Evolution of resistance to Bt crops: directional selection in structured environments

Abstract Widespread use of transgenic crops that express an insecticidal endotoxin from Bacillus thuringiensis increases the risk of evolution of resistance by the European corn borer and other insect pests. To delay resistance evolution, the high‐dose/refuge strategy is being implemented for Bt maize and Bt cotton. We develop a general modelling framework to understand the invasion and spread of alleles conferring resistance. We show that at least three processes are involved in explaining the effectiveness of the high‐dose/refuge strategy: the intensity of selection, assortative (non‐random) mating due to spatial subdivision, and variation in male mating success also due to spatial subdivision. Understanding these processes leads to a greater range of possible resistance management tactics. For example, efforts to encourage adults to leave their natal fields may have the unwanted effect of speeding rather than slowing resistance evolution. Furthermore, when Bt maize causes high mortality to susceptible target pests, spraying insecticides in refuges to reduce pest populations may not greatly disrupt resistance management.     

Host-plant mediated effects of transgenic maize on the insect parasitoid Campoletis sonorensis (Hymenoptera: Ichneumonidae)

Determining the impact of genetically modified (GM) crops on beneficial organisms is an important aspect of the environmental risk assessment of GM crops. In the present study, the impact of Bt maize expressing Cry1Ab on the development and behaviour of the parasitoid Campoletis sonorensis was compared to individuals reared on hosts fed conventionally bred plants partially resistant to the European corn borer (Ostrinia nubilalis Hübner) and on susceptible maize hybrids. Adult parasitoids reared on Bt maize-fed Spodoptera frugiperda larvae were significantly smaller (15–30%) than those reared in hosts fed either of the conventional maize hybrids. The magnitude of this effect was dependent on the size of the host at oviposition and its subsequent growth rate. The development time of C. sonorensis was not affected by the maize treatment. In choice tests, female parasitoids displayed no preference for hosts fed a specific maize hybrid. No Cry1Ab was detected within adult parasitoids.     

寄主植物对桃蛀螟生长发育及产卵选择行为的影响

为明确寄主植物对桃蛀螟生长发育及产卵选择行为的影响,利用实验种群生命表和二项产卵选择试验,研究了玉米、大豆、棉花和桃等4种寄主植物对桃蛀螟种群生长发育及产卵选择性的影响。结果表明:取食棉花的桃蛀螟幼虫存活率最低、幼虫历期最长,取食玉米的幼虫存活率最高、幼虫历期最短,取食桃和大豆的幼虫存活率和历期居于棉花处理组和玉米处理组之间;玉米处理组的桃蛀螟化蛹率、蛹重和蛹历期均为最高,棉花处理组为最低,大豆和桃处理组的这些参数均显著小于玉米处理组而大于棉花处理组;发育至成虫后,取食玉米的桃蛀螟羽化率显著高于其他3个处理组;取食桃的桃蛀螟成虫寿命(雌虫和雄虫)及个体发育历期均显著高于其他3种处理组;同时取食桃的桃蛀螟单雌产卵量最高,其次是玉米处理组,两者均显著高于大豆和棉花处理组。二项产卵选择试验结果显示,桃蛀螟雌蛾在棉花和玉米处理组、玉米和大豆处理组或棉花和大豆处理组间的落卵量差异不显著;但在包含桃的处理组中,桃蛀螟在棉花、玉米或大豆处理区的落卵量均显著高于桃处理区。上述结果表明,供试4种寄主植物中,桃蛀螟偏好在棉花、玉米和大豆上产卵,其中玉米对桃蛀螟的适合度相对较高,棉花对桃蛀螟的适合度相对较低。     

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.     

The role of herbivore‐ and plant‐related experiences in intraspecific host preference of a relatively specialized parasitoid

Parasitoids use odor cues from infested plants and herbivore hosts to locate their hosts. Specialist parasitoids of generalist herbivores are predicted to rely more on herbivorederived cues than plant-derived cues. Microplitis croceipes (Cresson)(Hymenoptera: Braconidae) is a relatively specialized larval endoparasitoid of Heliothis virescens (F.)(Lepidoptera: Noctuidae), which is a generalist herbivore on several crops including cotton and soybean. Using M. croceipes/H. virescens as a model system, we tested the following predictions about specialist parasitoids of generalist herbivores:(i) naive parasitoids will show innate responses to herbivore-emitted kairomones, regardless of host plant identity and (ii) herbivore-related experience will have a greater influence on intraspecific oviposition preference than plant-related experience. Inexperienced (naive) female M. croceipes did not discriminate between cotton-fed and soybean-fed H. virescens in oviposition choice tests, supporting our first prediction. Oviposition experience alone with either host group influenced subsequent oviposition preference while experience with infested plants alone did not elicit preference in M. croceipes, supporting our second prediction. Furthermore, associative learning of oviposition with host-damaged plants facilitated host location. I terestingly, naive parasitoids attacked more soybeathan cotton-fed host larvae in two-choice tests when a background of host-infested cotton odor was supplied, and vice versa. This suggests that plant volatiles may have created an olfactory contrast effect. We discussed ecological significance of the results and concluded that both plant- and herbivore-related experiences play important role in parasitoid host foraging.     

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.     

Oviposition acceptance and larval development of Chilo partellus stemborers in drought‐stressed wild and cultivated grasses of East Africa

Maternal host choices during oviposition by herbivorous insects determine the fitness of their offspring and may be influenced by environmental changes that can alter host‐plant quality. This is of particular relevance to ‘push‐pull’ cropping systems where host preferences are exploited to manage insect pest populations. We tested how drought stress in maize and companion plants that are used in these systems affect oviposition preference, larval feeding, and development of the spotted stemborer, Chilo partellus Swinhoe (Lepidoptera: Crambidae). Five host species were tested (all Poaceae): maize (Zea mays L.), Napier grass (Pennisetum purpureum Schumach), signal grass [Brachiaria brizantha (A. Rich) Stapf], Brachiaria cv. ‘Mulato’, and molasses grass [Melinis minutiflora (Beauv.)]. Under drought stress, maize experienced as much oviposition as control unstressed maize in choice and no‐choice experiments. Similarly, larval leaf damage was not significantly different in drought‐stressed vs. unstressed maize. In contrast, oviposition occurred less on drought‐stressed than on unstressed Napier and signal grass. Oviposition acceptance and leaf damage remained low in both drought‐stressed and unstressed molasses grass and Mulato. Larval survival and development remained high in drought‐stressed maize, but not in Napier, signal, and molasses grass and Mulato, where survival and development were low in both drought‐stressed and unstressed plants. Our results indicate that herbivore responses to drought‐stressed plants depend on the plant species and that drought stress can change host preference and acceptance rankings. In particular, trap‐crops such as Napier grass may not divert oviposition from the main maize crop under drought stress conditions.     

High dose refuge strategies and genetically modified crops – reply to Tabashnik et al.

Cultivating non‐toxic conventional crops (refuges) in the proximity to transgenic crops that produce Bacillus thuringienesis (Bt) toxins is widely recommended to delay pest adaptation to these toxins. Using a spatially structured model of resistance evolution, Vacher and co‐workers (Vacher, C., Bourguet, D., Rousset, F., Chevillon, C. & Hochberg, M.E. 2003. J. Evol. Biol. 16 : 378–387.) show that the percentage of refuge fields required for the sustainable control of pests can be reduced through intermediate levels of refuge field aggregation and by lowering the toxin dose produced by Bt plants. Tabashnik, B.E., Gould, F. & Carrière, Y. (2004 J. Evol. Biol doi: 10.1111/j1420–9101.2004.00695.x) call into question the results of Vacher et al. (2003) concerning the effect of toxin dose. They argue that these results arise from invalid assumptions about larval concentration–mortality responses for the insect considered, the cotton pest Heliothis virescens. We show here that the models presented by Vacher et al. (2003) and Tabashnik et al. (2004) both show inaccuracies in their definitions of genotypic fitness. The level of dominance estimated by Tabashnik et al. (2004) from larval mortality rates data is irrelevant to resistance evolution, and the fitness cost of resistance evolution, and the fitness cost of resistance is inaccurately integrated into their framework. Neverthless, the comments of Tabashnik et al. (2004) are very helpful in elucidating the definitions of genotypic fitness used in Vacher et al. (2003) and in pointing out the essential factors in predicting the evolution of insect resistance to Bt transgenic crops, namely, accurate estimations of the fitness cost of resistance, of the dominance level of this cost, and of the variations in the dominance level of the advantage conferred by the resistance with Bt toxin dose.     

No effect of host–parasite co‐evolution on host range expansion

Antagonistic co‐evolution between hosts and parasites (reciprocal selection for resistance and infectivity) is hypothesized to play an important role in host range expansion by selecting for novel infectivity alleles, but tests are lacking. Here, we determine whether experimental co‐evolution between a bacterium (Pseudomonas fluorescens SBW25) and a phage (SBW25Φ2) affects interstrain host range: the ability to infect different strains of P. fluorescens other than SBW25. We identified and tested a genetically and phenotypically diverse suite of co‐evolved phage variants of SBW25Φ2 against both sympatric and allopatric co‐evolving hosts (P. fluorescens SBW25) and a large set of other P. fluorescens strains. Although all co‐evolved phage had a greater host range than the ancestral phage and could differentially infect co‐evolved variants of P. fluorescens SBW25, none could infect any of the alternative P. fluorescens strains. Thus, parasite generalism at one genetic scale does not appear to affect generalism at other scales, suggesting fundamental genetic constraints on parasite adaptation for this virus.     

Bt抗虫棉秸秆还田对土壤养分特征的影响

【目的】研究转基因作物秸秆或残茬还田可能对土壤养分特性造成的影响。【方法】以不同抗虫水平Bt棉花和常规棉花(泗棉3号)为研究材料,分别在经过一、二个生长周期后将秸秆机械粉碎后原位还田,40 d后测定分析土壤中Bt蛋白含量及肥力相关的养分含量变化。【结果】Bt棉秸秆还田后,所有品种棉花土壤中Bt蛋白含量与还田前无显著增加,且转Bt基因棉与非转基因棉还田对土壤Bt蛋白含量的影响并无显著差异。同时,棉秸秆还田可显著提高土壤有机质、速效磷、碱解氮、速效钾、全氮、全磷和全钾含量,提升土壤pH值;增加幅度在不同抗虫水平Bt棉花间及与非转基因常规棉花品种间皆无显著性差异。【结论】秸秆还田对土壤肥力的提升与Bt棉的抗虫水平无关。“转Bt基因”不成为Bt棉秸秆还田提高土壤肥力的限制性因素,其秸秆还田不会对土壤肥力质量产生负面影响,可使土壤养分含量增加,有效提升土壤肥力。秸秆原位还田简单、无害又提升肥力,有条件作为转Bt基因植物秸秆无害化处理的理想方式。     

Host selection and utilisation of Sonchus oleraceus (Asteraceae) by Helicoverpa armigera (Lepidoptera: Noctuidae): A genetic analysis

The cotton bollworm (Helicoverpa armigera) prefers the common sowthistle (Sonchus oleraceus L.) to cotton (Gossypium hirsutum L.), sorghum (Sorghum bicolor L.) and maize (Zea mays L.) for oviposition in the field in Australia. Using the common sowthistle and cotton as host plants, we carried out this study to evaluate genetic variation in both oviposition preference and larval growth and genetic correlation between maternal preference and larval performance. There was a significant genetic component of phenotypic variation in both characters, and the heritability of oviposition preference was estimated as 0.602. Helicoverpa armigera larvae survived slightly better and grew significantly faster on common sowthistle than on cotton, but genetic correlation between maternal preference and larval growth performance was not detectable. Instead, larval growth performance on the two hosts changed with families, which renders the interaction between family and host plant significant. As a result, the genetic correlation between mean values of larval growth across the two host species was not different from zero. These results are discussed in the context of the relationship between H. armigera and the common sowthistle and the polyphagous behaviour of this insect in general.     

The relationship between host selection behaviour and offspring fitness in a koinobiont parasitoid

1. When host quality varies, optimal foraging theory assumes that parasitic wasps select hosts in a manner that increases their individual fitness. In koinobiont parasitoids, where the hosts continue developing for a certain period of time after parasitisation, host selection may not reflect current host quality but may be based on an assessment of future growth rates and resources available for the developing larvae. 2. When presented with hosts of uniform quality, the koinobiont parasitoid Leptomastix dactylopii exhibits a characteristic host‐selection behaviour: some hosts are accepted for oviposition on first encounter, while others are rejected several times before an egg is laid in them, a behaviour that is commonly associated with a changing host acceptance threshold during the course of a foraging bout. 3. The fitness of the offspring that emerged from hosts accepted immediately upon encounter was compared with the fitness of offspring emerged from hosts rejected several times before being accepted for oviposition. 4. The pattern of host acceptance and rejection was not related to any of the measured fitness parameters of the offspring emerging from these hosts (development time, size at emergence, sex ratio at emergence, and female offspring egg load). 5. While complex post facto adaptive explanations can be devised to explain the nature of such a time and energy consuming host selection process, it is suggested that physiological constraints on egg production or oviposition may provide an alternative, purely mechanistic, explanation for the results obtained.     

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.     

Vetiver grass (Vetiveria zizanioides (L.) Nash) as trap plant for Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) and Busseola fusca (Fuller) (Lepidoptera: Noctuidae)

The preference of lepidopterous stem borer moths to oviposit on certain wild host plants can be exploited in habitat management systems by using those hosts as trap crops. Vetiver grass (Vetiveria zizanioides (L.) Nash) was evaluated for its attractiveness and suitability to the pyralid Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) and the noctuid Busseola fusca (Fuller) (Lepidoptera: Noctuidae). Two choice tests were conducted in the laboratory and in the greenhouse to determine oviposition choice of C. partellus for maize, Vetiver and rice (Oryza sativa L.), and of B. fusca for Vetiver and maize. C. partellus larval survival was evaluated in green house studies. Results indicated that C. partellus chose Vetiver grass over maize though larval survival on Vetiver was extremely low. B. fusca did not show any host preference.     

Comparison of Host-Searching and Ovipositional Behavior of Cardiochiles nigriceps Viereck (Hymenoptera: Braconidae), a Parasitoid of Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae), in Tobacco and Cotton

To compare host-searching and ovipositional behavior of Cardiochiles nigriceps Viereck (CN) in tobacco and cotton, behavior of CN females was observed on Hellothis virescens (Febricius)-infested plants and recorded using a Psion Organizer II with Observer software. Hover, search, oviposit, preen, rest, walk, agony-search, drop-search, and miss-search were the nine behaviors observed for each plant species. Agony-search was a searching behavior exhibited when CN females were prevented from ovipositing in a host because they were unable to reach the host. Drop-search and miss-search were the searching behaviors that occurred by CN females after a host dropped from the plant and when CN females missed ovipositing in a host, respectively. Hovering, searching, oviposition, miss-search, resting, and walking were equally common for CN on tobacco and cotton. Agony-search was exhibited more on tobacco than cotton, while drop-search and preening occurred more on cotton than tobacco. Female CN spent more time hovering in tobacco than in cotton, indicating that these females preferred tobacco to cotton. Female parasitoids also spent more time in the main ovipositional sequence, hover–search–oviposit, in tobacco than in cotton, mainly because females hovered more in tobacco than in cotton. Frequency and time of behavioral sequences associated with experiencing frustration in ovipositing due to difficulty in reaching or finding a host and subsequent level of success in ovipositing in a new host were determined. Overall, the rate of successful oviposition by CN females was significantly higher in tobacco (87.7%) than in cotton (79.5%). For cotton and tobacco, the rate of successful oviposition by CN females was significantly reduced (ca. 40.0%) when they experienced frustration in ovipositing compared to the rate of ovipositional success (100%) for CN females when hosts were present and easily accessible. Encountering ovipositional frustration also increased the duration of time for successful oviposition in comparison to the time spent for problem-free oviposition in cotton and tobacco. Interestingly, with successful oviposition, the amount of time spent in a behavioral sequence was not significantly different for tobacco and cotton regardless of whether females experienced ovipositional frustration or not. However, female CN invested more time in tobacco than in cotton when they were unsuccessful in ovipositing due to the inaccessibility or disappearance of hosts. CN females' preference for tobacco over cotton probably gave them more motivation to continue searching for hosts, especially those not easily attained, on tobacco than on cotton. Nevertheless, CN females readily searched in cotton for HV even in the presence of host-infested tabacco. CN females possibly could be successful in maintaining levels of HV below an economic threshold when using a tobacco trap crop to protect cotton as the main crop.     

Oviposition, development, and reproduction of Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) fed on different hosts of economic importance

The host selection for oviposition by Spodoptera frugiperda (J.E. Smith) among corn, millet, cotton and soybean, and its relationship with the biological characteristics were investigated. Free and non-choice tests for oviposition using plots containing five plants each, from each host in plastic greenhouse, resulted in similar oviposition preference among the host plants. In addition, selected biological characteristics of S. frugiperda were determined in the laboratory with larvae feeding on host leaves, and the combination of leaf and cotton boll. Neonate larvae exhibited low success of colonization on cotton boll compared to the leaves of all other hosts. Spodoptera frugiperda fed only on cotton bolls exhibited longer larval and pupal development, and longer adult life span; however with similar egg production. Larvae fed cotton leaves during six days and then transferred to cotton bolls, however, exhibited development and reproduction similar to those reared on corn or only on cotton leaves. Therefore, the variations on immature stages of S. frugiperda were not related with host selection for oviposition which was similar among the studied hosts. Based on our data, the millet as a winter, rotational, and cover crop is a potential host for S. frugiperda, while leaves and cotton bolls were diets of intermediate suitability as compared to corn and soybean leaves.     

Impacts of Bt maize inoculated with rhizobacteria on development and food utilization of Mythimna separata

The current trend of increasing proportion of cultivation of transgenic Bt crops is pushing towards dramatic destabilization of the agroecosystem, thus raising severe concerns about the sustainability of transgenic Bt crops as an effective management tool for the control of target insect pests in the future. Rhizobacteria is the key biological regulator to ameliorate soil‐nitrogen utilization efficiency of crop plants, especially transgenic Bt crops. A laboratory study quantified the impacts of transgenic Bt maize (Line IE09S034 with Cry1Ie vs. non‐Bt maize cv. Xianyu335) inoculated with Azospirillum brasilense (AB) and Azotobacter chroococcum (AC) on the growth, development and food utilization of a target lepidopteran insect, Mythimna separata. The results showed that the inoculation of rhizobacteria significantly prolonged the larval lifespan and pupal duration, increased RCR and AD, reduced pupal weight, pupation rate, fecundity, RGR, ECD and ECI, and shortened adult longevity of M. separata fed on transgenic Bt maize, while exact opposite trends were found in these measured indexes of growth, development and food utilization for M. separata fed on non‐Bt maize inoculated with AB and AC compared with the buffer control in both years. Thus, the results clearly depicted that the inoculation of rhizobacteria had opposite influences on the growth, development and food utilization of M. separata fed on transgenic Bt maize. Presumably, rhizobacteria inoculation can be used to stimulate plant–soil‐nitrogen uptake and promote plant growth for transgenic Bt maize and non‐Bt maize, simultaneously increasing Bt toxin production and enhancing resistance efficiency against target lepidopteran pests for transgenic Bt maize.     

Non-target organism effects tests on Vip3A and their application to the ecological risk assessment for cultivation of MIR162 maize

Transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) provide economic, environmental and health benefits by maintaining or increasing crop yields with fewer applications of insecticide. To sustain these benefits, it is important to delay the evolution of insect resistance to the proteins, and to ensure that the proteins do not harm non-target organisms, particularly those that may control secondary pests that would otherwise flourish because of reduced insecticide applications. Vip3A is a Bt vegetative insecticidal protein that is active against lepidopterous pests. It has a different mode of action from other proteins for control of Lepidoptera in current Bt crops, and when combined with these proteins, it should help to delay the evolution of pest resistance to Bt crops. This paper presents data on the effects of Vip3A on non-target organisms, and an ecological risk assessment of MIR162 maize, which expresses Vip3Aa20. Laboratory studies indicate few adverse effects of Vip3A to non-target organisms: 11 of 12 species tested showed no adverse effects when exposed to high concentrations of Vip3A relative to estimated exposures resulting from cultivation of MIR162 maize. Daphnia magna exposed to Vip3Aa20 were unaffected in terms of survival or fecundity, but grew slightly more slowly than unexposed controls. The data indicate that cultivation of MIR162 maize poses negligible risk to non-target organisms, and that crops producing Vip3A are unlikely to adversely affect biological control organisms such that benefits from reduced insecticide applications are lost.     

Flight and oviposition behaviour of the African stem borer,Busseola fusca,on various host plant species

The African stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae), is an important pest of maize and sorghum in sub‐Saharan Africa. As in many other lepidopteran insects, the ability of B. fusca to recognize and colonize a variety of plants is based on the interaction between its sensory systems and the physical and chemical characteristics of its immediate environment. In this study, we tried to identify the behavioural steps of B. fusca leading to host selection and oviposition. Three Poaceae species commonly cultivated in Kenya for human consumption and animal forage were used in this study: the two most preferred hosts, maize (Zea mays L.) and sorghum [Sorghum bicolor (L.) Moench], and one non‐preferred host, Napier grass (Pennisetum purpureum Schumach). Wind tunnel observations revealed that volatiles produced by the different plant species did not appear to strongly influence the general orientation of B. fusca towards the plant, as similar behavioural steps were exhibited by the female moth regardless of the plant species involved. This indicated that the females were not able to recognize their preferred hosts from a distance. After landing, the female typically swept her ovipositor on the plant surface, simultaneously touching it with the tips of her antennae, and then oviposited. This behaviour was more frequently observed on maize and sorghum than on Napier grass, and indicated that both antennal and ovipositor receptors are used by the female moths to evaluate the plant surface before deciding to oviposit. As a result, the females laid more eggs on the two crops than on Napier grass. We conclude therefore that females recognized their preferred hosts only after landing. Tactile and contact‐chemoreception stimuli from the plants seemed to play a major role in oviposition decisions of B. fusca.     

The pollen beetle, Meligethes aeneus, changes egg production rate to match host quality

Motivation-based models dominate current theory concerning host plant selection for oviposition by herbivorous insects. A female searching for a host plant will be more likely to accept a host which is of inferior quality for her offspring if motivation is high, e.g. a large eggload or long time since last oviposition. This implies that insects will accumulate eggs if exposed to hosts of low acceptability and after a time lay eggs on such hosts. An alternative strategy for insects when exposed to less acceptable hosts is to stop producing, instead of accumulating, eggs. Thus, resources would be saved until a more acceptable host is found. If this hypothesis is true, a herbivorous insect would cease egg production when exposed to hosts of low acceptability and resume egg production when exposed to hosts of high acceptability. Previous exposure should not affect oviposition rate when an insect encounters a new host of a different quality. In an earlier study pollen beetles, Meligethes aeneus (F.) (Coleoptera: Nitidulidae), did not accumulate eggs in the absence of high quality hosts. In this study we monitored the daily oviposition rate of female pollen beetles on hosts plants of low (Sinapis alba L.), intermediate (Brassica nigra Koch) or high (B. napus L.) acceptability over a 5-day period. Individuals were then switched to an oviposition resource of a different acceptability. Beetles moved from high- to low-acceptability plants reduced their oviposition rate considerably. In the opposite case, low to high acceptability, the rate of oviposition increased markedly after the switch. When M. aeneus were moved from the high-acceptability host to that of intermediate acceptability oviposition rate was modified accordingly. However, when moved to the intermediate host from a host of low acceptability oviposition on B. nigra was much less than would normally be expected. A possible mechanism for this finding is discussed. M. aeneus, by adjusting oviposition rate to host acceptability, maximizes the average host quality for offspring, even at the cost of a lower egg-laying rate. Received: 5 October 1998 / Accepted: 20 April 1999