evolve, and the main target pest, Sesamia nonagrioides, has been exposed to Cry1Ab maize continuously since 1998. The cropping system in northeast Spain has some similar characteristics to those that probably led to rapid resistance failures in two other target noctuid maize pests. These include repeated cultivation of Bt maize in the same fields, low use of refuges, recurring exposure of larvae to non-high dose concentrations of Cry1Ab toxin during the first years of cultivation, low migratory potential, and production concentrated in an irrigated region with few alternative hosts. Available data reveal no evidence of resistance in S. nonagrioides after 16 years of use. We explore the possible reasons for this resistance management success using evolutionary models to consider factors expected to accelerate resistance, and those expected to delay resistance. Low initial adoption rates and the EU policy decision to replace Event 176 with MON 810 Bt maize were key to delaying resistance evolution. Model results suggest that if refuge compliance continues at the present 90%, Bt maize might be used sustainably in northeast Spain for at least 20 more years before resistance might occur. However, obtaining good estimates of the present R allele frequency and level of local assortative mating are crucial to reduce uncertainty about the future success of resistance management.
The Bacillus thuringiensis δ-endotoxins (Bt toxins) are widely used insecticidal proteins in engineered crops that provide agricultural, economic, and environmental benefits. The development of insect resistance to Bt toxins endangers their long-term effectiveness. Here we have developed a phage-assisted continuous evolution selection that rapidly evolves high-affinity protein-protein interactions, and applied this system to evolve variants of the Bt toxin Cry1Ac that bind a cadherin-like receptor from the insect pest Trichoplusia ni (TnCAD) that is not natively bound by wild-type Cry1Ac. The resulting evolved Cry1Ac variants bind TnCAD with high affinity (dissociation constant Kd = 11-41 nM), kill TnCAD-expressing insect cells that are not susceptible to wild-type Cry1Ac, and kill Cry1Ac-resistant T. ni insects up to 335-fold more potently than wild-type Cry1Ac. Our findings establish that the evolution of Bt toxins with novel insect cell receptor affinity can overcome insect Bt toxin resistance and confer lethality approaching that of the wild-type Bt toxin against non-resistant insects.
Crops expressing genes from Bacillus thuringiensis (Bt crops) are among the most successful technologies developed for the control of pests but the evolution of resistance to them remains a challenge. Insect resistant cotton and maize expressing the Bt Vip3Aa protein were recently commercialized, though not yet in Australia. We found that, although relatively high, the frequency of alleles for resistance to Vip3Aa in field populations of H. armigera in Australia did not increase over the past four seasons until 2014/15. Three new isofemale lines were determined to be allelic with previously isolated lines, suggesting that they belong to one common gene and this mechanism is relatively frequent. Vip3Aa-resistance does not confer cross-resistance to Cry1Ac or Cry2Ab. Vip3Aa was labeled with 125I and used to show specific binding to H. armigera brush-border membrane vesicles (BBMV). Binding was of high affinity (Kd = 25 and 19 nM for susceptible and resistant insects, respectively) and the concentration of binding sites was high (Rt = 140 pmol/mg for both). Despite the narrow-spectrum resistance, binding of 125I-labeled Vip3Aa to BBMV of resistant and susceptible insects was not significantly different. Proteolytic conversion of Vip3Aa protoxin into the activated toxin rendered the same products, though it was significantly slower in resistant insects.
Background:
The soybean looper (SBL), Chrysodeixis includens (Walker), is one of the most important soybean pests in Brazil. MON 87701 × MON 89788 soybean expressing Cry1Ac has been recently deployed in Brazil, providing high levels of control against the primary lepidopteran pests. To support insect resistance management (IRM) programs, the baseline susceptibility of SBL to Cry1Ac was assessed and resistance allele frequency was estimated based on an F2 screen.
Results:
The toxicity (LC50 ) of Cry1Ac ranged from 0.39 to 2.01 µg·ml(-1) of diet among all SBL field populations collected from crop seasons 2008/09 to 2012/13, which indicated approximately 5-fold variation. Cry1Ac diagnostic concentrations of 5.6 and 18 µg·ml(-1) of diet were established for monitoring purposes, and no shift in mortality has been observed. A total of 626 F2 family lines derived from SBL collected from locations across Brazil during crop season 2014/15 were screened for the presence of Cry1Ac resistance alleles. None of the 626 families survived on MON 87701 × MON 89788 soybean leaf tissue (joint frequency = 0.0004).
Conclusions:
SBL showed high susceptibility and low resistance allele frequency to Cry1Ac across the main soybean-producing regions in Brazil. These findings meet important criteria for effective IRM strategy.
Transgenic crops producing Bacillus thuringiensis (Bt) proteins kill key insect pests, providing economic and environmental benefits. However, the evolution of pest resistance threatens the continued success of such Bt crops. To delay or counter resistance, transgenic plant "pyramids" producing two or more Bt proteins that kill the same pest have been adopted extensively. Field populations of the pink bollworm (Pectinophora gossypiella) in the United States have remained susceptible to Bt toxins Cry1Ac and Cry2Ab, but field-evolved practical resistance to Bt cotton producing Cry1Ac has occurred widely in India. Here we used two rounds of laboratory selection to achieve 18,000- to 150,000-fold resistance to Cry2Ab in pink bollworm. Inheritance of resistance to Cry2Ab was recessive, autosomal, conferred primarily by one locus, and independent of Cry1Ac resistance. We created a strain with high resistance to both toxins by crossing the Cry2Ab-resistant strain with a Cry1Ac-resistant strain, followed by one selection with Cry2Ab. This multi-toxin resistant strain survived on field-collected Bt cotton bolls producing both toxins. The results here demonstrate the risk of evolution of resistance to pyramided Bt plants, particularly when toxins are deployed sequentially and refuges are scarce, as seen with Bt cotton and pink bollworm in India.
Transgenic Bt maize that produces less than a high-dose has been widely adopted and presents considerable insect resistance management (IRM) challenges. Western corn rootworm, Diabrotica virgifera virgifera LeConte, has rapidly evolved resistance to Bt maize in the field, leading to local loss of efficacy for some corn rootworm Bt maize events. Documenting and responding to this resistance has been complicated by a lack of rapid diagnostic bioassays and by regulatory triggers that hinder timely and effective management responses. These failures are of great concern to the scientific and agricultural community. Specific challenges posed by western corn rootworm resistance to Bt maize, and more general concerns around Bt crops that produce less than a high-dose of Bt toxin, have caused uncertainty around current IRM protocols. More than 15 years of experience with IRM has shown that high-dose and refuge-based IRM is not applicable to Bt crops that produce less than a high-dose. Adaptive IRM approaches and pro-active, integrated IRM-pest management strategies are needed and should be in place before release of new technologies that produce less than a high-dose. We suggest changes in IRM strategies to preserve the utility of corn rootworm Bt maize by 1) targeting local resistance management earlier in the sequence of responses to resistance and 2) developing area-wide criteria to address widespread economic losses. We also favor consideration of policies and programs to counteract economic forces that are contributing to rapid resistance evolution.
Western bean cutworm, Striacosta albicosta (Smith) (Lepidoptera: Noctuidae), is a native, univoltine pest of corn and dry beans in North America. The current degree-day model for predicting a specified percentage of yearly moth flight involves heat unit accumulation above 10°C after 1 May. However, because the moth’s observed range has expanded into the northern and eastern United States, there is concern that suitable temperatures before May could allow for significant S. albicosta development. Daily blacklight moth catch and temperature data from four Nebraska locations were used to construct degree-day models using simple or sine-wave methods, starting dates between 1 January and 1 May, and lower (−5 to 15°C) and upper (20 to 43.3°C) developmental thresholds. Predicted dates of flight from these models were compared with observed flight dates using independent datasets to assess model performance. Model performance was assessed with the concordance correlation coefficient to concurrently evaluate precision and accuracy. The best model for predicting timing of S. albicosta flight used simple degree-day calculations beginning on 1 March, a 3.3°C (38°F) lower threshold, and a 23.9°C (75°F) upper threshold. The revised cumulative flight model indicated field scouting to estimate moth egg density at the time of 25% flight should begin when 1,432 degree-days (2,577 degree-days °F) have accumulated. These results underscore the importance of assessing multiple parameters in phenological models and utilizing appropriate assessment methods, which in this case may allow for improved timing of field scouting for S. albicosta.
Bollgard® cotton, expressing Cry1Ac insecticidal protein, was approved for commercial planting in India in 2002, and by 2009 constituted 87% of the Indian crop, reducing losses from lepidopteran pests including pink bollworm (PBW), Pectinophora gossypiella. Inadequate control of PBW in fields of single-gene Bollgard cotton was reported in 2009; surveys revealed heavy infestations of PBW in Bollgard, restricted to Gujarat state, but not elsewhere in India.
Bioassays of PBW strains from Bollgard bolls showed that while susceptible PBW could not complete development to ≥3rd instar at 10.0 µg/ml Cry1Ac, 66.1% of larvae from Gujarat Bollgard strains could. A field-resistant strain, further selected in the laboratory, had susceptibility to Cry1Ac reduced by >2000-fold. Resistance to Cry1Ac did not confer cross-resistance to the Cry2Ab2 protein. In 2010, Bollgard fields in Gujarat continued to be infested with PBW and many Bollgard fields in the adjoining states of Maharashtra and Madhya Pradesh showed high level infestation by PBW.
Inadequate planting of refuges for PBW is the likely explanation for the field resistance to Bt cotton observed in Gujarat. These findings underscore the higher vulnerability of single-gene Bt products relative to dual-gene products expressing Cry1Ac and Cry2Ab2, and the increased risk of resistance evolution with low refuge compliance.
This article is protected by copyright. All rights reserved.
Brazil ranked second only to the United States in hectares planted to genetically modified crops in 2013. Recently corn producers in the Cerrado region reported that the control of Spodoptera frugiperda with Bt corn expressing Cry1Fa has decreased, forcing them to use chemicals to reduce the damage caused by this insect pest. A colony of S. frugiperda was established from individuals collected in 2013 from Cry1Fa corn plants (SfBt) in Brazil and shown to have at least more than ten-fold higher resistance levels compared with a susceptible colony (Sflab). Laboratory assays on corn leaves showed that in contrast to SfLab population, the SfBt larvae were able to survive by feeding on Cry1Fa corn leaves. The SfBt population was maintained without selection for eight generations and shown to maintain high levels of resistance to Cry1Fa toxin. SfBt showed higher cross-resistance to Cry1Aa than to Cry1Ab or Cry1Ac toxins. As previously reported, Cry1A toxins competed the binding of Cry1Fa to brush border membrane vesicles (BBMV) from SfLab insects, explaining cross-resistance to Cry1A toxins. In contrast Cry2A toxins did not compete Cry1Fa binding to SfLab-BBMV and no cross-resistance to Cry2A was observed, although Cry2A toxins show low toxicity to S. frugiperda. Bioassays with Cry1AbMod and Cry1AcMod show that they are highly active against both the SfLab and the SfBt populations. The bioassay data reported here show that insects collected from Cry1Fa corn in the Cerrado region were resistant to Cry1Fa suggesting that resistance contributed to field failures of Cry1Fa corn to control S. frugiperda.
This study provides the first genetic characterization of the soybean looper, Chrysodeixis includens (Walker, 1857), an important defoliating pest species of soybean crops in Brazil. Population genetic variability and the genetic structure of C. includens populations were evaluated by using ISSR markers with samples from the major soybean producing regions in Brazil in the growing seasons 2011/2012. Seven different primers were applied for population characterization of the molecular variability and genetic structure of 8 soybean looper populations from 8 states of Brazil. The seven ISSR loci generated 247 bands in 246 individuals of C. includens sampled. The expected heterozygosity (HE) in the populations varied between 0.093 and 0.106, while the overall HE was 0.099, indicating low genetic diversity. The analysis of molecular variance indicated that 98% of the variability was expressed among individuals within populations (FST = 0.021, p = 0.001). The low level of polymorphism over all populations, the high levels of gene flow, and the low genetic structure are indicatives of the exchange of genetic information between the different sampled regions. Population structuring suggests the presence of two major groups which do not correlate with their geographic sampling location in Brazil. These results may indicate recent recolonization of C. includens in Brazil or migration patterns following source-sink dynamics. Furthermore, the presence of two groups within C. includens suggests that a study on development of resistance or any other genetic-based trait needs to be evaluated on both groups, and pest management in soybean fields should be aware that differences may come to the control strategies they use.
The Bacillus thuringiensis vegetative insecticidal protein, Vip3A, represents a new family of Bt toxin and is currently applied to commercial transgenic cotton. To determine whether the Cry1Ac-resistant Helicoverpa armigera is cross-resistant to Vip3Aa protein, insecticidal activities, proteolytic activations and binding properties of Vip3Aa toxin were investigated using Cry1Ac-susceptible (96S) and Cry1Ac-resistant H. armigera strain (Cry1Ac-R). The toxicity of Vip3Aa in Cry1Ac-R slightly reduced compared with 96S, the resistance ratio was only 1.7-fold. The digestion rate of full-length Vip3Aa by gut juice extracts from 96S was little faster than that from Cry1Ac-R. Surface plasmon resonance (SPR) showed there was no significant difference between the binding affinity of Vip3Aa and BBMVs between 96S and Cry1Ac-R strains, and there was no significant competitive binding between Vip3Aa and Cry1Ac in susceptible or resistant strains. So there had little cross-resistance between Vip3Aa and Cry1Ac, Vip3A+Cry proteins maybe the suitable pyramid strategy to control H. armigera in China in the future.
Fall armyworm, Spodoptera frugiperda (J.E. Smith), is a major target species of Bt maize and Bt cotton in the United States. Field resistance to Cry1F maize in S. frugiperda has occurred in Puerto Rico, Brazil, and the southeast region of the United States. There is a great concern that the Cry1F-maize resistant S. frugiperda may move to the cotton fields and cause control problems to Bt cotton where both crops are planted. In this study, larval mortality and growth inhibition of Cry1F maize-susceptible (SS),-hetero-zygous (RS), and-resistant (RR) genotypes of S. frugiperda were evaluated using diet-incorporated bio-assays with six Bt proteins that have been used in commercialized and future Bt cotton plants: Cry1F, Cry1Ab, Cry1Ac, Cry2Ab2, Cry2Ae, and Vip3A. Compared to the mortality data, the measuring of growth inhibition was more sensitive in determining Bt susceptibility of the insect to all six Bt proteins. Overall, RS had a similar performance as SS for the six Bt proteins with a resistance ratio of <5.5-fold. The resistance ratio of RR calculated based on larval growth inhibition was 930-fold for Cry1F, 50.5-fold for Cry1Ab, and >172-fold for Cry1Ac. In contrast, all three insect genotypes were susceptible to Cry2Ab2, Cry2Ae, or Vip3A. The results showed that the Cry1F-maize resistant S. frugiperda was cross-resistant to Cry1Ab or Cry1Ac, but not cross-resistant to Cry2Ab2, Cry2Ae, or Vip3A. The strong cross-resistance among Cry1 proteins suggest that it is necessary to plant pyramided cotton traits that contain at least two active Bt genes for Cry1F-resistant S. frugiperda to ensure the continued success of the Bt cotton technology, especially in the areas where Bt resistance has already occurred.
In the United States of America, the western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is commonly managed with transgenic corn (Zea mays L.) expressing insecticidal proteins from the bacteria Bacillus thuringiensis Berliner (Bt). Colonies of this pest have been selected in the laboratory on each commercially available transformation event and several resistant field populations have also been identified; some field populations are also resistant. In this study, progeny of a western corn rootworm population collected from a Minnesota corn field planted to SmartStax® corn were evaluated for resistance to corn hybrids expressing Cry3Bb1 (event MON88017) or Cry34/35Ab1 (event DAS-59122-7) and to the individual constituent proteins in diet-overlay bioassays. Results from these assays suggest that this population is resistant to Cry3Bb1 and is incompletely resistant to Cry34/35Ab1. In diet toxicity assays, larvae of the Minnesota (MN) population had resistance ratios of 4.71 and >13.22 for Cry34/35Ab1 and Cry3Bb1 proteins, respectively, compared with the control colonies. In all on-plant assays, the relative survival of the MN population on the DAS-59122-7 and MON88017 hybrids was significantly greater than the control colonies. Larvae of the MN population had inhibited development when reared on DAS-59122-7 compared with larvae reared on the non-Bt hybrid, indicating resistance was incomplete. Overall, these results document resistance to Cry3Bb1 and an incomplete resistance to Cry34/35Ab1 in a population of WCR from a SmartStax® performance problem field.
In the never-ending war on insect pests, the widespread soil bacterium Bacillus thuringiensis ( Bt ) is one of the greatest heroes. Insecticidal crystalline (Cry) proteins and vegetative insecticidal proteins (Vips) from Bt are treasured for their effectiveness against some devastating pests and their safety for beneficial insects, wildlife, and people ( 1 ). Sprays containing Bt proteins have been used for more than 70 years and remain valuable in organic and conventional agriculture, forestry, and vector control ( 1 ). Crops genetically engineered to produce Bt proteins were introduced 20 years ago and quickly became a cornerstone of pest management. They have suppressed pest populations, reduced reliance on insecticide sprays, enhanced control by natural enemies, and increased farmer profits ( 2 – 4 ). In 2015, Bt crops were planted on 84 million hectares globally ( 5 ), including 81% of the corn and 84% of the cotton in the United States ( 6 ). But with increasingly rapid evolution of pest resistance to Bt crops ( 7 , 8 ) (see the figure), alternatives are urgently needed. On page [634][1] of this issue, Schellenberger et al. ( 9 ) report the discovery of tiny insecticidal proteins (Tips) from other soil bacteria that could be part of the solution.
[1]: /lookup/doi/10.1126/science.aaf6056
Western corn rootworm (WCR), Diabrotica virgifera virgifera, is one of the most significant pests of corn in the United States. Although transgenic solutions exist, increasing resistance concerns make the discovery of novel solutions essential. In order to find a novel protein with high activity and a new mode of action, a large microbial collection was surveyed for toxicity to WCR using in vitro bioassays. Cultures of strain ATX2024, identified as Chromobacterium piscinae, had very high activity against WCR larvae. The biological activity from the strain was purified using chromatographic techniques and fractions were tested against WCR larvae. Proteins in the final active fraction were identified by mass spectrometry and N-terminal sequencing and matched to the genome of ATX2024. A novel 58.9 kDa protein, identified by this approach, was expressed in a recombinant expression system and found to have specific activity against WCR. Transgenic corn events containing this gene showed good protection against root damage by WCR, with average scores ranging between 0.01 and 0.04 on the Iowa State node injury scale. Sequence analysis did not reveal homology to any known insecticidal toxin, suggesting that this protein may act in a novel way to control WCR. The new WCR active protein is named GNIP1Aa, for Gram Negative Insecticidal Protein.
Transgenic crops producing Bacillus thuringiensis- (Bt) insecticidal proteins (Bt crops) have provided useful pest management tools to growers for the past 20 years. Planting Bt crops has reduced the use of synthetic insecticides on cotton, maize and soybean fields in 11 countries throughout Latin America. One of the threats that could jeopardize the sustainability of Bt crops is the development of resistance by targeted pests. Governments of many countries require vigilance in measuring changes in Bt-susceptibility in order to proactively implement corrective measures before Bt-resistance is widespread, thus prolonging the usefulness of Bt crops.
Minnesota populations of Diabrotica virgifera virgifera LeConte, the western corn rootworm, surviving Cry3Bb1-expressing corn in the field and western corn rootworm populations assumed to be susceptible to all Bt proteins were evaluated for susceptibility to Cry3Bb1, mCry3A, eCry3.1Ab, and Cry34/35Ab1 in diet assays and three different plant-based assays. Rootworm populations originating from Cry3Bb1 fields and that consistently experienced greater than expected damage had increased survival and larval growth compared to control populations assayed on Cry3Bb1 as well as mCry3a and eCry3.1Ab. Cross resistance was documented between Cry3Bb1 and both mCry3A and eCry3.1Ab as single toxins. Despite very high resistance ratios in some comparisons, cross resistance was not complete and also varied with the population being evaluated, the trait measured, and the susceptible rootworm population used for comparison. Regardless of resistance and cross resistance, all proteins, even Cry3Bb1, retained some efficacy in terms of either reducing rootworm larval growth, protecting plants from damage, or both, for all rootworm populations evaluated. For one Cry3Bb1-selected population, a resistance ratio of 9.1-fold was found to Cry34/35Ab1 when evaluating EC50 values relative to a susceptible control population; however, resistance to Cry34/35Ab1 was not evident in all assays in this population. The United States Environmental Protection Agency recently suggested eliminating diet assays as part of the Bt resistance monitoring process. However, given the variability of responses of western corn rootworm populations to different proteins in different assays, both plant and diet assays are needed as options for detecting and fully characterizing resistance.
Bt cotton was initially deployed in Australia in the mid-1990s to control the polyphagous pest Helicoverpa armigera (Hübner) which was intractably resistant to synthetic chemistries. A conservative strategy was enforced and resistance to first generation single toxin technology was managed. A decade later, shortly after the release of dual toxin cotton, high baseline frequencies of alleles conferring resistance to one of its components prompted a reassessment of the thinking behind the potential risks to this technology. Several reviews detail the characteristics of this resistance and the nuances of deploying first and second generation Bt cotton in Australia. Here we explore recent advances and future possibilities to estimate Bt resistance in Australian pest species and define what we see as the critical data for enabling effective pre-emptive strategies. We also foreshadow the imminent deployment of three toxin (Cry1Ac, Cry2Ab, Vip3A) Bollgard 3 cotton, and examine aspects of resistance to its novel component, Vip3A, that we believe may impact on its stewardship.
Entomopathogenic bacteria produce insecticidal proteins that accumulate in inclusion bodies or parasporal crystals (such as the Cry and Cyt proteins) as well as insecticidal proteins that are secreted into the culture medium. Among the latter are the Vip proteins, which are divided into four families according to their amino acid identity. The Vip1 and Vip2 proteins act as binary toxins and are toxic to some members of the Coleoptera and Hemiptera. The Vip1 component is thought to bind to receptors in the membrane of the insect midgut, and the Vip2 component enters the cell, where it displays its ADP-ribosyltransferase activity against actin, preventing microfilament formation. Vip3 has no sequence similarity to Vip1 or Vip2 and is toxic to a wide variety of members of the Lepidoptera. Its mode of action has been shown to resemble that of the Cry proteins in terms of proteolytic activation, binding to the midgut epithelial membrane, and pore formation, although Vip3A proteins do not share binding sites with Cry proteins. The latter property makes them good candidates to be combined with Cry proteins in transgenic plants (
Bacillus thuringiensis
-treated crops [Bt crops]) to prevent or delay insect resistance and to broaden the insecticidal spectrum. There are commercially grown varieties of Bt cotton and Bt maize that express the Vip3Aa protein in combination with Cry proteins. For the most recently reported Vip4 family, no target insects have been found yet.
The primary strategy for delaying the evolution of pest resistance to transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt) entails refuges of plants that do not produce Bt toxins and thus allow survival of susceptible pests. Recent advances include using refuges together with Bt crop 'pyramids' that make two or more Bt toxins effective against the same pest, and planting seed mixtures yielding random distributions of pyramided Bt and non-Bt corn plants within fields. We conclude that conditions often deviate from those favoring the success of pyramids and seed mixtures, particularly against pests with low inherent susceptibility to Bt toxins. For these problematic pests, promising approaches include using larger refuges and integrating Bt crops with other pest management tactics.
Zea mays L. (maize) hybrids producing the Cry1F protein from Bacillus thuringiensis were first commercialized in the United States in 2003. These products demonstrated varying levels of moderate control, but not immunity to Striacosta albicosta (Smith) (Lepidoptera: Noctuidae) (western bean cutworm). Susceptibility of western bean cutworm to Cry1F protein was assessed in field populations collected in the mid- and western United States in 2003, 2004, 2013, and 2014 using diet bioassay. A meta-analysis of 32 western bean cutworm field collections assessed for susceptibility to Cry1F was conducted to investigate changes in susceptibility over time. Based on meta-analysis results, these data suggest a 5.2-fold increase in median lethal concentration (LC50) response to Cry1F in the 2013-2014 populations compared with collections that were assessed 10 yr earlier. Widespread use of Cry1F-producing maize hybrids over the past 10 yr may have contributed to favoring western bean cutworm populations with tolerance to the Cry1F protein.
Background:
Spodoptera frugiperda is one the main target pests of maize events expressing Vip3Aa20 protein from Bacillus thuringiensis (Bt) in Brazil. In this study, we selected a resistant strain of S. frugiperda on Bt maize expressing Vip3Aa20 protein and characterized the inheritance and fitness costs of the resistance.
Results:
The resistance ratio of Vip3Aa20-resistant strain of S. frugiperda was >3200-fold. Neonates of the Vip3Aa20-resistant strain were able to survive and emerge as fertile adults on Vip3Aa20 maize while larvae from susceptible and heterozygous strains did not survive. The inheritance of Vip3Aa20-resistant resistance was autosomal recessive and monogenic. Life history studies to investigate fitness cost revealed 11% reduction in the survival rate until adult stage and 50% lower reproductive rate of the Vip3Aa20-resistant strain compared with susceptible and heterozygous strains.
Conclusion:
This is the first characterization of S. frugiperda resistance to Vip3Aa protein. Our results provide useful information for resistance management programs designed to prevent or delay resistance evolution to Vip3Aa proteins in S. frugiperda.
Background:
The first Bt maize in Brazil was launched in 2008 and contained the MON 810 event, which expresses Cry1Ab protein. Although the Cry1Ab dose in MON 810 is not high against fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith), MON 810 provided commercial levels of control. To support insect resistance management in Brazil, the baseline and ongoing susceptibility of FAW was examined using protein bioassays, and the level of control and life history parameters of FAW were evaluated on MON 810 maize.
Results:
Baseline diet-overlay assays with Cry1Ab (16 µg cm(-2) ) caused 76.3% mortality to field FAW populations sampled in 2009. Moderate mortality 48.8%) and significant growth inhibition (88.4%) were verified in leaf disc bioassays. In greenhouse trials, MON 810 had significantly less damage than non-Bt maize. The surviving FAW larvae on MON 810 (22.4% ) had a 5.5-day increase in life cycle time and a 24% reduction in population growth rate. Resistance monitoring (2010-2015) showed a significant reduction in Cry1Ab susceptibility of FAW over time. Additionally, a significant reduction in the field efficacy of MON 810 maize against FAW was observed in different regions from crop season 2009 to 2013.
Conclusions:
The decrease susceptibility to Cry1Ab was expected, but the specific contributions to this resistance by MON 810 maize cannot be distinguished from cross-resistance to Cry1Ab caused by exposure to Cry1F maize. Technologies combining multiple novel insecticidal traits with no cross resistance to the current Cry1proteins and high activity against the same target pests should be pursued in Brazil and similar environments.
To delay evolution of resistance by insect pests, farmers are rapidly increasing their use of transgenic crops producing two or more Bacillus thuringiensis (Bt) toxins that kill the same pest. A key condition favoring durability of these "pyramided" crops is the absence of cross-resistance between toxins. Here we evaluated cross-resistance in the major lepidopteran pest Helicoverpa zea (Boddie) to Bt toxins used in pyramids. In the laboratory, we selected a strain of this pest with Bt toxin Cry1Ac followed by selection with MVP II, a formulation containing a hybrid protoxin that is identical to Cry1Ac in the active portion of the toxin and 98.5% identical overall. We calculated the resistance ratio as the EC50 (concentration causing mortality or failure to develop beyond the first instar of 50% of larvae) for the laboratory-selected strain divided by the EC50 for its field-derived parent strain that was not selected in the laboratory. The resistance ratio was 20.0-33.9 (mean=27.0) for MVP II, 57.0 for Cry1Ac, 51.3 for Cry1A.105, 22.4 for Cry1Ab, 3.3 for Cry2Ab, 1.8 for Cry1Fa, and 1.6 for Vip3Aa. Resistance ratios were 2.9 for DiPel ES and 2.0 for Agree VG, which are commercial Bt spray formulations containing Cry1Ac, other Bt toxins, and Bt spores. By the conservative criterion of non-overlap of 95% fiducial limits, the EC50 was significantly higher for the selected strain than its parent strain for MVP II, Cry1Ac, Cry1A.105, Cry1Ab, Cry2Ab and DiPel ES. For Cry1Fa, Vip3Aa, and Agree VG, significantly lower susceptibility to a high concentration indicated low cross-resistance. The resistance ratio for toxins other than Cry1Ac was associated with their amino acid sequence similarity to Cry1Ac in domain II. Resistance to Cry1Ac and the observed cross-resistance to other Bt toxins could accelerate evolution of H. zea resistance to currently registered Bt sprays and pyramided Bt crops.
Dominance of resistance has been one of major parameters affecting the rate of evolution of resistance to Bt crops. High-dose is the capacity of Bt crops to kill heterozygous insects and has been an essential component of the most successful strategy to manage resistance to these crops. Experiments were conducted to evaluate directly and indirectly if TC1507 event is high-dose to Spodoptera frugiperda (J E Smith).
About 8% of heterozygote neonate larvae were able to survive, complete larval development and emerge as normal adults on TC1507 leaves while susceptible larvae could not survive for five days. The estimated dominance of resistance was 0.15 ± 0.09 and significantly higher than zero; therefore, the resistance to Cry1F expressed in TC1507 was not completely recessive. A 25-fold dilution of TC1507 maize leaf tissue in an artificial diet was able to cause a maximum mortality of only 37% and to inhibit growth of 82% at seven days after larval infestation.
Resistance to Cry1F in TC1507 maize is incompletely recessive in S. frugiperda. TC1507 maize is not high-dose for S. frugiperda. Additional or alternative resistance management strategies, such as the replacement of single-trait Bt maize with pyramided Bt maize that produces multiple proteins targeting the same insect pests, should be implemented wherever this technology is in use and S. frugiperda is the major pest.
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The inheritance and phenotypic expression of resistance to Bacillus thuringiensis Cry1Ac-insecticidal protein were studied in selected populations of pink bollworm, Pectinophora gossypiella (Saunders), that were collected from Bollgard cotton in India. The individual populations in the pool were Cry1Ac-resistant and sourced from Cry1Ac-containing Bt cotton (Bollgard) hybrids in 2010.
Laboratory selection on diet with 1.0 µg/ mL of Cry1Ac protein increased the percentage reaching ≥ third instars from 7% in the F3 generation to 94% in the F15 generation, a 257-fold increase in median lethal concentration relative to the susceptible strain. The analysis of reciprocal genetic crosses between the Cry1Ac-resistant strain NKJ and a susceptible laboratory strain MRC showed dominance (h) of 0.22 indicating that the inheritance of Cry1Ac-resistance is partially recessive at Cry1Ac concentrations comparable to those in Bollgard. Analyses of backcrosses of F1 hybrid moths with NKJ and MRC indicated that resistance is autosomal. The Cry1Ac-resistant strain exhibited little or no cross-resistance to the Cry2Ab2 protein.
This is the first study of the dominance of Cry1Ac field-resistance in P. gossypiella. The results provide the basis for refining resistance management strategies for Bt cotton.
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The gene Cry1Ab derived from the entomopathogenic bacterium Bacillus thuringiensis is currently deployed commercially for control of Pyralid stem borers of maize in the USA. In this study various events of the Bt-gene were evaluated in maize inbred lines and hybrids for leaf feeding resistance to the local stem borers Busseola fusca and Chilo partellus, using artificially infested plants under greenhouse and field conditions. The event MON810 proved superior to all other events tested. C. partellus was more susceptible than B. fusca to the same events. Significant maternal effects were observed. Efficacy was not negatively affected by variations in the time of infestation. Hybrids did not respond equally to transformation with the same event, which was attributed to differences in adaptation to adverse environmental conditions.
The seasonal abundance of Busseola fusca moths at five localities in the maize production area of South Africa was monitored by means of Robinson light traps. Geographical variation in the flight pattern was shown to exist between localities from east to west. Both the time and magnitude of the three seasonal moth flights seem to be governed by climatic factors. It is pointed out that the severity and the time of occurrence of larval infestations in different localities are largely predetermined by the time of planting.
Since its discovery, RNA interference (RNAi) has revolutionized functional genomic studies due to its sequence-specific nature of post-transcriptional gene silencing. In this paper, we provide a comprehensive review of the recent literature and summarize the current knowledge and advances in the applications of RNAi technologies in the field of insect toxicology and insect pest management. Many recent studies have focused on identification and validation of the genes encoding insecticide target proteins, such as acetylcholinesterases, ion channels, Bacillus thuringiensis receptors, and other receptors in the nervous system. RNAi technologies have also been widely applied to reveal the role of genes encoding cytochrome P450 monooxygenases, carboxylesterases, and glutathione S-transferases in insecticide detoxification and resistance. More recently, studies have focused on understanding the mechanism of insecticide-mediated up-regulation of detoxification genes in insects. As RNAi has already shown great potentials for insect pest management, many recent studies have also focused on host-induced gene silencing, in which several RNAi-based transgenic plants have been developed and tested as proof of concept for insect pest management. These studies indicate that RNAi is a valuable tool to address various fundamental questions in insect toxicology and may soon become an effective strategy for insect pest management.
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