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Silicon reduces slug feeding on wheat seedlings
Abstract
Slugs are a serious pest of cereal crops, and recent emphasis in slug pest management has shifted from
solely chemical towards integrated approaches. The objective of the present research was to test if boosted silicon (Si) and calcium (Ca) levels in wheat seedlings can reduce slug grazing. Laboratory experiments were conducted in which wheat seedlings were grown firstly, with soluble Si and Ca (with and without additional mineral N) or secondly, with six levels of soluble Si, and consumption of leave sections by the field slug (Deroceras reticulatum) was measured. Boosted foliar Si concentrations reduced consumption significantly (P<0.001) compared to an untreated control and Ca treatments in a no-choice setting; a similar trend (P<0.10), but with a higher variability, was observed in a simultaneous choice setting. It is shown for the first time that increasing the nominal Si concentration of treatment solutions in a geometric series (from 0 to 6 g sodium metasilicate nonahydrate l-1) translated into a logarithmic increase in foliar Si concentrations (from 5.0 to 19.4 g Si kg-1 dry weight). When these leaves were offered simultaneously (choice setting), wheat leaves containing less than 10 g Si kg-1 were consumed preferentially by D. reticulatum (P\0.001), suggesting that Si concentrations as low as 1 % leaf dry weight may be effective at reducing grazing by slugs. It is concluded that boosting Si levels in cereals has potential as a novel tool in crop protection against pest slugs and snails. Various open research questions to advance this tool are identified.
... Increased silica or silicon content has been observed to cause general health problems in animals, including molluscs [37][38][39][40][41]. In slugs, increased silicon content was suggested to cause reduced consumption [42,43]. This may be due to decreased leaf digestibility or wear on the feeding apparatus, with similar effects observed in other invertebrates [41][42][43]. ...
... In slugs, increased silicon content was suggested to cause reduced consumption [42,43]. This may be due to decreased leaf digestibility or wear on the feeding apparatus, with similar effects observed in other invertebrates [41][42][43]. However, the Golden Apple Snail, Pomacea canaliculata, showed no feeding aversion to plants with higher silicon content, indicating that an increased silicon content may not affect feeding in all mollusc species and that reduced feeding may be due to other factors [44]. ...
Gastropod damage to crop plants has a significant economic impact on agricultural and horticultural industries worldwide, with the Grey Field Slug (Deroceras reticulatum (Müller)) considered the main mollusc pest in the United Kingdom and in many other temperate areas. The prevailing form of crop protection is pellets containing the active ingredient, metaldehyde. Metaldehyde can cause paralysis and death in the mollusc, depending on the amount ingested. The paralysing effects may result in reduced pellet consumption. A greater understanding of metaldehyde consumption may reveal an area that can be manipulated using novel molluscicide formulations. Novel pellet types included commercial metaldehyde pellets coated so that metaldehyde is released more slowly. In both laboratory and arena trials, an audio sensor was used to record individual slugs feeding on a variety of pellet types, including commercially available toxic pellets (metaldehyde and ferric phosphate) and novel metaldehyde formulations. The sensor was used to record the length of each bite and the total number of bites. There was no significant difference in the length of bites between pellet types in laboratory trials. Novel pellets were not consumed more than commercial pellet types. Commercial pellet types did not differ in consumption.
... reduced consumption rates due to higher phenolic content (e.g., Albrectsen et al., 2004;Fritz et al., 2001) and higher silicon content (e.g., Griffin et al., 2015). The phenolic compounds in the leaves impart a bitter taste and toxicity, while the higher amounts of silicon make the leaves sharper, harder, and less digestible, reducing their palatability. ...
The invasion of the terrestrial ecosystem by gastropods has immense negative impacts on ecosystem functions, health, and agricultural plants. The non‐native slug, Deroceras laeve (O. F. Müller, 1774), a recognised agricultural pest in native and invaded regions, was recently reported from the Darjeeling Himalayas, India. We assessed the plant palatability and preference of D. laeve using selected plants being farmed in the Darjeeling Himalayas (basil, cabbage, coriander, lettuce, mint, pumpkin leaf, spinach) by laboratory‐based choice and non‐choice experiments. Moreover, we evaluated the correlation between different chemical features of the plants with the consumption rate of D. laeve and observed the fecundity of D. laeve reared on different plant diets. The highest consumption rate of D. laeve was observed for pumpkin (12.99 ± 1.33 mg dry mass) and the lowest for mint (1.63 ± 0.13 mg dry mass), with a significant positive correlation between consumption rate and D. laeve body mass (for lettuce: R= 0.47, p= 0.0009 and other plant species: R= 0.52, p= 0.00004) was observed through the non‐choice experiment. Moreover, the consumption rate of the non‐native slug, D. laeve , was considerably higher than a controphic slug, Meghimatium bilineatum . In the choice experiment, D. laeve preferred lettuce, followed by pumpkin, cabbage and coriander, even in the presence of detritus and significantly avoided spinach, mint and basil. The consumption rate of D. laeve had a significant negative correlation with calcium (R= ‐0.49, p= 0.003) and potassium (R= ‐0.37, p= 0.03), and a positive correlation was observed for magnesium ( R = 0.37, p = 0.03). However, no correlation was observed for zinc, silica, total carbohydrate, protein and phenol content of the leaves. The highest fecundity of D. laeve was observed in the lettuce and cabbage diet, while no eggs were laid on the mint diet. Hence, the consumption of different leaves, irrespective of physical and chemical features (hairiness, amount of silicon, protein, carbohydrate and phenol), suitable life history traits, and suitable habitats, may facilitate D. laeve as a potential agricultural pest in the Darjeeling Himalayas, India.
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... Numerous studies have showed that Si can increase crop resistance to various phytophagous pests (Reynolds et al. 2009(Reynolds et al. , 2016Griffin et al. 2015). Both soil and foliar applications of calcium silicate-enhanced cucumber resistance to Bemisia tabaci and reduced its egg production, nymph growth and survival rate (Correa et al. 2005). ...
Silicon (Si) enhances rice resistance to various insect herbivores. However, the underlying mechanisms remain unclear. Whereas the salicylic acid (SA) signaling pathway plays a vital role in plant defense responses to sucking insects, its role in Si-enhanced rice resistance has not been investigated. Si transporter mutant OsLsi1 and mutants with antisense expression of ICS (as-ics) and NPR1 (as-npr1) in the SA pathway and their corresponding wild types (WT) were treated with and without Si to determine Si effects on rice resistance to brown planthopper (BPH), Nilaparvata lugens (Stål), as well as on SA accumulation, defense-related enzyme activity and gene expression. Si application significantly affected host preference of BPH, significantly reduced honeydew secretion and inhibited oviposition and hatch rate. Upon BPH infestation, SA content, transcript levels of BPH3, ICS1 and PAL4, and activities of POD, SOD, PPO and PAL were significantly higher in Si-treated than untreated plants. The defense responses were also faster. However, OsLsi1 mutant plants displayed higher susceptibility to BPH and minimal defense responses. Furthermore, simultaneous application of SA and Si in WT plants showed the highest resistance to BPH, but had no obvious effect on OsLsi1, antisense as-ics and as-npr1 plants. Our results suggest that Si enhances rice defense against the sucking insect BPH by defense priming and the Si-mediated priming involves SA signaling pathway.
... Supplementation of Si fertilizers enhances the defense mechanisms of plants against pathogens such as viruses, bacteria, fungi, and other organisms like nematodes, arthropods, vertebrates, and herbivores (Griffin et al. 2015;Reynolds et al. 2016). Si mitigates the biotic stress in plants by either acting as a physical barrier in the epidermal layer or by alleviating resistance to pathogens. ...
Silicon is the second most abundant element and accounts for 27–28% of the earth’s crust. Biological systems also contain significant amounts of silicon, as amorphous silica (SiO2·nH2O), and its soluble form, silicic acid (Si(OH)4). Plant dry biomass contains 0.1–10% of silicon. Despite its extensive distribution in plants, silicon is viewed as a quasi-essential element, as most of the plant species can live their entire life in its absence. Interestingly, even in higher amounts, silicon is harmless, noncorrosive, and nonpolluting to plants. It is typically accumulated in the epidermal cells, creating external dual layers of
silica-cuticle and silica-cellulose on leaves, stem, and hulls. It thus acts as a physical barrier in plants. Si also alleviates the stress-induced responses in plants. This chapter reviews how plants benefit from silicon under adverse environmental conditions.
... An increase in silicon content in sugarcane stalks due to application of a silicate source has been observed in earlier studies (Keeping et al. 2013;Vilela et al. 2014;Camargo et al. 2014b). Generally, the beneficial effect of this mineral occurs through deposition in the leaves and stems of plants, constituting a physical barrier that helps in resistance to agricultural pests such as herbivorous insects and slugs (Ma 2004;Moraes et al. 2005;Massey et al. 2006;Keeping et al. 2013;Griffin et al. 2015;Reynolds et al. 2016). ...
The sugarcane borer Diatraea saccharalis (Fabricius) (Lepidoptera: Crambidae) is a major pest of sugarcane (Saccharum sp.) (Glumiflorae: Poaceae) in Brazil and has been controlled efficiently with the parasitoid Cotesia flavipes (Cameron) (Hymenoptera: Braconidae). Addition of a silicon source to the soil induces resistance to insect pests in the host plant and can be integrated with other pest-management tactics such as biological control. This study evaluated the effects of silicon on the tritrophic interactions among sugarcane, D. saccharalis, and its natural enemy C. flavipes. Sugarcane varieties (one resistant and one susceptible to the sugarcane borer) cultivated with or without silicon constituted the four treatments. IACSP 96–2042 (susceptible variety) benefited from silicate addition as the length of tunnels bored by D. saccharalis was reduced by 43%. Tunnel length in the resistant variety (IACSP 96–3060) did not change with silicon application. The fresh mass and larval body size of the sugarcane borer were not affected by the treatments. Parasitism by C. flavipes on the sugarcane borer and the morphometric parameters of this parasitoid were not affected by silicon addition, nor were cane yield and quality parameters of the two sugarcane varieties. Treatment with silicon in the management of D. saccharalis was compatible with the parasitoid C. flavipes. This study constitutes the first field evaluation of the performance of C. flavipes combined with the effect of silicon as a resistance factor in D. saccharalis control in sugarcane.
... The ability of Melanopsis to perform compensatory feeding in Populus relative to Alnus is in line with the general perception that effects of accumulated silicon in plant tissues would be less intense for snails. Several studies have reported no effects of increased Si concentration on consumption rates on herbivorous and detritivores snails (Schaller 2013;Horgan et al. 2017; but see Griffin et al. 2015), which has been attributed to the renewable teeth of their radula making it less susceptible to wear caused by silicon (Horgan et al. 2017). However, despite this ability, we did not observe compensatory feeding between the quality classes of Populus. ...
The linkage between leaf‐litter and macroinvertebrate shredders is pivotal for stream food webs. Global change is predicted to decrease the nutritional quality of litter inputs to streams. However, little is known about shredder's ability to develop local interpopulation adaptations to face nutrient‐depleted leaf‐litter. We hypothesized that this adaptation could be present in populations receiving low‐quality leaf‐litter. We performed feeding tests on three abundant shredders species from lowland (a snail) and mountain (two insects) streams. Two populations of each species were derived from two subregions contrasting in average quality of litter inputs. Individuals were fed on four diets of contrasting quality: two leaf‐litter species with two qualities each, and their feeding rates, survivorship, growth, and energetic storage were evaluated. Results suggest that local population adaptation to low‐quality litters is not common, being essentially a fixed species trait that varies across species, in particular among snails and insects. The ability of the snail to cope with low‐quality litters suggests that ecosystem processes in lowland streams may resist reductions in litter quality. Conversely, potential alterations of riparian vegetation linked to global changes might disrupt mountain stream ecosystem functioning.
... Silicon serves as a biologically active and significant element for agriculture and is listed as 8th most common element in nature and the second most common element found in soil after molecular oxygen [19,20]. It is required for the growth of diatoms, sponge, and corals, and is also found to be associated with plant cell growth because Si enhances biotic resistance against bacteria, fungi, viruses, and herbivores [21][22][23][24]. Silicon is considered as quasi-essential [25] for plant development. ...
... Silicon serves as a biologically active and significant element for agriculture and is listed as 8th most common element in nature and the second most common element found in soil after molecular oxygen [19,20]. It is required for the growth of diatoms, sponge, and corals, and is also found to be associated with plant cell growth because Si enhances biotic resistance against bacteria, fungi, viruses, and herbivores [21][22][23][24]. Silicon is considered as quasi-essential [25] for plant development. ...
Silicon (Si) is not categorized as a biologically essential element for plants, yet a great number of scientific reports have shown its significant effects in various crop plants and environmental variables. Plant Si plays biologically active role in plant life cycle, and the significant impact depends on its bioaccumulation in plant tissues or parts. In particular, it has been investigated for its involvement in limited irrigation management. Therefore, this experiment was conducted to examine the effect of Si application in eco-physiological, enzymatic and non-enzymatic activities of sugarcane plants against water stress. Four irrigation levels, i.e., normal (100-95% of soil moisture), 80-75, 55-50, and 35-30% of soil moisture were treated for the sugarcane cultivar GT 42 plants supplied with 0, 100, 200, 300, 400 and 500 mg Si l −1 and exposed for 60 days after Si application. Under stress, reduction in plant length (~26-67%), leaf area-expansion (~7-51%), relative water content (~18-57%), leaf greenness (~12-35%), photosynthetic pigments (~12-67%), physiological responses such as photosynthesis (22-63%), stomatal conductance (~25-61%), and transpiration rate (~32-63%), and biomass production were observed in the plants without Si application. The drought condition also inhibited the activities of antioxidant enzymes like catalase (~10-52%), peroxidase (ca. 4-35), superoxide dismutase (10-44%) and enhanced proline (~73-410%), and malondialdehyde content (ca. 15-158%), respectively. However, addition of Si ameliorated drought induced damage in sugarcane plants. The findings suggest that the active involvement of Si in sugarcane responsive to water stress ranges from plant performance and physiological processes, to antioxidant defense systems.
... Application of soluble form of Si can reduce severity of plant diseases [20,22]. According to Griffin et al. [9] Si reduces slug feeding on wheat seedlings. An increased Si content in leaves might be a method of crop protection against slugs and snails. ...
Application of silicon (Si) could greatly boost wheat growth and yield by mitigating abiotic stress, especially drought.
Aim
The objective of this study was to determine the effect of silicon products in different forms and methods of its application on growth and yield of spring wheat cultivated under organic farming regime.
Methods
A field experiment was conducted in the period of 2017–2018 on a research farm. The impact of different methods of application of powder and liquid forms of Si products to soil, leaves and combined methods of application (to soil and leaves) on growth parameters and yielding of spring wheat was evaluated. The treatments consisted of the recommended doses of two Si products - Adesil and ZumSil at 10 kg and 0.3 l/ha, respectively. The effect of silicon was assessed by measuring emergence and height of plants, number and weight of ears, as well as the SPAD index and yield.
Results
The number of plants emergence (316–321 seedlings per 1 m²), the number of spikes and the height of plants were the highest after application of the liquid form of silicon, regardless of the method of application. The yield was statistically higher after the combined and foliar applications of liquid form of silicon (4.97 and 4.84 t/ha, respectively). The powder silicon form was similar effective when was used as the combined application (to soil and leaves) (5.35 t/ ha).
Conclusions
Si stimulates growth of wheat enhancing number of ears and plant height, increases yields of organically grown spring wheat, a liquid Si formulation (solution of monosilicic acid) is more effective than a powder Si formulation (diatomaceous earth). Soil and foliar Si application is more effective than soil or foliar application.
... Gomes et al. (2005) investigated the effects of Si fertilisation and previous exposure to aphids on the number of bugs on the leaves, and both treatments caused a reduction in S. graminum infesting the leaves. In a similar way Griffin et al (2015) showed that Si fertilisation of wheat plants increased Si content and decreased palatability to slugs. Finally, rabbits are an important mammalian pest in wheat fields. ...
Silicon (Si) is the second most abundant element in the Earth’s crust, and is an important component of mineral soils. Crystalline forms of Si are sparingly soluble and amorphous forms are somewhat more so. The main soluble form of Si in soils is monosilicic acid (H4SiO4) which is the form taken up by plant roots. Plants vary considerably in the amount of Si that they accumulate, with grasses and cereals being particularly high and most dicotyledons being much lower. Once in the plant Si is transported in the xylem and most of it is deposited in the leaf cell lumina and walls as solid amorphous silica. These deposits are commonly known as phytoliths.
Plant scientists do not consider Si to be an essential element, and most plants can complete their life cycles in its absence. However, Si is important in defence against grazing, pests and pathogens. It also has roles in the amelioration of abiotic stresses such as aluminium and heavy metal toxicity. Silicon fertilisation is very important for rice, and is also used for other crops such as sugarcane on occasions, but is rarely employed in Northern Europe. There are, however, some concerns that soluble Si availability may become depleted in some situations where plant amorphous silica is not returned to the soil. Most recently there has also been interest in the potential of phytoliths to sequester carbon in soils thereby helping to combat climate change.
This paper will describe the availability of Si in soils, the uptake and deposition of Si within plants, and the roles of Si in decreasing abiotic and biotic stresses. It will then focus on the management of Si in soils, worries about decreased Si availability in the future, and the possibility that phytoliths may be important for carbon sequestration.
(This paper was uploaded with the express permission of the International Fertiliser Society)
... Only two amendments were selected as only a limited number of samples could be analysed due to the consuming nature of the analysis: liquid metasilicate that was expected to result in maximum Si enrichment and rice husk ash that was known from concurrent research to release the most available Si among the solid amendments. The bark was processed and Si concentrations were measured using the methodology described in detail in Griffin et al. (2015). ...
Alternative methods of protection are required against feeding by the large pine weevil (Hylobius abietis) on the bark of conifer seedlings. Silicon (Si) has been shown to enhance the resistance of plants to insect herbivores. This study investigated the effects of low doses of Si-rich soil amendments on growth, mortality and bark feeding damage of Sitka spruce (Picea sitchensis) seedlings. Two-year old seedlings were grown, individually, in soil taken from a tree nursery treated with coal ash, peat ash, rice husk ash, slag, sodium metasilicate or a commercially available Si fertilizer (Pro-Tekt) and planted out on two reforestation sites in Ireland. Seedlings grew well (about 20% growth in terms of height, 66% in root collar diameter, after two growing seasons), and Si-rich amendments did not have a significant effect on growth or mortality. Bark feeding damage on Si-treated seedlings did not vary significantly from control seedlings. Bark Si concentrations were not significantly larger in treated seedlings than in control seedlings, but control seedlings already had comparatively high bark Si concentrations (560 mg kg⁻¹ dry tissue). In conclusion, Sitka spruce seedlings grown in the presence of Si-rich soil amendments prior to planting did not show greater resistance to weevil feeding under the present conditions.
... Just as plants must contend with numerous abiotic stress- ors, so too are they subject to stress from other organisms. Supplementing plants with Si has been shown to increase plant resistance to mammalian, arthropod and molluscan herbivores, fungal and bacterial pathogens, viruses and nematodes (Griffin et al., 2015;Rodrigues et al., 2015;Reynolds et al., 2016). One of the earliest mechanisms identified and associated with plant resistance to pests is the physical defence conferred by Si de- position in plant tissues in the form of phytoliths (largely com- posed of SiO 2 ) ( McNaughton and Tarrants, 1983;Katz, 2015). ...
Background:
Silicon (Si) is known to have numerous beneficial effects on plants, alleviating diverse forms of abiotic and biotic stress. Research on this topic has accelerated in recent years and revealed multiple effects of Si in a range of plant species. Available information regarding the impact of Si on plant defence, growth and development is fragmented, discipline-specific, and usually focused on downstream, distal phenomena rather than underlying effects. Accordingly, there is a growing need for studies that address fundamental metabolic and regulatory processes, thereby allowing greater unification and focus of current research across disciplines.
Scope and conclusions:
Silicon is often regarded as a plant nutritional 'non-entity'. A suite of factors associated with Si have been recently identified, relating to plant chemistry, physiology, gene regulation and interactions with other organisms. Research to date has typically focused on the impact of Si application upon plant stress responses. However, the fundamental, underlying mechanisms that account for the manifold effects of Si in plant biology remain undefined. Here, the known effects of Si in higher plants relating to alleviation of both abiotic and biotic stress are briefly reviewed and the potential importance of Si in plant primary metabolism is discussed, highlighting the need for a unifying research framework targeting common underlying mechanisms. The traditional approach of discipline-specific work on single stressors in individual plant species is currently inadequate. Thus, a holistic and comparative approach is proposed to assess the mode of action of Si between plant trait types (e.g. C3, C4 and CAM; Si accumulators and non-accumulators) and between biotic and abiotic stressors (pathogens, herbivores, drought, salt), considering potential pathways (i.e. primary metabolic processes) highlighted by recent empirical evidence. Utilizing genomic, transcriptomic, proteomic and metabolomic approaches in such comparative studies will pave the way for unification of the field and a deeper understanding of the role of Si in plants.
... Changes in calcium supply also have shown to affect the integrity of plant cell wall and tissue hardness, leading to decreased damages caused by bacterial or fungal infections [82,83] and damages caused by feeding insects. Increased rigidity of leaf tissues in response to calcium supply caused attrition to the mouthparts of the chewing insects Spodoptera exigua Hübner, Eldana saccharina Walker, and Deroceras reticulatum Müller [84][85][86], and might impede D. citri stylet penetration. ...
The biology and behaviour of the psyllid Diaphorina citri Kuwayama (Hemiptera: Sternorrhyncha: Liviidae), the major insect vector of bacteria associated with huanglongbing, have been extensively studied with respect to host preferences, thermal requirements, and responses to visual and chemical volatile stimuli. However, development of the psyllid in relation to the ontogeny of immature citrus flush growth has not been clearly defined or illustrated. Such information is important for determining the timing and frequency of measures used to minimize populations of the psyllid in orchards and spread of HLB. Our objective was to study how flush ontogeny influences the biotic potential of the psyllid. We divided citrus flush growth into six stages within four developmental phases: emergence (V1), development (V2 and V3), maturation (V4 and V5), and dormancy (V6). Diaphorina citri oviposition and nymph development were assessed on all flush stages in a temperature controlled room, and in a screen-house in which ambient temperatures varied. Our results show that biotic potential of Diaphorina citri is not a matter of the size or the age of the flushes (days after budbreak), but the developmental stage within its ontogeny. Females laid eggs on flush V1 to V5 only, with the time needed to commence oviposition increasing with the increasing in flush age. Stages V1, V2 and V3 were most suitable for oviposition, nymph survival and development, and adult emergence, which showed evidence of protandry. Flush shoots at emerging and developmental phases should be the focus of any chemical or biological control strategy to reduce the biotic potential of D. citri, to protect citrus tree from Liberibacter infection and to minimize HLB dissemination.
... Silicon can be especially useful when plants are under abiotic or biotic stress (Ma et al., 2001;Ma, 2004;Ahmed et al., 2013;Balakhnina & Borkowska, 2013;Van Bockhaven et al., 2013;Meharg & Meharg, 2015;Pontigo et al., 2015;Rizwan et al., 2015). Silicon can improve resistance of plants to diseases (Fauteux et al., 2005;Van Bockhaven et al., 2013) and herbivores, ranging from phytophagous insects (Laing et al., 2006;Reynolds et al., 2009Reynolds et al., , 2016Han et al., 2016) and slugs (Griffin et al., 2015) to mammals (Cotterill et al., 2007;Massey et al., 2007;Reynolds et al., 2012), in both monocotyledons and dicotyledons (Epstein, 2009). Such beneficial effects support an adaptive origin of mechanisms for Si uptake and accumulation in many plants. ...
Soil amendment with Silicon (Si) can increase plant resistance against insect herbivores, but the underlying mechanisms remain unclear. The mechanical resistance hypothesis (MRH) states that Si accumulated in epidermal cells directly and passively protects against herbivores by creating a mechanical barrier. The physiological resistance hypothesis (PRH) states that Si enhances resistance by activating plant biochemical and physiological processes. We tested both hypotheses by manipulating Si fertilization of the Si non-accumulator collard, Brassica oleracea L. cv. acephala (Brassicaceae). Then, we assessed functional and ultrastructural plant responses and the developmental and reproductive performance of the leaf-chewing larvae of the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), and the sap-sucking cabbage aphid, Brevicoryne brassicae L. (Hemiptera: Aphididae). There was a 20% increase in leaf Si content. Silicon deposition in epidermal cells was identified by confocal microscopy and directly coincided with lower performance of P. xylostella, but did not affect B. brassicae. On the other hand, we found no unequivocal evidence that Si-mediated changes in primary and secondary metabolism improved plant resistance against the insects. Negative mechanical effects of Si on the insects may have masked beneficial effects of increased water, nitrogen, and mineral contents in Si-treated collards. Silicon did not change leaf contents of hemicellulose, cellulose, and lignin. Although Si-mediated increases in leaf glucosinolates (GLS) correlated with lower larval performance and higher oviposition preference of P. xylostella, both P. xylostella and B. brassicae are highly specialized in overcoming such secondary metabolites. Thus, mechanical resistance may have impaired P. xylostella, rather than the Si-mediated increase in GLS. We suggest that the PRH may depend on the degree of insect feeding specialization, so that toxic Si-mediated defenses may be more efficient against unadapted polyphagous herbivores. For them, a toxic barrier may be added to the mechanical resistance.
... Though some physical defences of plants are useful against slugs and snails (the element silicon is particularly promising; (Wadham and Parry 1981;Griffin et al. 2015), chemical defences are more broadly relevant. Many constitutively expressed secondary metabolites that are toxic and/or repellent to herbivorous insects can also be effective against slugs, eliciting chemoreceptor responses from their tentacles (Birkett et al. 2004). ...
Terrestrial molluscs are some of the most important herbivores in temperate habitats. They tend to be generalists and can be serious pests in agricultural fields, particularly no-till fields used for field and forage crops; however, farmers have access to few commercially available solutions, and the existing ones present many disadvantages (e.g. reliability, cost, environmental concerns). In this paper, we review these current management options with a focus on agronomic crops, as well as the biotic factors that influence mollusc feeding, such as natural enemies, plant nutritional content, and chemical defences. These biotic factors all have important direct consequences on mollusc fitness and can be manipulated in agricultural settings. We then review evidence from the latest research in the field of nutritional ecology to propose the use of the Geometric Framework, a well-established nutritional approach, to measure nutrient regulation and performance of terrestrial molluscs and develop ecologically based management programs that also relies on susceptibility to natural enemies. To illustrate our point, we detail a specific strategy being used by farmers in the Mid-Atlantic US to manage slug populations; in this system, farmers are using cover crops terminated after the cash crop is planted (also called “planting green”) and this approach appears to harness slug nutritional preferences and natural enemies to manage slug populations.
... Silicon soil amendments have been shown to reduce damage to rice and wheat, Triticum aestivum 344 L., from the slug D. reticulatum (Wadham and Wynn Parry 1981; Griffin et al., 2015). Furthermore, it 345 has been suggested that using rice husk or ash as a soil amendment reduces apple snail damage in 346 farmers' fields in the Philippines; however, the suggested mechanism was direct mechanical injury to 347 the snails, rather than plant-mediated resistance (Abas, 2014). ...
This study examines the potential for silicon soil amendments and nitrogen to reduce apple snail, Pomacea canaliculata Lamarck, damage to rice (Oryza sativa L.) seedlings. A rate of 75 kg/ha of nitrogen applied as (NH4)2SO4 to rice (cv IR64 and cv YTH183) seedbeds increased seedling biomass, allowing the seedlings to gain critical stem thickness and avoid snail herbivory. These seedlings remained relatively large after exposure to snails in pot experiments, mainly because of faster growth rates, but also due to lower damage from snails to >21 day-old IR64 seedlings. Silicon applied as Na2O3Si·9H20 alone (without nitrogen) reduced seedling growth compared to control seedlings. When nitrogen and silicon were applied together, the addition of silicon resulted in reduced seedling growth in YTH183 compared to seedlings treated with nitrogen alone. However, the same effect was not noted for IR64 seedlings, indicating clear variety-specific responses to seedbed inputs. Regardless of variety, silicon-treated seedlings that were transplanted to snail-infested pots at 21 days after seeding (DAS) had lower biomass than seedlings without silicon despite silicon-treated IR64 seedlings having less snail damage than untreated controls. From an experiment conducted in snail-infested ponds, we found no difference between snail damage to silicon (SiO2)-treated and control cv IR50 seedlings. Although we did not determine silicon levels in plant tissues, our results indicate that the effects of silicon soil amendments are largely insufficient to reduce the impact of apple snails to young rice seedlings (≤21 DAS).
Increased use of chemical fertilizers to support crop production has resulted in global soil, water, and air pollution. It is generally agreed that the only solution is to improve the exploitation of beneficial bacteria that play a key role in increasing the supply of inaccessible minerals to plants in uninterrupted soil conditions. Main attention is paid to biotechnologies that are not completely used as tools for the production of biofertilizers, such as microbial co-immobilization and co-cultivation. Biotechnological processing and combined use of active microorganisms/organic compounds (biostimulants) such as plant extracts and exudates, not only promotes plant growth and development but also plant-bacterial interactions. This chapter focuses on the improvement in the techniques for development and formulation of microbial inoculants as well as the most significant potential and innovative strategies in this area are discussed.
Silicon as a defence against herbivory in grasses has gained increasing recognition and has now been studied in a wide range of species, at scales from individual plants in pots to plant communities in the field. The impacts of these defences have been assessed on herbivores ranging from insects to rodents to ungulates. Here, we review current knowledge of silicon mediation of plant–herbivore interactions in an ecological context.
The production of silicon defences by grasses is affected by both abiotic and biotic factors and by their interactions. Climate, soil type and water availability all influence levels of silicon uptake, as does plant phenology and previous herbivory. The type of defoliation matters and artificial clipping does not appear to have the same impact on silicon defence induction as herbivory which includes the presence of saliva. Induction of silicon defences has been demonstrated to require a threshold level of damage, both in the laboratory and in the field. In recent studies of vole–plant interactions, the patterns of induction were found to be quantitatively similar in glasshouse compared with field experiments, in terms of both the threshold required for induction and timing of the induction response.
The impacts of silicon defences differ between different classes of herbivore, possibly reflecting differences in body size, feeding behaviour and digestive physiology. General patterns are hard to discern however, and a greater number of studies on wild mammalian herbivores are required to elucidate these, particularly with an inclusion of major groups for which there are currently no data, one such example being marsupials.
We highlight new research areas to address what still remains unclear about the role of silicon as a plant defence, particularly in relation to plant–herbivore interactions in the field, where the effects of grazing on defence induction are harder to measure. We discuss the obstacles inherent in scaling up laboratory work to landscape‐scale studies, the most ecologically relevant but most difficult to carry out, which is the next challenge in silicon ecology.
As acreage of row crops managed with conservation tillage increases, more growers are encountering slugs, elevating their importance as crop pests. Slugs can eat virtually all crops and they inflict most of their damage during crop establishment and early growth in the spring and fall. This damage tends to be most severe under cool, wet conditions, which slow crop growth and favor slug activity. These mollusks are particularly troublesome within the Chesapeake Bay watershed where conservation tillage is strongly encouraged to minimize agricultural run-off into waterways that lead to the Bay. Slugs are challenging to control because of the limited number of management tactics that are available. We consider the species of slugs that are commonly found in mid-Atlantic field crop production and discuss their natural history, ecology, and some of the factors limiting their populations. We conclude with cultural, biological, and chemical management options, particularly for corn production, and suggest elements of a potential integrated management program for slugs.
The electronic version of this article is the definitive one. It is located here: Abstract Slugs are an important crop and ornamental plant pest throughout temperate regions in all sectors of the growing industry. Their pest status is set to increase as environmental considerations such as using reduced tillage and over-winter green crops, along with legislative changes to pesticide usage and more extreme weather patterns favour population growth. Consequently, the need for sustainable approaches to slug control will become ever-more important. This review focuses on biocontrol strategies, including nematodes, sciomyzid flies and microbes. First, an overview of the problems caused by slugs in agriculture and horticulture is given, highlighting the key pest species of temperate climates. This is followed by a brief description of the existing approaches to their control and the current position with regard to classical biological control of slugs is discussed in detail. Finally, future research needs are summarized, along with the challenges involved. Review Methodology: The literature for this review was sought by searching the databases CAB Reviews, CAB Abstracts and ISI Web of Knowledge along with a number of key texts. The older literature was identified using reference lists in the articles and books identified as above.
Oxalic acid is synthesized by a wide range of plants. A few of them are forage plants that can cause oxalate poisoning in ruminants under certain conditions. In this paper, the role of some agronomic, climatic and genetic factors in minimizing oxalate accumulation in forage plants has been discussed. Research indicates that the content of oxalate in forage can be controlled by fertilizer application. For example, nitrate application resulted in higher contents of soluble and insoluble oxalates than ammonium application. With an increased rate of potassium application, soluble oxalate content showed an increasing trend and insoluble oxalate content showed a decreasing trend. With an increased rate of calcium application, soluble oxalate content showed a decreasing trend and insoluble oxalate content showed a reverse trend. Other agronomic factors such as growing season, harvesting practices, plant maturity, plant species, plant variety and plant parts can also have a large effect on oxalate accumulation. However, the potential benefits of the above approaches for improving forage quality have not been fully exploited. In addition, there is still insufficient information to fully utilize means (e.g. plant nutrients, season and soil moisture) to minimize oxalate accumulation in forage plants. Therefore, more research is required for a better understanding of the interactions between oxalate and the above-mentioned factors in forage plants.
Background and aims:
Despite the selective pressure slugs may exert on seedling recruitment there is a lack of information in this context within grassland restoration studies. Selective grazing is influenced by interspecific differences in acceptability. As part of a larger study of how slug-seedling interactions may influence upland hay meadow restoration, an assessment of relative acceptability is made for seedlings of meadow plants to the slug, Deroceras reticulatum.
Methods:
Slug feeding damage to seedling monocultures of 23 meadow species and Brassica napus was assessed in microcosms over 14 d. The severity and rate of damage incurred by each plant species was analysed with a generalized additive mixed model. Plant species were then ranked for their relative acceptability.
Key results:
Interspecific variation in relative acceptability suggested seedlings of meadow species form a hierarchy of acceptability to D. reticulatum. The four most acceptable species were Achillea millefolium and the grasses Holcus lanatus, Poa trivialis and Festuca rubra. Trifolium pratense was acceptable to D. reticulatum and was the second highest ranking forb species. The most unacceptable species were mainly forbs associated with the target grassland, and included Geranium sylvaticum, Rumex acetosa, Leontodon hispidus and the grass Anthoxanthum odoratum. A strong positive correlation was found for mean cumulative feeding damage and cumulative seedling mortality at day 14.
Conclusions:
Highly unacceptable species to D. reticulatum are unlikely to be selectively grazed by slugs during the seedling recruitment phase, and were predominantly target restoration species. Seedlings of highly acceptable species may be less likely to survive slug herbivory and contribute to seedling recruitment at restoration sites. Selective slug herbivory, influenced by acceptability, may influence community-level processes if seedling recruitment and establishment of key functional species, such as T. pratense is reduced.
Silicon (Si) is one of the most abundant elements in the earth's crust, although its essentiality in plant growth is not clearly established. However, the importance of Si as an element that is particularly beneficial for plants under a range of abiotic and biotic stresses is now beyond doubt. This paper reviews progress in exploring the benefits at two- and three-trophic levels and the underlying mechanism of Si in enhancing the resistance of host plants to herbivorous insects. Numerous studies have shown an enhanced resistance of plants to insect herbivores including folivores, borers, and phloem and xylem feeders. Silicon may act directly on insect herbivores leading to a reduction in insect performance and plant damage. Various indirect effects may also be caused, for example, by delaying herbivore establishment and thus an increased chance of exposure to natural enemies, adverse weather events or control measures that target exposed insects. A further indirect effect of Si may be to increase tolerance of plants to abiotic stresses, notably water stress, which can in turn lead to a reduction in insect numbers and plant damage. There are two mechanisms by which Si is likely to increase resistance to herbivore feeding. Increased physical resistance (constitutive), based on solid amorphous silica, has long been considered the major mechanism of Si-mediated defences of plants, although there is recent evidence for induced physical defence. Physical resistance involves reduced digestibility and/or increased hardness and abrasiveness of plant tissues because of silica deposition, mainly as opaline phytoliths, in various tissues, including epidermal silica cells. Further, there is now evidence that soluble Si is involved in induced chemical defences to insect herbivore attack through the enhanced production of defensive enzymes or possibly the enhanced release of plant volatiles. However, only two studies have tested for the effect of Si on an insect herbivore and third trophic level effects on the herbivore's predators and parasitoids. One study showed no effect of Si on natural enemies, but the methods used were not favourable for the detection of semiochemical-mediated effects. Work recently commenced in Australia is methodologically and conceptually more advanced and an effect of Si on the plants' ability to generate an induced response by acting at the third trophic level was observed. This paper provides the first overview of Si in insect herbivore resistance studies, and highlights novel, recent hypotheses and findings in this area of research. Finally, we make suggestions for future research efforts in the use of Si to enhance plant resistance to insect herbivores.
Herbivores can affect the composition of grassland communities. We examined seedling herbivory by the introduced snail, Helix aspersa, on four grass species common in California grasslands. To assess preference, standardized tests using equal amounts of seedlings of the same age were conducted in addition to tests in which live seedlings were exposed to snail herbivory. Helix aspersa exhibited the greatest preference for native Bromus carinatus, while native Nassella pulchra and exotic B. hordeaceus were moderately preferred. Exotic Avena fatua was least preferred. Assessment of preference using both standardized and live seedling trials yielded consistent results, suggesting that preference is due to variation in seedling quality rather than differences in per-seedling biomass that are likely to exist under field conditions. Avena fatua is very common in California grasslands while B. carinatus is relatively rare. Our results suggest that the high level of snail preference for B. carinatus and aversion to A. fatua, coupled with severe effects on future growth due to complete defoliation of live seedlings, may reinforce patterns of establishment of these grasses observed in natural communities.
The herbivore defence system of true grasses (Poaceae) is predominantly based on silicon that is taken up from the soil and deposited in the leaves in the form of abrasive phytoliths. Silicon uptake mechanisms can be both passive and active, with the latter suggesting that there is an energetic cost to silicon uptake. This study assessed the effects of plant-available soil silicon and herbivory on the competitive interactions between the grasses Poa annua, a species that has previously been reported to accumulate only small amounts of silicon, and Lolium perenne, a high silicon accumulator.
Plants were grown in mono- and mixed cultures under greenhouse conditions. Plant-available soil silicon levels were manipulated by adding silicon to the soil in the form of sodium silicate. Subsets of mixed culture pots were exposed to above-ground herbivory by desert locusts (Schistocerca gregaria).
In the absence of herbivory, silicon addition increased biomass of P. annua but decreased biomass of L. perenne. Silicon addition increased foliar silicon concentrations of both grass species >4-fold. Under low soil-silicon availability the herbivores removed more leaf biomass from L. perenne than from P. annua, whereas under high silicon availability the reverse was true. Consequently, herbivory shifted the competitive balance between the two grass species, with the outcome depending on the availability of soil silicon.
It is concluded that a complex interplay between herbivore abundance, growth-defence trade-offs and the availability of soil silicon in the grasses' local environment affects the outcome of inter-specific competition, and so has the potential to impact on plant community structure.
This 19-chapter book discusses the biology (including reproduction, life history, feeding preferences and sexual behaviour) of molluscs as pests of horticultural, field and fodder crops, and outlines the development of appropriate mechanisms for the control of these pests (mainly biological, cultural and chemical). Two chapters review progress towards the development of chemical control strategies, one addressing the toxicology of chemicals, the other the deployment of molluscicides in baits. These chapters also highlight the statistical and biological procedures for screening and evaluating molluscicides which are not a component of the standard procedure of mollusc control. A series of chapters focus on specific crop situations, providing a synopsis of the current pest status of gastropod species or species groups.
This book presents a synthesis of current knowledge and research on the biology of terrestrial gastropod molluscs, which are of importance to human societies as food, medicine, crop pests, vectors of parasites, and as tools, personal ornamentation and currency in trade. It covers the morphology, phylogeny and systematics, structure and function of the various organ systems, feeding behaviour, life history strategies, behavioural ecology, population and conservation genetics, and soil biology and ecotoxicology of the terrestrial molluscs.
A controlled environment experiment was performed on plants from 2 Agropyron sndthii Rydb. (western wheatgass) populations to determine how defollltion at iweek intervals and graxing bis-tory affected total silicon accumulation in shoots, and how Si was distributed within the plant. Plants were collected from a heavily grazed, IO-year-old prairie dog colony and an ungazed, 40-year-old exciosure at Wind Cave National Park, South Dakota. After 18 weeks, the total amount of Si accumuhtted in shoots was similar in plants from both populations, regardless of whether or not the plants were clipped. However, the Si concentration in shoots was greater in nondefoliated than defoliated phmts of both populations because of Si dilution resulting from greater shoot production in defoliated pIants. In both populations, roots and leaf blades had the highest Si concentrations, rhizomes had the lowest concentra-tions, and sheaths, crowns, and belowground stems had interme-diate concentrations.
Silicon (Si), although not considered essential, has beneficial effects on plant growth which are mostly associated with the ability to accumulate amorphous (phytogenic) Si, e.g., as phytoliths. Phytogenic Si is the most active Si pool in the soil–plant system because of its great surface-to-volume ratio, amorphous structure, and high water solubility. Despite the high abundance of Si in terrestrial biogeosystems and its importance, e.g., for the global C cycle, little is known about Si fluxes between soil and plants and Si pools used by plants. This study aims at elucidating the contribution of various soil Si pools to Si uptake by wheat. As pH affects dissolution of Si pools and Si uptake by plants, the effect of pH (4.5 and 7) was evaluated. Wheat was grown on Si-free pellets mixed with one of the following Si pools: quartz sand (crystalline), anorthite powder (crys-talline), or silica gel (amorphous). Silicon content was measured in aboveground biomass, roots, and soil solution 4 times in intervals of 7 d. At pH 4.5, plants grew best on anorthite, but pH did not significantly affect Si-uptake rates. Total Si contents in plant biomass were significantly high-er in the silica-gel treatment compared to all other treatments, with up to 26 mg g –1 in above-ground biomass and up to 17 mg g –1 in roots. Thus, Si uptake depends on the conversion of Si into plant-available silicic acid. This conversion occurs too slowly for crystalline Si phases, there-fore Si uptake from treatments with quartz sand and anorthite did not differ from the control. For plants grown on silica gel, real Si-uptake rates were higher than the theoretical value calculated based on water transpiration. This implies that Si uptake by wheat is driven not only by passive water flux but also by active transporters, depending on Si concentration in the aqueous phase, thus on type of Si pool. These results show that Si uptake by plants as well as plant growth are significantly affected by the type of Si pool and factors controlling its solubility.
Since the beginning of the nineteenth century, silicon (Si) has been found in significant concentrations in plants. Despite
the abundant literature which demonstrates its benefits in agriculture, Si is generally not considered as an essential element.
The integration of Si in agricultural practices is, however, effective in a few countries. Silicon fertilization by natural
silicates has the potential to mitigate environmental stresses and soil nutrient depletion and as a consequence is an alternative
to the extensive use of phytosanitary and NPK fertilizers for maintaining sustainable agriculture. This review focuses on
recent advances on the mechanisms of Si accumulation in plants and its behavior in soil. Seven among the ten most important
crops are considered to be Si accumulators, with concentration of Si above 1% dry weight. New approaches using isotopes and
genetics have highlighted the mechanisms of uptake and transfer of Si in planta. There is a general agreement on an uptake of dissolved silica as H4SiO4 and precipitation as amorphous silica particles (the so-called phytoliths), but the mechanism, either active or passive,
is still a matter of debate. The benefits of Si are well demonstrated when plants are exposed to abiotic and biotic stresses.
The defense mechanisms provided by Si are far from being understood, but evidences for ex planta and in planta processes are given indicating multiple combined effects rather than one single effect. Phytoliths that are located mainly
in shoots of monocots return to the soil through litterfall if the plants are not harvested and contribute to the biogeochemical
cycle of Si. According to recent progress made on the understanding of the biogeochemical cycle of Si and the weathering process
of silicate minerals, phytoliths may significantly contribute to the resupply of Si to plants. We suggest that straw of crops,
which contains large amounts of phytoliths, should be recycled in order to limit the depletion of soil bioavailable Si.
Silica (Si) is of great concern to agronomists because it has a beneficial effect on plant resistance to various stresses, enabling yield optimization in economically important crop species. Yet biogenic silica (BSi) cycling in soils controls a large part of the Si export fluxes to rivers and oceans. Despite the importance of agricultural-harvest-related Si removal, previous studies have not addressed this topic thoroughly. By performing a detailed quantification of agricultural Si export in Western Europe's Scheldt River basin, we show that harvest not only disrupts BSi cycling but also introduces an agricultural Si pathway, with major export Si fluxes as compared with BSi production in climax forest communities and grasslands. Harvesting substantially changes terrestrial Si cycling because reconstitution of BSi to soils in litter fall is prevented. The agricultural Si loop clearly constitutes an important flow of BSi out of terrestrial ecosystems one that is currently unrecognized in global biogeochemical Si cycling.
A serious omission in ecological methodology
is the absence of a rigorous statistical procedure to analyse
multiple-choice feeding-preference experiments. A
sample of 21 studies in the littoral marine context shows
that results from such experiments are used to study
a variety of conceptual issues, ranging from nutritional
biology to ecosystem dynamics. A majority of such studies
have been incorrectly analysed. The analytical problem
has two facets: (1) lack of independence in the simultaneous
offer of food types and (2) the existence of autogenic
changes particular to each food type. Problem
(2) requires the use of control arenas without the consumer.
A recent advance allows the rigorous analysis
of experiments with two food types offered simultaneously.
Here I propose a method for the multiple-choice case.
For the first problem I suggest the use of multivariate
statistical analysis, providing both a parametric and a
nonparametric procedure. The second problem is solved
using basic statistical theory. I analyse data from an experiment
with the sea urchin Tetrapygus niger feeding
on three species of algae: Ulva nematoidea, Gymnogongrus
furcellatus, and Macrocystis pyrifera. The parametric
and nonparametric procedures yielded similar results,
and showed that when offered the three species of algae
T. niger does not feed at random but shows a preference
for U. nematoidea. The method requires that the number
of replicates in the treatment and control arenas be the
same, and greater than the number of food types. The
method is useful for other kinds of multiple-choice experiments.
Plant structural traits often act as defenses against herbivorous insects, causing them to avoid feeding on a given plant or tissue. Mineral crystals of calcium oxalate in Medicago truncatula Gaertn. (Fabaceae) leaves have previously been shown to be effective deterrents of lepidopteran insect feeding. They are also inhibitors of conversion of plant material into insect body mass during or after consumption. Growth of beet armyworm, Spodoptera exigua Hübner (Lepidoptera: Noctuidae), larvae was correspondingly greater on calcium oxalate-defective (cod) mutants of M. truncatula with lower levels of crystal accumulation. Data presented here show that insects feeding on M. truncatula leaves with calcium oxalate crystals experience greater negative effects on growth and mandible wear than those feeding on artificial diet amended with smaller amorphous crystals from commercial preparations. Commercial calcium oxalate can be added to insect artificial diet at levels up to 7.5-fold higher than levels found in wild-type M. truncatula leaves with minimal effect on insect growth or lepidopteran mandibles. These data suggest that negative impacts of calcium oxalate in the diet of larvae are due to physical factors, and not toxicity of the compound, as high levels of the commercial crystals are readily tolerated. In contrast to the dramatic physical effects that M. truncatula-derived crystals have on insect mandibles, we could detect no damage to insect peritrophic gut membranes due to consumption of these crystals. Taken together, the data indicate that the size and shape of prismatic M. truncatula oxalate crystals are important factors in determining effects on insect growth. If manipulation of calcium oxalate is to be used in developing improved insect resistance in plants, then our findings suggest that controlling not only the overall amount, but also the size and shape of crystals, could be valuable traits in selecting desirable plant lines.
The title of this essay declares that silicon does have roles in plants and all participants in this conference know that that is so. This knowledge, however, is not shared by the general community of plant biologists, who largely ignore the element. This baffling contrast is based on two sets of experience. First, higher plants can grow to maturity in nutrient solutions formulated without silicon. That has led to the conventional wisdom that silicon is not an essential element, or nutrient, and thus can be disregarded. Second, the world's plants do not grow in the benign environment of solution culture in plant biological research establishments. They grow in the field, under conditions that are often anything but benign. It is there, in the real world with its manifold stressful features, that the silicon status of plants can make a huge difference in their performance. The stresses that silicon alleviates range all the way from biotic, including diseases and pests, to abiotic such as gravity and metal toxicities. Silicon performs its functions in two ways: by the polymerization of silicic acid leading to the formation of solid amorphous, hydrated silica, and by being instrumental in the formation of organic defence compounds through alteration of gene expression. The silicon nutrition of plants is not only scientifically intriguing but also important in a world where more food will have to be wrung from a finite area of land, for that will put crops under stress.
A stopping rule for an experiment defines when (under what conditions) the experiment is terminated. I investigated the stopping
rules used in numerous multiple–choice feeding-preference experiments and also examined a recently proposed method for analyzing
the data arising from such experiments. All of the surveyed experiments imposed stopping rules which result in a random total
food consumption. If an acceptable quantification of preference is relative consumption of different food types, then the
proposed analysis will likely misstate the information about preference conveyed by the data. This is due to the fact that
the method may confound differences in preferences among food types with differences in the total consumption across trials.
I discuss this issue in detail and present an alternative procedure which is appropriate under all stopping regimes when preference
is quantified through relative consumption. The procedure I suggest uses an index which is a multivariate generalization of
the preference index suggested by Kogan and Goeden (Ann Entomol Soc 1970; 63: 1175–1180) and Kogan (Ann Entomol Soc 1972;
65: 675–683) and which is analogous to a selection index for discrete food units proposed by Manly (Biometrics 1974; 30: 281–294).
The importance of invertebrate herbivores in regulating plant communities remains unclear, due to the absence of long-term exclusion experiments. An experiment in an English grassland involving long-term exclusions of insect and mollusc herbivores, along with rabbit fencing, showed strong, but opposing, effects of the invertebrate herbivores. Plant species richness declined and biomass increased following insect exclusion, due to increased dominance by a grass species, whereas mollusc exclusion led to increased herbs abundance. The two herbivores had a compensatory interaction: molluscs had no effects in the absence of insects and large insect effects depended on the absence of molluscs. The effects of invertebrate exclusion became apparent only after 8 years, and would have been seriously underestimated in shorter studies. Our results suggest that theorists and conservation managers need to shift from their historic focus on vertebrate herbivory, to a recognition that invertebrates can be equally important drivers of plant community structure.
1. Silica in the leaves of grasses can act as a defence against both vertebrate and invertebrate herbivores. The mechanisms by which silica affects herbivore performance are not well characterized. Here we expose an insect herbivore Spodoptera exempta to high-silica diets and test two mechanisms by which silica has been proposed to act as a defence. First, that silica reduces the digestibility of leaves and second, that silica causes wear to insect mandibles, both of which could potentially impact on herbivore performance. 2. Silica reduced the efficiency with which S. exempta converted ingested food to body mass and the amount of nitrogen absorbed from their food, leading to reduced insect growth rates. The measure of how efficiently herbivores utilize digested food (ECD) was unaffected by silica. 3. These effects occurred even with short-term exposure to silica-rich diets, but they also increased markedly with the duration of exposure and affected late instars more than early instar larvae. This appears to be due to the progressive impacts of silica with longer exposure times and suggests that herbivores cannot adapt to silica defences, nor do they develop a tolerance for silica with age. 4. Exposure to silica-rich diets caused increased mandible wear in S. exempta. This effect was extremely rapid, occurring within a single instar, further reducing feeding efficiency and growth rates. These effects on insect growth and feeding efficiency are nonreversible, persisting after the herbivore has switched diets. Up to a third of this residual impact can be explained by the degree of mandible wear caused by previous silica-rich diets. 5. The impacts of silica on S. exempta larvae were progressive with exposure time and could not be compensated for, even by switching to a different diet. Thus, herbivores cannot easily adapt to physical defences such as silica, suggesting this defence will have major implications for herbivore fitness.
Calcium oxalate (CaOx) crystals are distributed among all taxonomic levels of photosynthetic organisms from small algae to angiosperms and giant gymnosperms. Accumulation of crystals by these organisms can be substantial. Major functions of CaOx crystal formation in plants include high-capacity calcium (Ca) regulation and protection against herbivory. Ultrastructural and developmental analyses have demonstrated that this biomineralization process is not a simple random physical-chemical precipitation of endogenously synthesized oxalic acid and environmentally derived Ca. Instead, crystals are formed in specific shapes and sizes. Genetic regulation of CaOx formation is indicated by constancy of crystal morphology within species, cell specialization, and the remarkable coordination of crystal growth and cell expansion. Using a variety of approaches, researchers have begun to unravel the exquisite control mechanisms exerted by cells specialized for CaOx formation that include the machinery for uptake and accumulation of Ca, oxalic acid biosynthetic pathways, and regulation of crystal growth.
Systemic application of sodium silicate can significantly enhance the levels of leaf silica in winter wheat (Triticum aestivum L. cv. Mercia), suggesting that this material could reduce the palatability of plants to vertebrate herbivores (e.g. rabbits, Oryctolagus cuniculus L.). A bioassay was developed using hydroponically grown wheat plants. Plants treated with sodium silicate were significantly more resistant to grazing by wild rabbits than untreated plants, with severe, potentially lethal feeding damage being reduced by over 50%. Further studies were carried out to develop more practical techniques for boosting silica levels in plants using silicon-rich 'fertilisers' including calcium silicate and calcium silicate slag (CSS). Silica levels were elevated in the plant 1.9-2.8 times over the control through the application of various silicon materials, in line with those of the hydroponic treatment. Encouragingly, levels of silica were elevated even in young wheat plants, which are most vulnerable to rabbit damage, and in a range of wheat varieties. The use of CSS is particularly promising because of its lower cost in comparison with calcium silicate, and it has a proven track record in slag fertilisation of rice and sugar cane crops. At the optimum CSS application rate of 3 g silicon L(-1) soil, wheat silica levels were approximately doubled, with no detrimental impacts on long-term growth or yield.
the solution was transferred to 100 ml polypropylene volumetric flasks and brought to volume using deionised water. Si analysis: colourimetry The amount of Si present in the solution was mea-sured using the Heteropoly Blue method
- Hcl
- Finally
HCl. Finally, the solution was transferred to 100 ml polypropylene volumetric flasks and brought to volume using deionised water. Si analysis: colourimetry The amount of Si present in the solution was mea-sured using the Heteropoly Blue method (Eaton et al. 1995).
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