Oviposition and feeding avoidance in Helicoverpa armigera (Hübner) against transgenic Bt cotton

Helicoverpa armigera (Hübner), the major target pest of transgenic Bacillus thuringiensis (Bt) cotton, remains susceptible to Bt cotton in China at present. Behavioural avoidance by ovipositing females might lead to reduced exposure to Bt cotton and minimize selection for physiological resistance. We examined the behavioural responses of H. armigera to Bt and non‐Bt cottons to determine whether behavioural avoidance to Bt cotton may be present. In oviposition choice tests, the number of eggs on non‐Bt cotton plants was significantly higher than on Bt cotton plants. Similarly, in no‐choice tests, Bt cotton plants attracted significantly fewer eggs compared with non‐Bt cotton plants. H. armigera neonates showed higher dispersal and lower establishment on Bt cotton than on non‐Bt cotton. First instars were found to feed consistently on non‐Bt cotton leaves, creating large feeding holes, but only produced tiny feeding holes on Bt cotton leaves. The H. armigera population used in this study showed avoidance of oviposition and feeding on Bt cotton. Our results provide important insights into one possible mechanism underlying the durability of Bt cotton resistance and may be useful for improving strategies to sustain the effectiveness of Bt crops.     

Survival of Helicoverpa armigera larvae on and Bt toxin expression in various parts of transgenic Bt cotton (Bollgard II) plants

There is no conclusive evidence that Helicoverpa spp. (Lepidoptera: Noctuidae) in Australia have evolved significant levels of resistance to Bollgard II^® cotton (which expresses two Bt toxin genes, cry1Ac and cry2Ab). However, there is evidence of surviving larvae on Bollgard II cotton in the field. The distribution and survival of early‐instar Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) larvae were examined on whole Bollgard II and non‐Bt cotton plants in greenhouse bioassays. The expression of Cry toxins in various parts of Bollgard II plants was compared to the survival of larvae in those locations. Only 1% of larvae survived after 6 days on greenhouse‐grown Bollgard II plants compared to 31% on non‐Bt cotton plants. Overall, and across all time intervals, more larvae survived on reproductive parts (squares, flowers, and bolls) than on vegetative parts (leaves, stems, and petioles) on Bollgard II plants. The concentration of Cry1Ac toxin did not differ between plant structures, whereas Cry2Ab toxin differed significantly, but there was no relationship between the level of expression and the location of larvae. This study provides no evidence that lower expression of Cry toxins in the reproductive parts of plants explains the survival of H. armigera larvae on Bollgard II cotton.     

Next‐generation transgenic cotton: pyramiding RNAi and Bt counters insect resistance

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 limit 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.     

Feeding behaviour of Helicoverpa armigera larvae on insect-resistant transgenic cotton and non-transgenic cotton

Abstract: Feeding behaviour of Helicoverpa armigera Hübner (Lep.; Noctuidae) larvae on non‐transgenic Bacillus thuringiensis (Bt) cotton (Gossypium hirsutum L.), Zhong 30, and transgenic cowpea trypsin inhibitor (CpTI)‐Bt cotton, SGK 321, and non‐transgenic cotton, Shiyuan 321, was investigated in both choice tests and no‐choice tests. The results of choice tests suggested that neonates have the ability to detect and avoid transgenic cotton. In the choice tests of neonates with both transgenic and non‐transgenic cotton leaves, a significantly greater proportion of larvae and higher consumption were observed on non‐transgenic cotton than on the transgenic Bt or CpTI‐Bt cotton. In the choice tests with leaves of two transgenic cotton lines, the proportion of neonates on leaf discs of the two lines was not significantly different, but there was significantly higher consumption on CpTI‐Bt transgenic cotton than that on Bt transgenic cotton. In addition, significantly more neonates were found away from the leaf discs, lower consumption and higher mortality were achieved in the choice test with two transgenic cotton leaves than in the choice tests containing non‐transgenic cotton leaves. Leaves and buds were examined in choice tests of fourth instars. It appeared that fourth instars were found in equal numbers on transgenic and non‐transgenic cotton, except when larvae were exposed to leaves for 3 h. However, the total consumption on transgenic cotton was lower than that of the non‐transgenic cotton, so fourth instars may still have the capacity to detect transgenic cotton and reduce feeding on it, although they showed no preference on either transgenic or non‐transgenic cotton. More larvae were found off diet in the treatments with leaves than that of buds, and the number of injured leaf discs by per fourth instar was significantly higher than that of buds in choice tests, suggesting that leaf is a less preferred organ for H. armigera larvae, elicited more larval movements. Similarly, in no‐choice tests of fifth instars, significantly fewer feeding time and more moving time occurred on leaf than that of bud, boll and petal. When cotton line was considered, compared with non‐transgenic cotton, significantly lower feeding time and higher resting time occurred on the two transgenic cottons. Overall, H. armigera larvae have the ability to detect the transgenic Bt and CpTI‐Bt cottons or the less preferred organs and selectively feed more on the non‐transgenic cotton or the preferred organs, especially the neonates, which have a high capacity for avoiding transgenic cotton.     

Predation of cotton bollworm by green lacewings in the presence of cotton aphid as alternative prey on transgenic Bt and conventional cotton

Experiments were conducted in small arenas and on whole plants to explore the effect of cotton aphids, Aphis gossypii Glover (Hemiptera: Aphididae), as alternative prey on the predation of Helicoverpa armigera Hübner (Lepidoptera: Noctuidae) larvae by green lacewing larvae, Mallada signatus Schneider (Neuroptera: Chrysopidae). Transgenic Bt (Bollgard II^®) and conventional cotton plants were included to explore potential differences in the predator's performance on these cotton types. In small arenas, the presence of 20 aphids reduced predation on H. armigera larvae by 22% (from 5.5 to 3.3 of 10) by a single lacewing larva over a 24‐h period. The presence of H. armigera reduced predation on aphids by ca. 29% (from 16.8 to 11.0 of 20) over 24 h. On whole plants, the presence of alternative prey had no effect on the number of H. armigera larvae or aphids remaining after 3 days. The presence of H. armigera larvae alone, without the predator, caused a 24% reduction in the numbers of aphids on conventional, but not on Bt cotton plants. The combination of Bt cotton and lacewing larvae caused a 96.6% removal of early‐stage H. armigera larvae, a statistically significant increase over the addition of the proportions (91.6%) removed by each factor measured separately, providing evidence of synergism. These studies suggest that the presence of aphids as alternative prey would not necessarily disrupt the predation by green lacewing on larvae of H. armigera, especially on Bt cotton.     

Consumptive and non‐consumptive effects of wolf spiders on cotton bollworms

Larvae of the cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) that survive on genetically modified Bt cotton (Gossypium hirsutum L., Malvaceae) contribute to the risk of widespread resistance to Bt toxins. Current resistance management techniques include pupae busting, which involves deep tilling of the soil to kill overwintering pupae. Unfortunately, pupae busting runs counter to soil and water conserving techniques, such as minimum tillage. This problem could be relieved with biological control methods, whereby predators attack either larvae going to ground to pupate or moths emerging from the ground. We found that the wolf spider Tasmanicosa leuckartii (Thorell) (Araneae: Lycosidae), a common inhabitant of Australian cotton agroecosystems, is an effective predator of H. armigera, attacking and killing most larvae (66%) and emerging moths (77%) in simple laboratory arenas. Tasmanicosa leuckartii also reduced the number of emerging moths by 66% on average in more structurally complex glasshouse arenas. Males, females, and late‐instar juveniles of T. leuckartii were similarly effective. Tasmanicosa leuckartii also imposed non‐consumptive effects on H. armigera, as when a spider was present larvae in the laboratory areas spent less time on the cotton boll and more time on the soil and more mass was lost from the cotton boll. Increased loss of boll mass likely reflects changes in H. armigera foraging behavior induced by the presence of spiders (indirect non‐consumptive effects). Helicoverpa armigera spent more time as pupae when the spider was present in simple laboratory arenas, but not in more complex glasshouse enclosures. Overall, results indicate that T. leuckartii spiders can be effective predators of H. armigera late instars and moths but also suggest that, under some conditions, the presence of spiders could increase the damage to individual cotton bolls.     

Comparing the predatory performance of green lacewing on cotton bollworm on conventional and Bt cotton

We compared the survival of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) eggs and larvae on Bt and conventional cotton, in the presence or absence of the generalist predator, green lacewing larvae, Mallada signatus, (Schneider) (Neuroptera: Chrysopidae). In small arenas, green lacewings consumed a similar number of H. armigera eggs (ave. 15.8 ± 1.3 on conventional, 12.6 ± 1.4 on Bt cotton per predator over 24 h) and larvae (ave. 6.8 ± 0.7 conventional, 6.5 ± 0.8 Bt per predator over 24 h) whether on Bt or conventional cotton leaves. Likewise, similar numbers of eggs were consumed by each lacewing larva searching whole plants of either Bt (ave. 15.5 ± 0.6 of 49 over 24 h) or conventional (ave. 13.6 ± 1.1 of 49 over 24 h). On conventional plants over 72 h, survival of H. armigera larvae was 72.8% and decreased to 37.7% when lacewings were present, giving a net consumption rate of 35.1% (8.6 prey per predator over 72 h). On Bt cotton plants, 13.6% of the H. armigera larvae survived after 72 h and this decreased to 1.7% when lacewings were present. This combination of mortality factors operated synergistically. Helicoverpa armigera larvae moved to fruiting structures on conventional or Bt cotton but failed to survive in the squares (young flower buds) when the impacts of Bt and lacewings were combined. The removal of first to second instar H. armigera larvae from squares of Bt cotton by predators has the potential to reduce immediate pest damage and, perhaps more importantly, remove potentially Bt‐resistant genotypes.     

Fitness of Bt‐resistant cabbage loopers on Bt cotton plants

Development of resistance to the insecticidal toxins from Bacillus thuringiensis (Bt) in insects is the major threat to the continued success of transgenic Bt crops in agriculture. The fitness of Bt‐resistant insects on Bt and non‐Bt plants is a key parameter that determines the development of Bt resistance in insect populations. In this study, a comprehensive analysis of the fitness of Bt‐resistant Trichoplusia ni strains on Bt cotton leaves was conducted. The Bt‐resistant T. ni strains carried two genetically independent mechanisms of resistance to Bt toxins Cry1Ac and Cry2Ab. The effects of the two resistance mechanisms, individually and in combination, on the fitness of the T. ni strains on conventional non‐Bt cotton and on transgenic Bt cotton leaves expressing a single‐toxin Cry1Ac (Bollgard I) or two Bt toxins Cry1Ac and Cry2Ab (Bollgard II) were examined. The presence of Bt toxins in plants reduced the fitness of resistant insects, indicated by decreased net reproductive rate (R₀) and intrinsic rate of increase (r). The reduction in fitness in resistant T. ni on Bollgard II leaves was greater than that on Bollgard I leaves. A 12.4‐day asynchrony of adult emergence between the susceptible T. ni grown on non‐Bt cotton leaves and the dual‐toxin‐resistant T. ni on Bollgard II leaves was observed. Therefore, multitoxin Bt plants not only reduce the probability for T. ni to develop resistance but also strongly reduce the fitness of resistant insects feeding on the plants.     

Development of a novel‐type transgenic cotton plant for control of cotton bollworm

The transgenic Bt cotton plant has been widely planted throughout the world for the control of cotton budworm Helicoverpa armigera (Hubner). However, a shift towards insect tolerance of Bt cotton is now apparent. In this study, the gene encoding neuropeptide F (NPF) was cloned from cotton budworm H. armigera, an important agricultural pest. The npf gene produces two splicing mRNA variants—npf1 and npf2 (with a 120‐bp segment inserted into the npf1 sequence). These are predicted to form the mature NPF1 and NPF2 peptides, and they were found to regulate feeding behaviour. Knock down of larval npf with dsNPF in vitro resulted in decreases of food consumption and body weight, and dsNPF also caused a decrease of glycogen and an increase of trehalose. Moreover, we produced transgenic tobacco plants transiently expressing dsNPF and transgenic cotton plants with stably expressed dsNPF. Results showed that H. armigera larvae fed on these transgenic plants or leaves had lower food consumption, body size and body weight compared to controls. These results indicate that NPF is important in the control of feeding of H. armigera and valuable for production of potential transgenic cotton.     

Feeding of fall armyworm,Spodoptera frugiperda,on Bt transgenic cotton and its isoline

Studies on insect food intake and utilization are important for determining the degree of insect/plant association and host species’ resistance, and also for helping design pest management programs by providing estimates of potential economic losses, techniques for mass breeding of insects, and identifying physiological differences between species. We studied the feeding and development of fall armyworm, Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae), on transgenic (Bt) and non‐transgenic (non‐Bt) cotton. The larvae of S. frugiperda fed on Bt cotton had a longer development period (23.0 days) than those fed on non‐Bt cotton (20.2 days). Survivorship of S. frugiperda larvae fed on Bt cotton (74.1%) was lower than that of larvae fed on non‐Bt cotton (96.7%). Pupal weight of larvae fed on Bt cotton (0.042 g) was lower than that of larvae fed on non‐Bt cotton (0.061 g). The cotton cultivar significantly affected food intake, feces production, metabolization, and food assimilation by S. frugiperda larvae. However, it did not affect their weight gain. Intake of Bt‐cotton leaf (0.53 g dry weight) per S. frugiperda larva was lower than the intake of non‐Bt‐cotton leaf (0.61 g dry weight). Larvae fed on Bt‐cotton leaves produced less feces (0.25 g dry weight) than those fed on non‐Bt‐cotton leaves (0.37 g dry weight). Weight gain per S. frugiperda larva fed on Bt‐cotton leaves (0.058 g dry weight) was similar to the weight gain for larvae fed on non‐Bt‐cotton leaves (0.056 g dry weight). The cotton cultivar significantly affected the relative growth, consumption, and metabolic rates, as well as other nutritional indices: the figures were lower for larvae fed on Bt‐cotton leaves than for larvae fed on non‐transgenic cotton leaves.     

Effects of elevated CO2 and transgenic Bt cotton on plant chemistry, performance, and feeding of an insect herbivore, the cotton bollworm

Effects of elevated atmospheric CO₂ (double‐ambient CO₂) on the growth and metabolism of cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), fed on transgenic Bacillus thuringiensis (Berliner) (Bt) cotton [Cry1A(c)], grown in open‐top chambers, were studied. Two levels of CO₂ (ambient and double‐ambient) and two cotton cultivars (non‐transgenic Simian‐3 and transgenic GK‐12) were deployed in a completely randomized design with four treatment combinations, and the cotton bollworm was reared on each treatment simultaneously. Plants of both cotton cultivars had lower nitrogen and higher total non‐structural carbohydrates (TNC), TNC:Nitrogen ratio, condensed tannin, and gossypol under elevated CO₂. Elevated CO₂ further resulted in a significant decrease in Bt toxin level in GK‐12. The changes in chemical components in the host plants due to increased CO₂ significantly affected the growth parameters of H. armigera. Both transgenic Bt cotton and elevated CO₂ resulted in a reduced body mass, lower fecundity, decreased relative growth rate (RGR), and decreased mean relative growth rate in the bollworms. Larval life‐span was significantly longer for H. armigera fed transgenic Bt cotton. Significantly reduced larval, pupal, and adult moth weights were observed in the bollworms fed elevated CO₂‐grown transgenic Bt cotton compared with those of bollworms reared on non‐transgenic cotton, regardless of the CO₂ level. The efficiency of conversion of ingested food and of digested food of the bollworm were significantly reduced when fed transgenic Bt cotton, but there was no significant CO₂ or CO₂× cotton cultivar interaction. Approximate digestibility of larvae reared on transgenic cotton grown in elevated CO₂ was higher compared to that of larvae fed non‐transgenic cotton grown at ambient CO₂. The damage inflicted by cotton bollworm on cotton, regardless of the presence or absence of insecticidal genes, is predicted to be more serious under elevated CO₂ conditions because of individual compensatory feeding on host plants caused by nitrogen deficiency.     

Preference of Bt‐resistant and susceptible Busseola fusca moths and larvae for Bt and non‐Bt maize

The sustainability of genetically engineered insecticidal Bacillus thuringiensis Berliner (Bt) maize, Zea mays L. (Poaceae), is threatened by the evolution of resistance by target pest species. Several Lepidoptera species have evolved resistance to Cry proteins expressed by Bt maize over the last decade, including the African maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae). The insect resistance management (IRM) strategy (i.e., the high‐dose/refuge strategy) deployed to delay resistance evolution is grounded on certain assumptions about the biology and ecology of a pest species, for example, the interactions between the insect pest and crop plants. Should these assumptions be violated, the evolution of resistance within pest populations will be rapid. This study evaluated the assumption that B. fusca adults and larvae select and colonize maize plants at random, and do not show any preference for either Bt or non‐Bt maize. Gravid female B. fusca moths of a resistant and susceptible population were subjected to two‐choice oviposition preference tests using stems of Bt and non‐Bt maize plants. Both the number of egg batches as well as the total number of eggs laid on each stem were recorded. The feeding preference of Bt‐resistant and susceptible neonate B. fusca larvae were evaluated in choice test bioassays with whorl leaf samples of specific maize cultivars. Although no differential oviposition preference was observed for either resistant or susceptible female moths, leaf damage ratings indicated that neonate larvae were able to detect Bt toxins and that they displayed feeding avoidance behaviour on Bt maize leaf samples.     

Effect of water-deficit stress on cotton plants expressing the Bacillus thuringiensis toxin

Bacillus thuringiensis (Bt) crops require a high dosage of Bt toxin to delay development of insect resistance, in particular, when the refuge strategy is applied. This strategy is threatened by plant developmental and environmental factors that might reduce Bt toxin concentration and Bt efficacy in Bt crops. Growth of Bt (Cry1Ac) cotton under prolonged, moderate water deficit as a single stress factor was evaluated. Bt cotton plants were analysed for physiological performance, Bt toxin concentration and Bt efficacy. For performance analysis, leaf and total plant dry weight and leaf area were measured. Bt toxin concentration was determined by an immuno‐assay. Effects of Bt toxin on growth and mortality of African cotton bollworm, Helicoverpa armigera, larvae were measured in different plant organs. Leaves from young plants exposed for 30 days to moderate water deficit had both higher Bt toxin concentrations and were more effective against larvae than leaves, flowers or bolls from mature flowering plants exposed to 60 days of moderate water deficit. Although growth of Bt cotton plants under moderate water‐deficit conditions decreased Bt concentrations in leaves, flowers and bolls, this had no effect on efficacy against first‐instar cotton bollworm larvae. No significant evidence was found that moderate water deficit, as a single stress factor, decreases Bt efficacy in Bt cotton.     

Expression of Cry1Aa in cassava improves its insect resistance against Helicoverpa armigera

Lepidopteran insects affect cassava production globally, especially in intercropping system. The expression of Cry toxins in transgenic crops has contributed to an efficient control of insect pests, leading to a significant reduction in chemical insecticide usage. Helicoverpa armigera is a Lepidopteran pest that feeds on a wide range of plants like cotton and cassava. In the present study, transgenic cassava plants over-expressing Cry1Aa, which we named as Bt cassava, were developed and used to evaluate its efficacy against H. armigera as a model. Insect feeding assays were carried out to test the effects of Bt cassava leaves on the development and survival of H. armigera. Significant reduction (P < 0.05) in the survival and weight were detected on larvae fed with Bt cassava leaves in comparison with those fed with wild-type cassava leaves. The higher expression of Cry1Aa in transgenic cassava caused the lethal effect in larvae, in contrast to the normal growth and development of adults and pupation observed when fed with wild-type leaves. Morphological observation on the larval midguts showed that the consumption of Bt cassava affected the gut integrity of H. armigera. The columnar cells of the midgut epithelium were dramatically damaged and showed loose or disordered structure. Their cytoplasms become highly vacuolated and contained disorganized microvilli. Our study demonstrated that the transgenic cassava expressing the Cry1Aa is effective in controlling H. armigera. Our Bt transgenic cassava plant would provide a long-term beneficial effect on all crops in intercropping system, which in-turn, will be profitable to the farmers.     

Determining the major Bt refuge crops for cotton bollworm in North China

Evaluation of the effectiveness of refuge strategies involved in cotton bollworm Bt resistance management would be aided by technologies that allow monitoring and quantification of key factors that affect the process under field conditions. We hypothesized that characterization of stable carbon and nitrogen isotopes in adult bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) moths may aid in determining the larval host that they developed upon. We found moths reared from larvae fed on peanut, soybean or cotton, respectively, could be differentiated using isotopic analyses that also corresponded to their respective larval host origins. These techniques were then used to classify feral second‐generation bollworm moths caught in Bt cotton (Gossypium hirsutum) fields into different populations based on their isotopic signatures. In 2006–2007 feral moths captured in Bt cotton fields predominantly correlated with the peanut (Arachis hypogea) having served as their larval host, indicating this is the most important refuge crop for Bt‐susceptible bollworm individuals (providing 58%?64% individuals) during independent moth peaks for the second generation in North China. The remaining feral moths correlated with soybean (Glycine max) (0?10%); other C₃ plant (20%?22%) and non‐C₃ plant (12%?14%) host types also provided some Bt‐sensitive moths. Field observations showed that peanut constitutes the primary refuge crop contributing to sustaining Bt‐susceptible moths dispersing into cotton in North China. These results suggest that peanut may be a more effective refuge to sustain Bt‐susceptible bollworm individuals and reduce the risk of development of a Bt‐resistant biotype.     

Behavioral Response of <Emphasis Type="Italic">Helicoverpa armigera</Emphasis> (Lepidoptera: Noctuidae) to Cotton with and Without Expression of the CrylAc δ-Endotoxin Protein of <Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> Berliner

Behavioral responses of larvae and adults of cotton bollworm, Helicoverpa armigera to cotton with (GK-12) and without (SI-3) expression of the CrylAc -endotoxin protein of Bacillus thuringiensis (Bt) Berliner were observed during 2001 and 2002. Our results showed that 8.3% individuals fed with flowers and bolls of GK-12 could develop from neonate to pupa; however, pupal weight decreased by 48.6% and duration of development was delayed by 7.6 days compared with those fed with flowers–bolls of SI-3. Deterrence index (DI) of larvae decreased in later instars, which indicated that the Bt toxin decreased with age. Feeding frequency of 4th-instar larvae on GK-12 leaves decreased by 38.8%, but movement frequency increased by 37.1%. Larvae moved at least one plant away by the age of 10 days in both pure and mixed plantings of SI-3 and GK-12 in the field. Adults preferred to lay eggs on SI-3. The total number of eggs deposited on SI-3 plants in 3 days were about 232 and 95% greater than that on GK-12 plants at bud–flower stage and flower–boll stage, respectively. Based on the behavior of larva and adults in response to the transgenic cotton, the potential effect of refuge strategy in resistance management of H. armigera is discussed.     

Impacts of elevated CO2 on expression of plant defensive compounds in Bt‐transgenic cotton in response to infestation by cotton bollworm

• 1 The allocation of defensive compounds of transgenic Bt (cv. GK‐12) and nontransgenic cotton (cv. Simian‐3) grown in elevated CO₂ in response to infestation by cotton bollworm Helicoverpa armigera (Hübner) was studied in closed‐dynamics CO₂ chambers.
• 2 A significant reduction in foliar nitrogen content and Bt toxin protein occurred when transgenic Bt cotton grew under elevated CO₂. A significantly higher carbon/nitrogen ratio as well as condensed tannin and gossypol contents was observed for transgenic Bt (cv. GK‐12) and nontransgenic cotton in elevated CO₂, in partial support of the carbon nutrient balance hypothesis as a result of limiting nitrogen and excess carbon in cotton plants in response to elevated CO₂.
• 3 The CO₂ level and infestation time significantly affected the foliar nitrogen, condensed tannin, gossypol and Bt toxin protein contents of cotton plants after feeding by H. armigera. The interaction between CO₂ levels × cotton variety had a significant effect on foliar nitrogen content after injury by H. armigera.

     

The effect of stem diameter on European corn borer behavior and survival: potential consequences for IRM in Bt-corn

The ability of non‐crop plants to support complete development of insect pests is an important factor for determining the impact of those plants on resistance management programs for transgenic crops. We assessed the effect of one physical factor, plant stem diameter, on the ability of plants to support full development of the European corn borer (ECB), Ostrinia nubilalis Hübner (Lepidoptera: Crambidae), the target pest of transgenic Bt‐corn. In the field, European corn borer larvae were significantly more likely to tunnel and survive in plants with larger stem diameters. Larvae were 40× more likely to survive on corn, the largest plant tested, compared to many of the smaller plants. In the laboratory, larvae were more likely to survive in and less likely to abandon the largest diet‐filled artificial stems that varied only in stem diameter. In conditions simulating those that an ECB larvae would encounter upon abandoning a host, larvae survived up to three weeks and were able to locate corn as a new host with a significantly higher frequency than would be expected if they were foraging randomly. These results indicate that the probability of ECB larval survival to maturity on a plant other than corn is relatively low and thus these smaller stemmed non‐corn plants may not make a substantial contribution to the pool of susceptible adults. Conversely, since more mature larvae are not as susceptible as neonates, any larvae that partially develop on non‐corn plants and subsequently colonize Bt‐corn may not be exposed to a lethal dose of the toxin. Since some proportion of the individuals that survive could be partially resistant heterozygotes the presence of non‐corn host plants could facilitate the development of resistant ECB populations.     

棉铃虫对Bt生物农药早期抗性及与转Bt基因棉抗虫性的关系

用饲料感染法建立了棉铃虫Helicoverpa rmigera(Hubmer)敏感品系(SUS1)对Bt生物农药的敏感毒力基线和区分剂量,1995年测定了五省六县棉铃虫初孵幼虫对Bt生物农药的敏感性,结果表明：山东阳谷、河北邯郸、河南新乡、安徽萧县及江苏丰县棉铃虫已产生早期抗性,抗性个体百分率为5%～10%,与敏感品系相比,LC₅₀值稍有增加,但斜率b值明显变小;而江苏东台棉铃虫仍属敏感。这是国内外首次诊测到棉铃虫对Bt生物农药抗性。用棉叶喂饲法测定比较了转Bt基因棉花品系对不同种群棉铃虫的抗虫性效果,结果表明：用早期抗性的阳谷和新乡棉铃虫初孵幼虫接虫5d后平均死亡率较敏感品系下降16%～29%,说明棉铃虫对Bt农药与转Bt生物基因棉花品系间存在交互抗性。还讨论了Bt农药的抗性治理对策。     

Effects of climate change and crop planting structure on the abundance of cotton bollworm,Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae)

The interactions between plants and insects play an important role in ecosystems. Climate change and cropping patterns can affect herbivorous pest insect dynamics. Understanding the reasons for population fluctuations can help improve integrated pest management strategies. Here, a 25‐year dataset on climate, cropping planting structure, and the population dynamics of cotton bollworms (Helicoverpa armigera) from Bachu County, south Xinjiang, China, was analyzed to assess the effects of changes in climate and crop planting structure on the population dynamics of H. armigera. The three generations of H. armigera showed increasing trends in population size with climate warming, especially in the third generation. The relative abundances of the first and second generations decreased, but that of the third generation increased. Rising temperature and precipitation produced different impacts on the development of different generations. The population numbers of H. armigera increased with the increase in the non‐Bacillus thuringiensis (Bt) cotton‐planted area. Asynchrony of abrupt changes existed among climate change, crop flowering dates, and the phenology of H. armigera moths. The asynchronous responses in crop flowering dates and phenology of H. armigera to climate warming would expand in the future. The primary factors affecting the first, second, and third generations of moths were T_mean in June, the last appearance date of the second generation of moths, and the duration of the third generation of moths, respectively. To reduce the harm to crops caused by H. armigera, Bt cotton should be widely planted.