Effect of an entomopathogen on adaptation of Heliothis virescens populations to transgenic host plants

The role of an entomopathogen in evolution of herbivore adaptation to partially resistant host plants was examined using a tritrophic system in the laboratory. We hypothesized that a pathogen should interact with herbivore behavior to accelerate herbivore adaptation to toxic plants: individuals not adapted to toxin tend to move more on toxic plants, and therefore are more likely to encounter a lethal dose of pathogen, further increasing the probability that they will be eliminated by selection. Heliothis virescens (F.) (Noctuidae) was selected for adaptation to transgenic tobacco containing a sublethal concentration of Bacillus thuringiensis Berliner toxin under two treatment regimes: larvae placed on plants treated with infective conidia of the entomopathogenic fungus, Nomuraea rileyi (Farlow) Samson, and larvae placed on plants without fungus.Selection was initiated with a genetically heterogeneous strain created by crossing two laboratory strains of H. virescens, one not adapted to B. thuringiensis toxin, and one highly adapted (>1000-fold) to toxin. This cross was performed four times to create four independent populations. Selection was initiated with F2 offspring from each cross and continued for 8–10 consecutive generations. Adaptation to toxin within each treatment population was quantified every generation by measuring survival and growth of larvae on artificial diet containing a low concentration of B. thuringiensis toxin.In three of four replicates, H. virescens populations exposed to N. rileyi adapted to B. thuringiensis toxin more quickly than populations not exposed. These results supported our hypothesis that the pathogen should accelerate adaptation to toxic plants. However, this hypothesis was contradicted by the result in one replicate, in which the population not exposed to fungus adapted to toxic plants faster. This opposite result could not be explained, but it suggests that there may be substantial variation in herbivore evolution in tritrophic systems.H. virescens populations selected in the presence of fungus and in the absence of fungus did not differ in feeding or in mortality when placed on leaf disks treated with conidia. Thus, populations exposed to N. rileyi on plants for 7–8 generations displayed neither physiological nor behavioral adaptation to N. rileyi.     

转抗虫基因植物对蜜蜂的影响

苏云金杆菌 (Bacillusthuringiensis,Bt)毒蛋白基因、蛋白酶抑制剂基因是广泛用于植物抗虫基因工程的两大类基因。Bt毒蛋白对蜜蜂没有明显毒害作用 ,但对草蛉、瓢虫等有益昆虫的繁殖、发育具有不良影响 ,而且在花粉中表达 ,因此转Bt基因植物对蜜蜂的影响有待于进一步研究。蛋白酶抑制剂浓度高时 ,对蜜蜂具有明显的毒害作用。随着基因工程技术的发展 ,蛋白酶抑制剂基因表达水平的提高 ,转基因植物必将对蜜蜂产生一些不良影响。蜜蜂仅取食植物的花蜜和花粉 ,可以采用不同的启动子 ,使抗虫基因只在害虫取食部位表达 ,而在花蜜和花粉中不表达 ,以确保既能抗虫 ,又对蜜蜂安全     

Effects of natural enemies on the rate of herbivore adaptation to resistant host plants

The potential of natural enemies to influence the rate of herbivore adaptation to resistance factors in plants is examined using conceptual and mathematical models. Results indicate that natural enemies could increase or decrease the rate of herbivore adaptation. The specific behavioral and physiological effects of a resistance factor on the herbivore, as well as the behavior of the natural enemy, and the population dynamics of the natural enemy/herbivore system are important in assessing the extent to which the natural enemies will affect the rate of herbivore adaptation to a resistance factor. Herbivore adaptation to partial resistance in a host-plant is generally expected to be slower than adaptation to high levels of resistance, even in the presence of natural enemies, if genetic variance is not limiting.

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Résumé Des modèles conceptuels et mathématiques ont servi à examiner l'influence du potentiel d'ennemis naturels sur le taux d'adaptation d'herbivores à des facteurs de résistance des plantes. Les résultats montrent que les ennemis naturels peuvent augmenter ou réduire le taux d'adaptation des herbivores. Les effets spécifiques (comportementaux et physiologiques) d'un facteur de résistance sur l'herbivore, aussi bien que le comportement de l'ennemi naturel et la dynamique de population du système ennemi naturel/herbivore permettent de déterminer le niveau de l'effet des ennemis naturels sur le taux d'adaptation de l'herbivore au facteur de résistance. Une adaptation de l'herbivore à une résistance partielle à une plante est généralement envisagée comme plus lente qu'une adaptation à des hauts niveaux de résistance, même en présence d'ennemis naturels, mais il peut y avoir des exceptions.

     

天敌与转Bt基因抗虫植物的协同控害作用

本文概述了转Bt基因抗虫植物对天敌的影响,特别是天敌与转Bt基因抗虫植物的协同控害作用及其对寄生昆虫抗生发展的影响研究进展。已有的研究表明转Bt基因抗虫植物对天敌并无明显不利的影响,但转Bt基因抗虫植物与天敌的协同控害作用表现出拮抗性、加和性及增效性等多种形式,这种协同作用可能还将影响到害虫对转Bt基因抗虫植物的速率。     

The relative importance of resources and natural enemies in determining herbivore abundance: thistles, tephritids and parasitoids

1. The relative importance of host-plant resources and natural enemies in influencing the abundance of insect herbivores was investigated in potted plant and natural population experiments, using tephritid (Diptera: Tephritidae) flies, their host plant, creeping thistle Cirsium arvense, and their Hymenoptera parasitoids. 2. Experimental manipulation of host-plant quality (i.e. levels of host-plant nutrients) and resource availability (i.e. the number of buds) increased tephritid abundance. There was no evidence that the seed-feeding tephritid fly Xyphosia miliaria preferentially oviposited on fertilized C. arvense. 3. At low thistle densities, X. miliaria showed a constant rate of resource exploitation. At higher thistle densities, a threshold was detected, above which additional buds were not attacked. 4. Parasitism attack was variable across host (tephritid) densities but levels of parasitism were consistently higher on the fertilized thistles. 5. Experimental manipulation of host-plant quality and resource availability (quantity) not only directly affects the tephritid population but also, indirectly, leads to high rates of parasitism. Both chemical and physical characteristics of host plants affect the performance of natural enemies. 6. Both top-down and bottom-up forces act to influence tephritid abundance, with bottom-up influences appearing to be the most important.     

Differences in the midgut proteolytic activity of two Heliothis virescens strains,one susceptible and one resistant to Bacillus thuringiensis toxins

The development of resistance to Bacillus thuringiensis toxic proteins is a growing concern because it could threaten both conventional and gene transfer use of this environmentally safe biological insecticide. The most common mechanism of resistance involves changes in binding affinity of toxin receptors in the insect midgut membrane. This has not been the case in Heliothis virescens. We have investigated changes in midgut proteolytic activity as a possibility to explain the resistance observed in this insect species. We have developed an improvement of known methods to demonstrate proteolytic activity in crude extracts. Using this method we have found differences in the proteolytic activity profile of midgut extracts of a susceptible and a resistant H. virescens strain. We also have studied the in vitro processing of CrylA(b) toxin and protoxin by midgut contents of both strains. SDS-PAGE of the in vitro degradation products showed differences between the strains. The resistant strain degrades protoxin more slowly and processes the active toxin more quickly than the susceptible strain. © 1996 Wiley-Liss, Inc.     

Foraging behavior of Campoletis sonorensis in response to Heliothis virescens and cotton plants

Wind tunnel flight behavior of inexperienced female Campoletis sonorensis (Cameron) (Hymenoptera; Ichneumonidae) in response to its larval host Heliothis virescens (F.) (Lepidoptera: Noctuidae) feeding on the host plant cotton (Gossypium hirsutum L.) is described. The flight behavioral sequence was determined by quantification of frequencies of observed behaviors and probabilities of first-order behavioral transitions. Comparison of inexperienced C. sonorensis flights to undamaged and damaged cotton indicated that stimuli from undamaged plants alone are adequate to elicit the complete flight behavioral sequence observed in response to H. virescens feeding on cotton.Parasitoid foraging behavior was also analyzed after landing on the stimulus. This behavior appeared to be random in its initial stages, but became sequential after location of evidence of a host. Analysis of foraging on undamaged and 3 treatments of damaged cotton resulted in the determination that parasitoids tend to remain on damaged plants longer than undamaged plants although no significant difference was detected. C. sonorensis spent a greater percentage of their time foraging on host damaged plants than on undamaged plants.

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Résumé Les comportements d'évaluation de l'effectif d'hôtes potentiels et de la position des hôtes par C. sonorensis (Hyméno.: Iccheumonidae) ont été quantifiés pour déterminer les séquences significatives des événements comportementaux. La localisation de la colonie potentielle d'hôtes est apparue comme une séquence régulière des événements comportementaux. Une fois que le parasitoïde a localisé une colonie potentielle, on a constaté que la recherche au hasard des hôtes se poursuit jusqu'à ce qu'il ait la démonstration qu'il s'agit d'une plante attaquée. La localisation par le parasitoïde d'un hôte certain a consitué une autre séquence régulière des événements comportementaux terminant la localisation de l'hôte.L'influence de pieds de coton intacts, de pieds abîmés mécaniquement et de pieds occupés par des chenilles du 3ème stade de l'hôte et de plantes dont les chenilles ont été retirées juste avant l'expérience a été déterminée en modifiant la composition du complexe hôte/plante. Des femelles naïves de C. sonorensis ont montré en présence de pieds de coton intacts apparemment toutes les séquences comportementales de vol impliquées dans la localisation d'une colonie potentielle d'hôtes. Une fois que le parasitoïde a atteint la colonie potentielle d'hôtes, la présence de dégâts de l'hôte n'a pas modifié le temps passé sur la plante, mais a modifié le temps consacré à la prospection.

     

An ecological cost of plant defence: attractiveness of bitter cucumber plants to natural enemies of herbivores

Plants produce defences that act directly on herbivores and indirectly via the attraction of natural enemies of herbivores. We examined the pleiotropic effects of direct chemical defence production on indirect defence employing near‐isogenic varieties of cucumber plants (Cucumis sativus) that differ qualitatively in the production of terpenoid cucurbitacins, the most bitter compounds known. In release–recapture experiments conducted in greenhouse common gardens, blind predatory mites were attracted to plants infested by herbivorous mites. Infested sweet plants (lacking cucurbitacins), however, attracted 37% more predatory mites than infested bitter plants (that produce constitutive and inducible cucurbitacins). Analysis of the headspace of plants revealed that production of cucurbitacins was genetically correlated with large increases in the qualitative and quantitative spectrum of volatile compounds produced by plants, including induced production of (E )‐β‐ocimene (3E )‐4,8‐dimethyl‐1,3,7‐nonatriene, (E,E)‐α‐farnesene, and methyl salicylate, all known to be attractants of predators. Nevertheless, plants that produced cucurbitacins were less attractive to predatory mites than plants that lacked cucurbitacins and predators were also half as fecund on these bitter plants. Thus, we provide novel evidence for an ecological trade‐off between direct and indirect plant defence. This cost of defence is mediated by the effects of cucurbitacins on predator fecundity and potentially by the production of volatile compounds that may be repellent to predators.     

Effect of Bt corn leaf suspension on food consumption by Chilo partellus and life history parameters of its parasitoid Cotesia flavipes under laboratory conditions

Laboratory scale experiments were conducted in order to assess the potential effect of Bacillus thuringiensis‐corn leaf material on the parasitized herbivore Chilo partellus Swinhoe (Lepidoptera: Crambidae) and on its parasitoid Cotesia flavipes (Cameron) (Hymenoptera: Braconidae). Food consumption and relative consumption rate of parasitized hosts exposed to Bt‐corn leaf‐material were strongly reduced compared to the control. The number of hosts allowing parasitoid larvae to complete their development was also reduced in the Bt group. Moreover, the fresh weight of parasitoid pupae and the dry weight of parasitoid adults was lower than in the control. Only in the Bt group, were strong negative correlations found between food intake by the host, and the number of parasitoid cocoons. Strong positive correlations were also only found in the Bt group, between food intake and parasitoid development time. As effects of Bt on the oviposition behaviour of C. flavipes could be excluded, differences between the Bt group and the control could only be due to the effect of Bt toxin on the parasitoid larva developing inside the host. Whenever food consumption can be measured, the methods used in this study are proposed as a model for future risk assessments on different types of insect‐resistant transgenic plants, herbivores, parasitoids, and predators.     

Bacillus thuringiensis: a century of research, development and commercial applications

Bacillus thuringiensis (Bt) is a soil bacterium that forms spores during the stationary phase of its growth cycle. The spores contain crystals, predominantly comprising one or more Cry and/or Cyt proteins (also known as δ-endotoxins) that have potent and specific insecticidal activity. Different strains of Bt produce different types of toxin, each of which affects a narrow taxonomic group of insects. Therefore, Bt toxins have been used as topical pesticides to protect crops, and more recently the proteins have been expressed in transgenic plants to confer inherent pest resistance. Bt transgenic crops have been overwhelmingly successful and beneficial, leading to higher yields and reducing the use of chemical pesticides and fossil fuels. However, their deployment has attracted some criticism particularly with regard to the potential evolution of pest-resistant insect strains. Here, we review recent progress in the development of Bt technology and the countermeasures that have been introduced to prevent the evolution of resistant insect populations.     

Responses of Microplitis croceipes to host and nonhost plants of Heliothis virescens in a wind tunnel

The relative attractiveness of velvet leaf (Abutilon theophrasti Medicus), cotton (Gossypium hirsutum L.) (host plants) and groundcherry (Physalis angulata L.) (nonhost plant), and cotton plants with or without nectaries and with or without glands to Microplitis croceipes Cresson (Hymenoptera: Braconidae) was determined in a wind tunnel. Female parasitoids flew significantly more to glandless than to glanded cotton; response to nectaried and nectariless cotton was similar. Velvet leaf and cotton were favored significantly over groundcherry; parasitoids being equally responsive to both host plant species. Addition of larval frass alone or in combination with host larvae significantly improved the attractivity of the nonhost plant (groundcherry) to the parasitoids. There was no difference in attractiveness of groundcherry terminals with or without host larvae. Parasitoid search time was significantly increased with addition of larval frass. In the presence of cotton, however, kairomone-treated groundcherry remained unattractive.

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Résumé L'influence des nectaires et des glandes du coton, et l'attractivité des feuilles veloutées du coton (plante hôte) et de Physalis angulata sur Microplitis croceipes, ont été déterminées dans des expériences avec tunnel à vent. Les résultats ont montré que les parasitoïdes femelles sont significativement plus attirés par les cotons sans glandes que par les cotons glanduleux, tandis que la présence ou l'absence de nectaires ne modifie pas l'attractivité du coton. Elles réagissent de la même façon aux feuilles veloutées. P. angulata traité avec des crottes de chenilles, présenté seul ou en combinaison avec du coton, attire plus de femelles que des pousses de P. angulata non traitées. L'addition de chenilles hôtes seules n'améliore pas significativement l'attractivité de la plante. P. angulata traité avec des kairomones est demeuré inattractif quand il était proposé en même temps que du coton.

     

Population-scale laboratory studies of the effect of transgenic plants on nontarget insects

Studies of the effects of insect-resistant transgenic plants on beneficial insects have, to date, concentrated mainly on either small-scale "worst case scenario" laboratory experiments or on field trials. We present a laboratory method using large population cages that represent an intermediate experimental scale, allowing the study of ecological and behavioural interactions between transgenic plants, pests and their natural enemies under more controlled conditions than is possible in the field. Previous studies have also concentrated on natural enemies of lepidopteran and coleopteran target pests. However, natural enemies of other pests, which are not controlled by the transgenic plants, are also potentially exposed to the transgene product when feeding on hosts. The reduction in the use of insecticides on transgenic crops could lead to increasing problems with such nontarget pests, normally controlled by sprays, especially if there are any negative effects of the transgenic plant on their natural enemies. This study tested two lines of insect-resistant transgenic oilseed rape (Brassica napus) for side-effects on the hymenopteran parasitoid Diaeretiella rapae and its aphid host, Myzus persicae. One transgenic line expressed the delta-endotoxin Cry1Ac from Bacillus thuringiensis (Bt) and a second expressed the proteinase inhibitor oryzacystatin I (OC-I) from rice. These transgenic plant lines were developed to provide resistance to lepidopteran and coleopteran pests, respectively. No detrimental effects of the transgenic oilseed rape lines on the ability of the parasitoid to control aphid populations were observed. Adult parasitoid emergence and sex ratio were also not consistently altered on the transgenic oilseed rape lines compared with the wild-type lines.     

The impact of transgenic plants on natural enemies: a critical review of laboratory studies

We reviewed laboratory tests which studied the impact of genetically modified plants on arthropod natural enemies. A total of 18 species of predators and 14 species of parasitoids have been tested, most in only a few experiments. Certain groups (braconid wasps) or species (the green lacewing, Chrysoperla carnea) have attracted much effort, while representatives of others, including whole orders (e.g., Diptera), have never had a species tested. We conclude that laboratory tests are not the ‘worst case’ scenarios intended by the experimental designs, and are not often ecologically realistic: they typically provided ad libitum feeding, no prey choice, single prey type, no combination of stress factors and usually uniform temperatures. None of these are representative of field conditions, yet most could be easily mimicked in more complex laboratory tests. In most cases (94.6%), the studies were unable to indicate the level of power required to detect any impact. Small sample size and large variability are factors that mask all but very large differences in potential effects. For predators, 126 parameters were quantified, most commonly including survival/mortality (37 cases), development time (22), and body mass/size (20). For parasitoids, 128 parameters were quantified, the majority involving lectins or proteinase inhibitors. Most frequent measurements were: fecundity (23 experiments), adult longevity, extent of parasitism (17 each), body size, mortality, and larval development time. An aggregative scoring (summarising all quantified parameters) indicated that the laboratory tests quantified a remarkable number of cases (30% for predators, 39.8% for parasitoids), where the impacts of the genetically modified plant were significantly negative. These involve various parameters, organisms, test methods, and significance levels, but collectively they indicate that the use of genetically modified crops may result in negative effects on the natural enemies of crop pests.     

Effects of transgenic Bt corn litter on the earthworm Lumbricus terrestris

A 200-day study was carried out to investigate the impact of transgenic Bacillus thuringiensis (Bt) corn on immature and adult Lumbricus terrestris in the field and in the laboratory. Another objective of this study was to develop test methods that could be used for standard testing of the impact of transgenic plants on different earthworm species in the field and in the laboratory. For this purpose two different experiments were involved, a laboratory experiment with adult L. terrestris and a field experiment with immature L. terrestris. No lethal effects of transgenic Bt corn on immature and adult earthworms were observed. Immature L. terrestris in the field had a very similar growth pattern when fed either (Bt+) or (Bt-) corn litter. No significant differences in relative weights of (Bt+) and (Bt-) corn-fed adult L. terrestris were observed during the first 160 days of the laboratory trial, but after 200 days adult L. terrestris had a significant weight loss of 18% of their initial weight when fed (Bt+) corn litter compared to a weight gain of 4% of the initial weight of (Bt-) corn-fed earthworms. Further studies are necessary to see whether or not this difference in relative weight was due to the Bt toxin or other factors discussed in the study. Degradation of Cry1Ab toxin in corn residues was significantly slower in the field than at 10 degrees C in the laboratory. Enzyme-linked immunosorbent assay results indicated that earthworms in both experiments were exposed to the Bt toxin throughout the whole experimental time.     

Effects of crop type on Bacillus thuringiensis toxicity and residual activity against Trichoplusia ni in greenhouses

We assessed the efficacy and persistence of a Bacillus thuringiensiskurstaki (Btk) formulation (Dipel) against Trichoplusia ni (Hubner) (Lep., Noctuidae), the cabbage looper, on three greenhouse vegetable crops (tomato, bell pepper and cucumber). First, T. ni larvae were fed leaf discs treated with Btk to assess how Btk toxicity varies with host plant. Secondly, T. ni larvae were fed leaf discs harvested from plants that had been sprayed with Btk 1, 5 and 9 days previously to assess the residual activity of Btk toxicity in greenhouse environments. Mortality of T. ni larvae fed tomato leaf discs was significantly higher than T. ni fed cucumber or pepper leaf discs. The toxicity of Btk had declined by less than 50% after 9 days, which suggests that Btk persistence is lengthy in greenhouse environments. No crop effects on the residual activity of Btk were found. These results demonstrate that the greenhouse environment and the crop should be considered when using Btk for insect management on greenhouse crops.     

Susceptibility of field populations of sugarcane borer from non‐Bt and Bt maize plants to five individual Cry toxins

Abstract Sugarcane borer, Diatraea saccharalis (F.), is a major target of transgenic maize expressing Bacillus thuringiensis (Bt) proteins in South America and the US mid‐south region. Resistance development in target pest populations is a major threat to the sustainable use of Bt crops. In our field trials in 2009, a significant number of live borers and plant injury from D. saccharalis were observed in an experimental SmartStax? maize line. The objective of this study was to assess the relative susceptibility of two field populations of D. saccharalis collected from non‐Bt and Bt maize plants containing SmartStax? traits to five individual Cry proteins. The five Bt proteins included two proteins (Cry1A.105 and Cry2Ab2) that were expressed in SmartStax? maize plants and three other common Bt proteins (Cry1Aa, Cry1Ab and Cry1Ac) that were not produced in SmartStax?. Larval mortality and growth inhibition on Bt diet of the fourth generation after field collections were evaluated 7 days after release of neonates on the diet surface. The laboratory bioassays showed that 50% lethal concentration (LC₅₀) values for Cry1A.105 and Cry2Ab2 for the population originated from Bt plants were 3.55‐ and 1.34‐fold greater, respectively, than those of the population collected from non‐Bt plants. In contrast, relative to the population from non‐Bt plants, the LC₅₀ of the population sampled from Bt plants were 3.85‐, 2.5‐ and 1.64‐fold more sensitive to Cry1Aa, Cry1Ab and Cry1Ac, respectively. The results did not provide clear evidence to conclude that the observed field survival of D. saccharalis on Bt plants was associated with increased levels of resistance.