The impact of Bt maize as a natal host on adult head capsule width in field populations of western corn rootworm

The success of the current resistance management plan for transgenic maize, Zea mays L. (Poaceae), targeting the rootworm complex hinges upon high rates of mating between resistant and susceptible beetles. However, differences in the fitness of adult beetles could result in assortative mating, which could, in turn, change the rate of resistance evolution. Adult head capsule widths of naturally occurring populations of western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), were examined in a variety of refuge configurations. Beetles were classified into treatments based on the hybrid – non‐Bt refuge or Bt maize targeting larval rootworms (hereafter Bt‐RW maize) – and location – proximity to other Bt‐RW or refuge plants – of the natal host plant. Treatments included the following: a refuge plant surrounded by other refuge plants, a refuge plant located near a Bt‐RW plant, a Bt‐RW plant surrounded by Bt‐RW plants, and a Bt‐RW plant located near a refuge plant. The mean head capsule width of males emerging from Bt‐RW plants was significantly smaller than the mean head capsule width of males emerging from refuge plants. These results indicate that males emerging from Bt‐RW maize plants may be exposed to sublethal doses of the Bt toxin as larvae. No differences were detected between females emerging from refuge plants compared with Bt‐RW plants. Overall mean head capsule width decreased as the season progressed, regardless of treatment. The diminished head capsule width of western corn rootworm males emerging from Bt‐RW maize may act to enhance resistance management, particularly in a seed mix refuge system.     

Colonization preference of Euschistus servus and Nezara viridula in transgenic cotton varieties,peanut, and soybean

Producers of Bt cotton, Gossypium hirsutum L. (Malvaceae), in the southeastern USA face significant losses from highly polyphagous stink bug species. These problems may be exacerbated by crop rotation practices that often result in cotton, peanut, Arachis hypogaea L., and soybean, Glycine max (L.) Merrill (both Fabaceae), growing in close proximity to one another. Because all of these crops are hosts for the major pest stink bug species in the region, we experimentally examined colonization preference of these species among the crops to clarify this aspect of their population dynamics. We planted peanut, soybean, Bt cotton, and glyphosate‐tolerant (RR) non‐Bt cotton at three sites over 3 years in replicated plots ranging from 192 to 1 323 m² and calculated odds ratios for colonization of each crop for Nezara viridula (L.) and Euschistus servus (Say) (both Hemiptera: Pentatomidae). In four of five experiments, both E. servus and N. viridula preferred soybean significantly more often than Bt cotton, non‐Bt cotton, and peanut. Neither N. viridula nor E. servus showed any preference between non‐Bt and Bt cotton in any experiment. Both species had higher numbers in Bt and non‐Bt cotton relative to peanut; this was not significant for any single experiment, but analyses across all experiments indicated that N. viridula preferred Bt and non‐Bt cotton significantly more often than peanut. Our results suggest that soybean in the landscape may function as a sink for stink bug populations relative to nearby peanut and cotton when the soybean is in the reproductive stage of development. Stink bug preference for soybean may reduce pest pressure in near‐by crops, but population increases in soybean could lead to this crop functioning as a source for later‐season pest pressure in cotton.     

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.     

Spatial and temporal dynamics of oviposition behavior of bollworm and three of its predators in Bt and non-Bt cotton fields

Host plants exhibiting insect resistance traits have long been known to influence within‐plant distributions of pests and their natural enemies. Sites and timing of egg deposition are particularly important for synchrony of predators and their prey in the field. Temporal and spatial distribution of eggs of the cotton bollworms [Heliothis virescens (F.) and Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae)] and that of the predators Geocoris punctipes (Say) (Heteroptera: Geocoridae), Chrysoperla rufilabris (Burmeister) (Neuroptera: Chrysopidae), and Micromus spec. (Neuroptera: Hemerobiidae) were determined during three cotton seasons, from 2002 to 2004, by collecting and examining plants throughout each season. Comparisons also were made between Bt and non‐Bt cotton to investigate possible changes in oviposition behavior on Bt cotton. The study was conducted in commercial fields with insecticide use to manage pests when economic thresholds were exceeded in both cotton types. Egg densities for predators and bollworms varied among years, but were similar on Bt and non‐Bt cottons. Oviposition of bollworms and G. punctipes correlated spatially within plants, with most eggs laid on structures in the top five nodes of cotton plants and on the three outermost leaves on lateral branches regardless of cotton type. Bollworm oviposition dynamics exhibited two peaks within the season (early July and early August). Eggs of all predators and bollworms collected from the field and incubated in the laboratory had high hatching rates throughout each season (74–100%). Temporal association of predator with bollworm oviposition showed a significant correlation with green lacewings, a delay of 10 days for big‐eyed bugs, and no correlation with brown lacewings. Furthermore, Bt cotton plants exerted no significant effect on temporal or spatial patterns of oviposition of bollworms or the predators, indicating no change in oviposition behavior of bollworm females within plant structures after almost one decade of widespread planting of Bt cotton.     

Nosema pyrausta and Cry1Ab‐incorporated diet led to decreased survival and developmental delays in European corn borer

The high dose/refuge strategy for delaying evolution of resistance to Bt maize [Zea mays L. (Poaceae)] relies on random mating between resistant European corn borers, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae), and susceptible O. nubilalis from the refuge. However, differences in developmental rate caused by feeding on Bt maize, or infection with the microsporidium Nosema pyrausta Paillot (Microsporida: Nosematidae) may result in assortative mating. Developmental delays and mortality caused by infection with N. pyrausta and feeding on Bt maize were quantified alone and in combination in Cry1Ab‐resistant and susceptible O. nubilalis. Feeding on Cry1Ab‐incorporated diet significantly increased number of days from hatch to pupation and decreased survival in the resistant population. Infection with N. pyrausta increased mortality and lengthened development in both the resistant and susceptible populations. The combination of Cry1Ab‐incorporated diet and infection with N. pyrausta in resistant O. nubilalis lengthened development and increased mortality to a greater extent than either factor alone. Greater larval delays of resistant O. nubilalis feeding on Bt maize could lead to temporal isolation from adults emerging from refuge maize. The resulting assortative mating would hasten the evolution of resistance. Developmental delays caused by infection with N. pyrausta may increase the likelihood of mating between resistant and infected susceptible adults emerging from refuge maize, producing infected offspring that are also more susceptible to Bt maize.     

Antixenosis and larval and adult dispersal in the Mediterranean corn borer,Sesamia nonagrioides,in relation to Bt maize

Sesamia nonagrioides Lefèbvre (Lepidoptera: Noctuidae) is a key pest of maize [Zea mays L. (Poaceae)] and a main target of Bt maize in the Mediterranean area. To choose the most suitable non‐Bt refuge strategy for preventing or delaying resistance development in this maize borer, we examined its biology and behaviour. No antixenotic effects were found on numbers of eggs and egg batches per plant in choice (Bt vs. non‐Bt plants) and no‐choice assays. However, a greater ratio of young larvae dispersed from Bt than from non‐Bt plants. In addition, larvae that hatched on Bt plants tended to disperse more than those that hatched on non‐Bt plants, particularly during young growth stages. Many adults, especially females, could fly at least up to 400 m, as was found in a dispersal study with rubidium‐marked adults. The stimulation of larval dispersal by the Bt trait and the dispersal capacity of adults might compromise the efficacy of seed mixtures as an insecticide resistance management strategy.     

Effects of Bacillus thuringiensisδ-endotoxin-fed Helicoverpa armigera on the survival and development of the parasitoid Campoletis chlorideae

With the deployment of transgenic crops expressing δ‐endotoxins from Bacillus thuringiensis (Bt) for pest management, there is a need to generate information on the interaction of crop pests with their natural enemies that are important for regulation of pest populations. Therefore, we studied the effects of the Bt δ‐endotoxins Cry1Ab and Cry1Ac on the survival and development of the parasitoid Campoletis chlorideae Uchida (Hymenoptera: Ichneumonidae) reared on Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) larvae fed on Bt toxin‐intoxicated artificial diet. The H. armigera larvae fed on artificial diet impregnated with Cry1Ab and Cry1Ac at LC₅₀ (effective concentration to kill 50% of the neonate H. armigera larvae) and ED₅₀ (effective concentration to cause a 50% reduction in larval weight) levels before and after parasitization resulted in a significant reduction in cocoon formation and adult emergence of C. chlorideae. Larval period of the parasitoid was prolonged by 2 days when fed on Bt‐intoxicated larvae. No adverse effects were observed on female fecundity. The observed effects appeared to be indirect in nature, because no Bt proteins were detected through enzyme‐linked immunosorbent assay in the C. chlorideae larvae, cocoons, or adults fed on Cry1Ab‐ or Cry1Ac‐treated H. armigera larvae. The effects of Bt toxin proteins on C. chlorideae were due to early mortality of H. armigera larvae, that is, before completion of parasitoid larval development.     

Dispersal capacity in the Mediterranean corn borer, Sesamia nonagrioides

Corn (Zea mays L.) borers are the primary target of Bacillus thuringiensis Berliner (Bt) transgenic maize. Management of corn borer resistance to Bt requires information on larval and adult dispersal capacities, a feature that is particularly unknown in Sesamia nonagrioides Lefèbvre (Lepidoptera: Noctuidae), the most damaging corn borer in Spain. Larval dispersal was studied over a 3 year period by infesting plants with egg masses and dissecting the neighbouring plants 7, 14, and 32 days later to measure larval dispersal at several ages. The number and age of larvae were recorded in the dissected plants. Only mature larvae dispersed in significant numbers; they moved at least to rows adjacent to those containing the infested plant, and down the row five plants. The percentage of larvae that dispersed from the infested plant was density‐dependent. Adult dispersal was studied with directional light and pheromone uni‐traps over 5 and 3 year periods, respectively. Directional light traps were placed in the margins between Bt and non‐Bt maize fields, half oriented towards each of the two kinds of maize field. Pheromone traps were placed in the Bt and non‐Bt fields at increasing distances (0–100 m) from the border. The numbers of males and females caught in directional light traps were not different in traps oriented towards Bt or non‐Bt fields, but the number of males caught in the third flight in Bt fields was lower than in non‐Bt fields. These results suggest that males from adjacent Bt and non‐Bt fields mate indiscriminately with females emerging in any of the two kinds of maize fields. However, male movement in the third flight may not be sufficient to randomly distribute males between the two fields.     

A Challenge for the Seed Mixture Refuge Strategy in Bt Maize: Impact of Cross-Pollination on an Ear-Feeding Pest,Corn Earworm

To counter the threat of insect resistance, Bacillus thuringiensis (Bt) maize growers in the U.S. are required to plant structured non-Bt maize refuges. Concerns with refuge compliance led to the introduction of seed mixtures, also called RIB (refuge-in-the-bag), as an alternative approach for implementing refuge for Bt maize products in the U.S. Maize Belt. A major concern in RIB is cross-pollination of maize hybrids that can cause Bt proteins to be present in refuge maize kernels and negatively affect refuge insects. Here we show that a mixed planting of 5% nonBt and 95% Bt maize containing the SmartStax traits expressing Cry1A.105, Cry2Ab2 and Cry1F did not provide an effective refuge for an important above-ground ear-feeding pest, the corn earworm, Helicoverpa zea (Boddie). Cross-pollination in RIB caused a majority (>90%) of refuge kernels to express ≥ one Bt protein. The contamination of Bt proteins in the refuge ears reduced neonate-to-adult survivorship of H. zea to only 4.6%, a reduction of 88.1% relative to larvae feeding on ears of pure non-Bt maize plantings. In addition, the limited survivors on refuge ears had lower pupal mass and took longer to develop to adults.     

Stay or move: how Bt‐susceptible Helicoverpa armigera neonates behave on Bt cotton plants

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) larvae occasionally have been reported to survive at management threshold levels in fields of Bollgard II^® cotton, Gossypium hirsutum L. (Malvaceae). The pattern and degree of larval survival is not easily predicted but depends on the ability of first instars to establish on host plants. Experiments were conducted with Bacillus thuringiensis Berliner (Bt)‐susceptible and Bt‐resistant larvae of H. armigera to understand how physiologically Bt‐susceptible H. armigera survive on Bt cotton plants, and examine how their first meal influences survival rates. In assays using cotton plant parts, both strains of larvae displayed similar tendencies to drop‐off specific plant parts of Bt and non‐Bt cotton. However, significantly more Bt‐susceptible larvae dropped off young leaves, mature leaves, and squares of Bt cotton compared to non‐Bt cotton plants. Egg cannibalism significantly improved the survival of Bt‐susceptible H. armigera larvae on Bt cotton plants. Larvae were more likely to eat live aged eggs, than newly laid or dead eggs. Survival significantly improved when larvae cannibalized eggs before feeding on Bt leaves. The behavior of Bt‐susceptible larvae with respect to drop‐off and egg cannibalism may help enhance their survival on Bt cotton plants.     

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.     

Bacillus thuringiensis plants expressing Cry1Ac,Cry2Ab and Cry1F are not toxic to the assassin bug,Zelus renardii

Cotton‐ and maize‐producing insecticidal crystal (Cry) proteins from the bacterium, Bacillus thuringiensis (Bt), have been commercialized since 1996. Bt plants are subjected to environmental risk assessments for non‐target organisms, including natural enemies that suppress pest populations. Here, we used Cry1F‐resistant Spodoptera frugiperda (J.E. Smith) and Cry1Ac and Cry2Ab‐resistant Trichoplusia ni (Hübner) as prey for the assassin bug, Zelus renardii (Kolenati), a common predator in maize and cotton fields. In tritrophic studies, we assessed several fitness parameters of Z. renardii when it fed on resistant S. frugiperda that had fed on Bt maize expressing Cry1F or on resistant T. ni that had fed on Bt cotton expressing Cry1Ac and Cry2Ab. Survival, nymphal duration, adult weight, adult longevity and female fecundity of Z. renardii were not different when they were fed resistant‐prey larvae (S. frugiperda or T. ni) reared on either a Bt crop or respective non‐Bt crops. ELISA tests demonstrated that the Cry proteins were present in the plant at the highest levels, at lower levels in the prey and at the lowest levels in the predator. While Z. renardii was exposed to Cry1F and Cry1Ac and Cry2Ab when it fed on hosts that consumed Bt‐transgenic plants, the proteins did not affect important fitness parameters in this common and important predator.     

Effect of Cry1F maize on the behavior of susceptible and resistant Spodoptera frugiperda and Ostrinia nubilalis

Understanding the behavior of pests targeted with Bacillus thuringiensis Berliner (Bt) crops is important to define resistance management strategies. Particularly the study of larval movement between plants is important to determine the feasibility of refuge configurations. Exposure to Bt maize, Zea mays L. (Poaceae), has been suggested to increase larval movement in lepidopteran species but few studies have examined the potential for resistance to interact with behavioral responses to Bt toxins. Choice and no‐choice experiments were conducted with Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae) and Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) to determine whether Cry1F resistance influences neonate movement. Leaf discs of Cry1F maize and the corresponding isoline were used to characterize behavioral responses. In both experiments, the location (on or off of plant tissues) and mortality of susceptible and Cry1F resistant neonates was recorded for 5 days, but the analysis of larvae location was performed until 7 h. Our results indicated no strong difference between resistant and susceptible phenotypes in S. frugiperda and O. nubilalis, although a small percentage of susceptible neonates in both species abandoned maize tissue expressing Cry1F. However, significant behavioral differences were observed between species. Ostrinia nubilalis exhibited increased movement between leaf discs, whereas S. frugiperda selected plant tissue within the first 30 min and remained on the chosen plant regardless of the presence of Cry1F. Spodoptera frugiperda reduced larval movement may have implications to refuge configuration. This study represents the first step toward understanding the effects of Cry1F resistance on Lepidoptera larval behavior. Information regarding behavioral differences between species could aid in developing better and more flexible resistance management strategies.     

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.     

Aqueous washings of host plant surface influence the growth inhibitory effects of δ‐endotoxins of Bacillus thuringiensis against Helicoverpa armigera

Biological activity of the bacterium Bacillus thuringiensis Berliner (Bt) against insect pests is influenced by the host plants. To understand the underlying mechanism of variation in biological activity of Bt on host plants, we studied the effect of chemicals from the surface of chickpea (Cicer arietinum L., Fabaceae) leaves (ICCC 37 and ICC 506EB), sorghum [Sorghum bicolor (L.) Moench, Poaceae] grain (ICSV 745 and IS 18698), pigeon pea [Cajanus cajan (L.) Millsp., Fabaceae] pods (ICPL 87 and ICPL 332WR), and cotton (Gossypium hirsutum L., Malvaceae) squares (RCH 2 and Bt RCH 2), on which Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) feeds under natural conditions. Surface chemicals extracted in water from host plant leaves were added to the standard artificial diet containing a commercial formulation of Bt or Cry1Ac. Data were recorded on larval and pupal weights, pupation, adult emergence, larval and pupal periods, adult longevity, and fecundity. Weights of H. armigera at 5 days after initiation of the experiment were significantly reduced on artificial diets containing Bt + pod washings of ICPL 87 and ICPL 332WR, grain washings of ICSV 745, or square washings of RCH 2, and Cry1Ac + leaf‐surface washings of ICC 506EB. Pupal weights were lower on diets containing leaf‐surface washings of ICCC 37 + Bt than on standard artificial diet. Larval periods were prolonged on diets containing Bt + leaf‐surface washings of ICCC 37, pod washings of ICPL 87, and square washings of RCH 2, and on standard artificial diet + Cry1Ac. Pupation was significantly higher on standard artificial diet + Cry1Ac than on diets with Bt + grain washings of ICSV 745 and Cry1Ac + square washings of RCH 2 and Bt RCH 2. Adult emergence was lowest on diets with square washings of RCH 2 + Bt and grain washings of ICSV 745 + Cry1Ac. The results suggested that leaf‐surface washings play an important role in biological activity of Bt/Cry1Ac against H. armigera.     

Genetic structure of European and Mediterranean maize borer populations on several wild and cultivated host plants

Target pests may become resistant to Bacillus thuringiensis (Bt) toxins produced by trangenic maize (Zea mays L.). Untreated refuge areas are set aside to conserve high frequencies of susceptibility alleles: a delay in resistance evolution is expected if susceptible individuals from refuges mate randomly with resistant individuals from Bt fields. In principle, refuges can be toxin‐free maize or any other plant, provided it hosts sufficiently large pest populations mating randomly with populations from Bt‐maize fields. Our aim was to examine the suitability of several cultivated or weedy plants [pepper (Capsicum frutescens L.), sorghum (Sorghum spec.), sunflower (Helianthus annuus L.), cocklebur (Xanthium spec.), cantaloupe (Cucumis melo L.), and hop (Humulus lupulus L.)] as refuges for Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) and Sesamia nonagrioides Lefebvre (Lepidoptera: Noctuidae), two major maize pests in southern Europe. Larvae of both species were collected on these plants. Their genetic population structure was examined at several allozyme loci. We found little or no evidence for an influence of geographic distance, but detected a significant host‐plant effect on the genetic differentiation for both species. Ostrinia nubilalis populations from sunflower, pepper, cocklebur, and sorghum appear to belong to the same genetic entity as populations collected on maize, but to differ from populations on hop. Accordingly, females from pepper and cocklebur produced exclusively the ‘Z’ type sexual pheromone, which, in France, characterizes populations developing on maize. Qualitatively, these plants (except hop) could thus serve as refuges for O. nubilalis; however, they may be of little use quantitatively as they were found much less infested than maize. Sesamia nonagrioides populations on maize and sorghum reached comparable densities, but a slight genetic differentiation was detected between both. The degree of assortative mating between populations feeding on both hosts must therefore be assessed before sorghum can be considered as a suitable refuge for this species.     

Response of multiple generations of beet armyworm,Spodoptera exigua (Hübner), feeding on transgenic Bt cotton

Development, reproduction and food utilization of three successive generations of beet armyworm, Spodoptera exigua (Hübner), fed on transgenic and non‐transgenic Bt cotton were examined. Significantly longer larval life‐span and lower pupal weight were observed in three successive generations of S. exigua fed on transgenic Bt cotton compared with non‐transgenic Bt cotton. Significantly higher survival rate and adult fecundity of S. exigua were found in three successive generations of S. exigua fed on transgenic Bt cotton compared with non‐transgenic Bt cotton. The survival rate and adult fecundity of S. exigua were occurred significant increase in the third generation compared with the first generation after feeding on transgenic Bt cotton. Significantly lower consumption, frass and relative growth rate (RGR) were observed in three successive generations of S. exigua fed on transgenic Bt cotton compared with non‐transgenic Bt cotton. Cotton variety significantly affected all indices of larval consumption and utilization in three successive generations of S. exigua, except for efficiency of conversion of ingested food. However, beet armyworm generation only significantly affected RGR of S. exigua. The results of this study indicated food quality on the diet‐utilization efficiency of S. exigua was different along with beet armyworm generation. Measuring multigenerational development and food utilization of S. exigua at individual and population level in response to Bacillus thuringiensis (Bt) can provide a more meaningful evaluation of long‐term population dynamics than experiments on a single generation. It is imperative to develop an appropriate multigenerational pest management tactic to monitor the field population dynamics of non‐target pests (e.g., beet armyworm) in agricultural Bt cotton ecosystem.     

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.     

Growth and development of Bacillus thuringiensis Cry1Ab-susceptible and Cry1Ab-resistant sugarcane borer on diet and conventional maize plants

The sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae), is a dominant maize borer pest and a major target of Bacillus thuringiensis (Bt)‐maize in Louisiana and the Gulf Coast area of Texas (USA). Growth and development of D. saccharalis on non‐toxic diet, diet treated with three low concentrations (0.01, 0.05, and 0.1 μg g^?1) of Cry1Ab toxin, and on non‐Bt maize plants were compared for five insect genotypes: a Bt‐susceptible strain (BT‐SS), a Cry1Ab‐resistant strain (BT‐RR), a back‐crossed and re‐selected resistant strain (BT‐R’R’), and two F₁ progeny of the BT‐SS and BT‐R’R’ strains. Fitness of the five genotypes was examined by infesting neonates on diet with/without Cry1Ab toxin in the laboratory and on intact non‐Bt maize plants in the greenhouse. Biological parameters measured were neonate‐to‐pupa development time and pupation rate, larval survival, larval and pupal weight, and sex ratio. Larvae of BT‐SS and BT‐R’R’ on non‐toxic diet and non‐Bt maize plants grew normally and there were no significant differences between the two strains in all measured parameters, suggesting a lack‐of‐fitness cost of the Cry1Ab resistance in D. saccharalis. Except for the development time on non‐Bt diet, all other parameters on both non‐Bt diet and non‐Bt maize plants were similar among the five genotypes. Larval development of BT‐SS was significantly affected on diet treated with Cry1Ab toxin at 0.05 and 0.1 μg g^?1, whereas the effect to BT‐RR and BT‐R’R’ was not significant. Pupal weight and sex ratio reared on Cry1Ab‐diet were similar and there were no significant differences among the five genotypes. Neonate‐to‐pupation rate decreased as Cry1Ab concentrations increased but the decrease was more significant for BT‐SS than for the other four genotypes. The lack‐of‐fitness costs of Bt resistance in D. saccharalis imply a greater challenge in managing Bt resistance for this maize borer species.