[show abstract][hide abstract] ABSTRACT: Resistance to traditional synthetic pesticides has become one of the major driving forces altering the development of integrated
pest management (IPM) programs worldwide. Early definitions of resistance focused on the “development of strains capable of
surviving a dose lethal to a majority of individuals in a normal population” (cited by ffrench-Constant and Roush, 1990).
Sawicki (1987) proposed a definition that “resistance (is) a genetic change in response to selection by toxicants that may
impair control in the field.”
[show abstract][hide abstract] ABSTRACT: Neurophysiological studies were used to characterize the resistance mechanism in a new cyclodiene-resistant strain of Drosophila melanogaster. Suction electrode recordings were taken from the peripheral nerves of transected larval central nervous system. Treatment of nerve preparations with 1 mM GABA reduced the spontaneous firing of peripheral nerves. This inhibition was effectively reversed within 10 min by exposing preparations from susceptible insects (Oregon-R wild type) to 10 μM dieldrin. In contrast, 30 min incubations with 10 μM dieldrin had no effect on preparations from resistant individuals. At 10 μM, picrotoxinin was also effective in antagonizing the action of GABA in susceptible nerve preparations. In recordings from resistant insects (n = 3), picrotoxinin displayed either no antagonism of GABA-dependent inhibition, weak antagonism of GABA, or hyperexcitation indistinguishable from those of susceptible preparations. These results demonstrate that cyclodiene resistance in the Maryland strain of D. melanogaster is present at the level of the nerve, and that the resistance extends to picrotoxinin, albeit at a reduced level. The possible role of an altered GABA receptor in this resistance is discussed.
[show abstract][hide abstract] ABSTRACT: Chemically inducible production of Bacillus thuringiensis (Bt) toxins in transgenic plants may provide considerable benefits in preventing or delaying the evolution of insect resistance to Bt crops by creating within-plant temporal refuges. We examined the effect of inducible cry1Ab expression on survival of different genotypes (RR, RS, and SS) of diamondback moth, Plutella xylostella (L.), in transgenic broccoli, Brassica oleracea L., plants transformed with a PR-1a/cry1Ab expression cassette. Spraying leaves of these plants with the inducer acibenzolar-s-methyl [= benzo (1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester] (ASM) resulted in high levels of Bt toxin, and detached leaves from fully induced plants caused 100% mortality to all instars of P. xylostella SS and RS genotypes. When plants infested with larvae were treated with ASM, only a few larvae that were nearing completion of their development were able to survive the induction process. Signal transduction from ASM-treated leaves to new plant tissue also was evaluated using a larval assay. New foliage that emerged after plants were induced remained toxic to > or = 80% of RS larvae up to the fourth new leaf. In whole plant tests, however, induced plants remained protected from larval damage for > or = 3 wk. Uninduced PR-1a/cry1Ab plants seemed to produce low levels of Bt that were undetected by an enzyme-linked immunosorbent assay but that resulted in significant fitness costs for susceptible insects. The suitability of PR-1a/cry1Ab broccoli plants for insect resistance management and the requirements of an appropriate inducible promoter are discussed.
Journal of Economic Entomology 01/2006; 98(6):2188-94. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: Transgenic plants expressing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) were grown on over 13 million ha in the United States and 22.4 million ha worldwide in 2004. Preventing or slowing the evolution of resistance by insects ("resistance management") is critical for the sustainable use of Bt crops. Plants containing two dissimilar Bt toxin genes in the same plant ("pyramided") have the potential to delay insect resistance. However, the advantage of pyramided Bt plants for resistance management may be compromised if they share similar toxins with single-gene plants that are deployed simultaneously. We tested this hypothesis using a unique model system composed of broccoli plants transformed to express different Cry toxins (Cry1Ac, Cry1C, or both) and a synthetic population of the diamondback moth (Plutella xylostella) carrying genes for resistance to Cry1Ac and Cry1C at frequencies of approximately 0.10 and 0.34, respectively. After 24-26 generations of selection in the greenhouse, the concurrent use of one- and two-gene plants resulted in control failure of both types of Bt plants. When only two-gene plants were used in the selection, no or few insects survived on one- or two-gene Bt plants, indicating that concurrent use of transgenic plants expressing a single and two Bt genes will select for resistance to two-gene plants more rapidly than the use of two-gene plants alone. The results of this experiment agree with the predictions of a Mendelian deterministic simulation model and have important implications for the regulation and deployment of pyramided Bt plants.
Proceedings of the National Academy of Sciences 07/2005; 102(24):8426-30. · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: Transgenic plants expressing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) were first commercialized in 1996 amid concern from some scientists, regulators and environmentalists that the widespread use of Bt crops would inevitably lead to resistance and the loss of a 'public good,' specifically, the susceptibility of insect pests to Bt proteins. Eight years later, Bt corn and cotton have been grown on a cumulative area >80 million ha worldwide. Despite dire predictions to the contrary, resistance to a Bt crop has yet to be documented, suggesting that resistance management strategies have been effective thus far. However, current strategies to delay resistance remain far from ideal. Eight years without resistance provides a timely opportunity for researchers, regulators and industry to reassess the risk of resistance and the most effective strategies to preserve Bt and other novel insect-resistant crops in development.
[show abstract][hide abstract] ABSTRACT: Negative cross-resistance (NCR) toxins that hitherto have not been thought to have practical uses may indeed be useful in the management of resistance alleles. Practical applications of NCR for pest management have been limited (i) by the scarcity of high toxicity NCR toxins among pesticides, (ii) by the lack of systematic methodologies to discover and develop such toxins, as well as (iii) by the lack of deployment tactics that would make NCR attractive. Here we present the concept that NCR toxins can improve the effectiveness of refuges in delaying the evolution of resistance by herbivorous insect pests to transgenic host plants containing insecticidal toxins. In our concept, NCR toxins are deployed in the refuge, and thus are physically separated from the transgenic plants containing the primary plant-protectant gene (PPPG) encoding an insecticidal toxin. Our models show: (i) that use of NCR toxins in the refuge dramatically delays the increase in the frequency of resistance alleles in the insect population; and (ii) that NCR toxins that are only moderately effective in killing insects resistant to the PPPG can greatly improve the durability of transgenic insecticidal toxins. Moderately toxic NCR toxins are more effective in minimizing resistance development in the field when they are deployed in the refuge than when they are pyramided with the PPPG. We explore the potential strengths and weaknesses of deploying NCR toxins in refuges.
Journal of Theoretical Biology 01/2005; 231(4):461-74. · 2.35 Impact Factor
[show abstract][hide abstract] ABSTRACT: Preventing insect pests from developing resistance to Bacillus thuringiensis (Bt) toxins produced by transgenic crops is a major challenge for agriculture. Theoretical models suggest that plants containing two dissimilar Bt toxin genes ('pyramided' plants) have the potential to delay resistance more effectively than single-toxin plants used sequentially or in mosaics. To test these predictions, we developed a unique model system consisting of Bt transgenic broccoli plants and the diamondback moth, Plutella xylostella. We conducted a greenhouse study using an artificial population of diamondback moths carrying genes for resistance to the Bt toxins Cry1Ac and Cry1C at frequencies of about 0.10 and 0.20, respectively. After 24 generations of selection, resistance to pyramided two-gene plants was significantly delayed as compared with resistance to single-gene plants deployed in mosaics, and to Cry1Ac toxin when it was the first used in a sequence. These results have important implications for the development and regulation of transgenic insecticidal plants.
[show abstract][hide abstract] ABSTRACT: Transgenic crops that produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) grew on >62 million ha worldwide from 1996 to 2002. Despite expectations that pests would rapidly evolve resistance to such Bt crops, increases in the frequency of resistance caused by exposure to Bt crops in the field have not yet been documented. In laboratory and greenhouse tests, however, at least seven resistant laboratory strains of three pests (Plutella xylostella [L.], Pectinophora gossypiella [Saunders], and Helicoverpa armigera [Hübner]) have completed development on Bt crops. In contrast, several other laboratory strains with 70- to 10,100-fold resistance to Bt toxins in diet did not survive on Bt crops. Monitoring of field populations in regions with high adoption of Bt crops has not yet detected increases in resistance frequency. Resistance monitoring examples include Ostrinia nubilalis (Hübner) in the United States (6 yr), P. gossypiella in Arizona (5 yr), H. armigera in northern China (3 yr), and Helicoverpa zea (Boddie) in North Carolina (2 yr). Key factors delaying resistance to Bt crops are probably refuges of non-Bt host plants that enable survival of susceptible pests, low initial resistance allele frequencies, recessive inheritance of resistance to Bt crops, costs associated with resistance that reduce fitness of resistant individuals relative to susceptible individuals on non-Bt hosts ("fitness costs"), and disadvantages suffered by resistant strains on Bt hosts relative to their performance on non-Bt hosts ("incomplete resistance"). The relative importance of these factors varies among pest-Bt crop systems, and violations of key assumptions of the refuge strategy (low resistance allele frequency and recessive inheritance) may occur in some cases. The success of Bt crops exceeds expectations of many, but does not preclude resistance problems in the future.
Journal of Economic Entomology 08/2003; 96(4):1031-8. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: Transgenic plants expressing insecticidal proteins from the bacterium, Bacillus thuringiensis (Bt), are revolutionizing agriculture. Bt, which had limited use as a foliar insecticide, has become a major insecticide because genes that produce Bt toxins have been engineered into major crops grown on 11.4 million ha worldwide in 2000. Based on the data collected to date, generally these crops have shown positive economic benefits to growers and reduced the use of other insecticides. The potential ecological and human health consequences of Bt plants, including effects on nontarget organisms, food safety, and the development of resistant insect populations, are being compared for Bt plants and alternative insect management strategies. Scientists do not have full knowledge of the risks and benefits of any insect management strategies. Bt plants were deployed with the expectation that the risks would be lower than current or alternative technologies and that the benefits would be greater. Based on the data to date, these expectations seem valid.
Annual Review of Entomology 02/2002; 47:845-81. · 13.59 Impact Factor
[show abstract][hide abstract] ABSTRACT: Experimental evaluation of the effectiveness of resistance management tactics is vital to help provide guidelines for the deployment of transgenic insecticidal crops. Transgenic broccoli expressing a Cry1Ac gene of Bacillus thuringiensis (Bt) and the diamondback moth, Plutella xylostella (L.), were used in greenhouse tests to evaluate the influence of size and placement of nontransgenic refuge plants on changes in resistance allele frequency and pest population growth. In the first test with an initial Cry1Ac-resistance (R) allele frequency of 0.007, P. xylostella were introduced into cages with the following treatments: 0, 3.3, 10, 20, and 100% refuge plants. Results after four generations showed that resistance could be delayed by increasing the proportion of refuge plants in the cage. Population growth was also influenced by refuge size with the highest populations occurring in treatments that had either no refuge plants or all refuge plants. In the second test, we evaluated the effect of refuge placement by comparing 20% separate and 20% mixed refuges. P. xylostella with an initial frequency of resistant alleles at 0.0125 were introduced into cages and allowed to cycle; later generations were evaluated for resistance and population growth. Separating the refuge had a pronounced effect on delaying resistance and slowing establishment of resistant larvae on Bt plants. Combining information from both trials, we found a strong negative correlation between the number of larvae on Bt plants and the mortality of the population in leaf dip bioassays. Results from larval movement studies showed that separate refuges delayed resistance better than mixed refuges because they conserved relatively more susceptible alleles than R alleles and did not increase the effective dominance of resistance.
Journal of Economic Entomology 03/2001; 94(1):240-7. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: A field-collected colony of the diamondback moth, Plutella xylostella, had 31-fold resistance to Cry1C protoxin of Bacillus thuringiensis. After 24 generations of selection with Cry1C protoxin and transgenic broccoli expressing a Cry1C protein, the resistance that developed was high enough that neonates of the resistant strain could complete their entire life cycle on transgenic broccoli expressing high levels of Cry1C. After 26 generations of selection, the resistance ratios of this strain to Cry1C protoxin were 12,400- and 63,100-fold, respectively, for the neonates and second instars by a leaf dip assay. The resistance remained stable until generation 38 (G38) under continuous selection but decreased to 235-fold at G38 when selection ceased at G28. The Cry1C resistance in this strain was seen to be inherited as an autosomal and incompletely recessive factor or factors when evaluated using a leaf dip assay and recessive when evaluated using Cry1C transgenic broccoli. Saturable binding of (125)I-Cry1C was found with brush border membrane vesicles (BBMV) from both susceptible and Cry1C-resistant strains. Significant differences in Cry1C binding to BBMV from the two strains were detected. BBMV from the resistant strain had about sevenfold-lower affinity for Cry1C and threefold-higher binding site concentration than BBMV from the susceptible strain. The overall Cry1C binding affinity was just 2.5-fold higher for BBMV from the susceptible strain than it was for BBMV from the resistant strain. These results suggest that reduced binding is not the major mechanism of resistance to Cry1C.
Applied and Environmental Microbiology 10/2000; 66(9):3784-9. · 3.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Several important crops have been engineered to express toxins of Bacillus thuringiensis (Bt) for insect control. In 1999, US farmers planted nearly 8 million hectares (nearly 20 million acres) of transgenic Bt crops approved by the EPA. Bt-transgenic plants can greatly reduce the use of broader spectrum insecticides, but insect resistance may hinder this technology. Present resistance management strategies rely on a "refuge" composed of non-Bt plants to conserve susceptible alleles. We have used Bt-transgenic broccoli plants and the diamondback moth as a model system to examine resistance management strategies. The higher number of larvae on refuge plants in our field tests indicate that a "separate refuge" will be more effective at conserving susceptible larvae than a "mixed refuge" and would thereby reduce the number of homozygous resistant (RR) offspring. Our field tests also examined the strategy of spraying the refuge to prevent economic loss to the crop while maintaining susceptible alleles in the population. Results indicate that great care must be taken to ensure that refuges, particularly those sprayed with efficacious insecticides, produce adequate numbers of susceptible alleles. Each insect/Bt crop system may have unique management requirements because of the biology of the insect, but our studies validate the need for a refuge. As we learn more about how to refine our present resistance management strategies, it is important to also develop the next generation of technology and implementation strategies.
[show abstract][hide abstract] ABSTRACT: Leaf bioassays and oviposition choice tests were used to investigate the effects of transgenic broccoli expressing Cry1Ac toxin of Bacillus thuringiensis (Berliner) on susceptible and resistant Plutella xylostella (L.) larvae. Survival of susceptible 2nd instars on Cry1Ac-expressing broccoli declined from 99.1 to 19.2% at 24 and 72 h, respectively, and larvae exhibited an average weight loss of 0.2 mg/10 larvae at 24 h. Larvae that evolved resistance to foliar sprays of B. thuringiensis subsp. kurstaki in the field, however, showed no debilitating effects from the Cry1Ac broccoli. Survival of resistant larvae at 24 and 72 h was 98.6 and 90.8%, respectively, and weight gain at 24 h was 1.7 mg/10 larvae, none of which was significantly different from survival or weight gain on control plants. In oviposition choice tests, both susceptible and resistant females were unable to discriminate between Cry1Ac and normal broccoli, laying ≈38-41 eggs per plant per 2 females. Comparing mortality of susceptible larvae on 2 lines of transgenic broccoli (J1R and K20) derived from independent transformation events, we found that the majority of the variance (43.2%) in toxin expression was caused by transformation. Depending upon the transformation, plant could be a significant source of variation but toxin expression within plant was always fairly uniform. Our data indicate that resistance to sprays of B. thuringiensis can confer resistance to plants when similar toxins are involved.
Journal of Economic Entomology 01/1999; 92(1):47-55. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: Three resistance management strategies for field-sprayed commercial formulations of Bacillus thuringiensis Berliner subspecies were tested in field cages during the dry and rainy seasons of 1995 in Honduras. A local field population of Plutella xylostella (L.) with a 21-fold resistance to B. thuringiensis subsp. Kurstaki (Javelin), but no resistance to B. thurillgiensis subsp. Aizawa (Xentari), was selected for 5-6 generations with 16 field applications of a high (1.12 kg/ha) or low (0.3 kg/ha) dose of Javelin, a high or low dose of Javelin in the presence or absence of a refuge (25%), and Xentari (1.12 kg/ha). Resistance to Javelin increased ≍1.9-4.4 times, but was significant only with the 1.12 kg/ ha rate of Javelin irrespective of the presence or absence of a refuge. Field selection with Javelin at 0.3 kg/ha or Xentari did not cause a significant increase in resistance to B. thuringiensis subsp. Kurstaki, nor did P. xylostella selected with Xentari evolve resistance to B. thuringiensis subsp. aizawai. During the same period, the LC50 of Javelin in P. xylostella left un selected did not decrease. Although the rate of resistance increase was lower for lower doses of Javelin, a smaller proportion of marketable cabbage was produced in comparison with higher doses of Javelin or Xentari. Our data suggest that the deliberate inclusion of a refuge may reduce the proportion of marketable produce, and may affect use of this resistance management strategy in both sprayed B. thuringiensis and transgenic crops expressing B. thuringiensis toxins.
Journal of Economic Entomology 11/1997; 90(6):1462-1470. · 1.60 Impact Factor