The area devoted to growing transgenic plants expressing insecticidal Cry proteins derived from Bacillus thuringiensis (Bt) is increasing worldwide. A major concern with the adoption of Bt crops is their potential impact on nontarget organisms including biological control organisms. Regulatory frameworks should advocate a step-wise (tiered) approach to assess possible nontarget effects of Bt crops. Laboratory and glasshouse studies have revealed effects on natural enemies only when Bt-susceptible, sublethally damaged herbivores were used as prey or host, with no indication of direct toxic effects. Field studies have confirmed that the abundance and activity of parasitoids and predators are similar in Bt and non-Bt crops. In contrast, applications of conventional insecticides have usually resulted in negative impacts on biological control organisms. Because Bt-transgenic varieties can lead to substantial reductions in insecticide use in some crops, they can contribute to integrated pest management systems with a strong biological control component.

Transgenic crops expressing Bacillus thuringiensis toxins and biological control.

Insect resistance, based on Bacillus thuringiensis (Bt) endotoxins, is the second most widely used trait (after herbicide resistance) in commercial genetically modified (GM) crops. Other modifications for insect resistance, such as proteinase inhibitors and lectins, are also being used in many experimental crops. The extensive testing on nontarget plant-feeding insects and beneficial species that has accompanied the long-term and wide-scale use of Bt plants has not detected significant adverse effects. GM plants expressing other insect-resistant proteins that have a broader spectrum of activity have been tested on only a limited number of nontarget species. Little is known about the persistence of transgene-derived proteins in soil, with the exception of Bt endotoxins, which can persist in soil for several months. Bt plants appear to have little impact on soil biota such as earthworms, collembolans, and general soil microflora. Further research is required on the effects of GM plants on soil processes such as decomposition. Assessment of nontarget impacts is an essential part of the risk assessment process for insect-resistant GM plants.

Effects of plants genetically modified for insect resistance on nontarget organisms

The ubiquitous nature of Bacillus thuringiensis (Bt), a Gram-positive bacterium capable of producing crystal proteins with insecticidal activity during sporulation, is now being mirrored in major crops plants that have been engineered through recombinant DNA to carry genes responsible for producing these crystal proteins and providing host plant resistance to major lepidopteran and coleopteran pests. In 2007, the 11th year of commercial production, Bt maize and Bt cotton were commercially produced on a total of 42 million hectares in 20 countries. Assessment of environmental safety has been and continues to be a key element of transgenic crop technology. This review focuses on two environmental elements, effects on non-target invertebrates and changes in insecticide use patterns since the adoption of Bt maize and cotton. Meta-analyses of the extant literature on invertebrate non-target effects reveals that the pattern and extent of impact varies in relation to taxonomy, ecological or anthropomorphic guild, route of exposure and the non-Bt control against which effects are gauged. Hazards identified in the laboratory may not always manifest in the field and the minor negative effects of Bt crops demonstrated in the field pale in comparison with alternative pest suppression measures based on insecticides. The efficacy of Bt maize and cotton against major pest species has been associated with an estimated 136.6million kg global reduction in insecticide active ingredient used between 1996 and 2006 (29.9% reduction). Benefits vary by country and region and are heavily weighted towards cotton production, which has historically been one of the largest users of insecticides in the world.

/pdf/impacts-of-bt-crops-on-non-target-invertebrates-and-1qzcg3c0im.pdf

Impacts of Bt crops on non-target invertebrates and insecticide use patterns

With a projected population of 10 billion by 2050, an immediate priority for agriculture is to achieve increased crop yields in a sustainable and cost-effective way. The concept of using a transgenic approach was realized in the mid-1990s with the commercial introduction of genetically modified (GM) crops. By 2010, the global value of the seed alone was US $11.2 billion, with commercial biotech maize, soya bean grain and cotton valued at approximately US $150 billion. In recent years, it has become evident that insect-resistant crops expressing δ-endotoxin genes from Bacillus thuringiensis have made a significant beneficial impact on global agriculture, not least in terms of pest reduction and improved quality. However, because of the potential for pest populations to evolve resistance, and owing to lack of effective control of homopteran pests, alternative strategies are being developed. Some of these are based on Bacillus spp. or other insect pathogens, while others are based on the use of plant- and animal-derived genes. However, if such approaches are to play a useful role in crop protection, it is desirable that they do not have a negative impact on beneficial organisms at higher trophic levels thus affecting the functioning of the agro-ecosystem. This widely held concern over the ecological impacts of GM crops has led to the extensive examination of the potential effects of a range of transgene proteins on non-target and beneficial insects. The findings to date with respect to both commercial and experimental GM crops expressing anti-insect genes are discussed here, with particular emphasis on insect predators and parasitoids.

/pdf/insect-resistant-biotech-crops-and-their-impacts-on-2237ms5w27.pdf

Insect-resistant biotech crops and their impacts on beneficial arthropods

Recent studies have shown that transgenic insect resistant plants can have negative effects on non-target herbivores as well as on beneficial insects. The study of tritrophic interactions gives insight into the complex mechanisms of food webs in the field and can easily be incorporated into a tiered risk assessment framework. We investigated the effects of transgenic maize (Zea mays) expressing insecticidal proteins derived from Bacillus thuringiensis (Bt maize) on Spodoptera littoralis, a non-target herbivore, and on the hymenopteran parasitoid Cotesia marginiventris. In a laboratory study, S. littoralis larvae were reared for their whole lifespan on a mixture of leaves and stems from 2-4-week old Bt maize plants. S. littoralis survival, developmental times and larval weights were significantly affected by Bt maize diet. However, adult moths, which survived development on Bt maize, were the same size as the adults from the control group. C. marginiventris survival, developmental times and cocoon weights were significantly negatively affected if their S. littoralis host larva had been fed Bt maize. ELISA tests confirmed that S. littoralis larvae ingest high amounts of CrylA(b) toxin while feeding on Bt maize. In S. littoralis pupae and in C. marginiventris cocoon silk, only traces of the toxin could be detected. No toxin was found in S. littoralis and C. marginiventris adults. Thus the toxin is not accumulating in the trophic levels and in fact appears to be excreted. Our results suggest that the effects on C. marginiventris when developing in susceptible S. littoralis larvae are indirect (host mediated). The biological relevance of those results and the significance of this study in risk assessment are discussed.

Effects of Bt maize on the herbivore Spodoptera littoralis (Lepidoptera: Noctuidae) and the parasitoid Cotesia marginiventris (Hymenoptera: Braconidae)

Various aspects were studied of the brood size and sex allocation strategies, and of size-fitness relationships in Parallorhogas pyralophagus (Marsh), a gregarious ectoparasitoid of Eoreuma loftini Dyar. Brood size was significantly correlated with host size; larger hosts were allocated larger broods. Brood sex ratios were fixed precisely at 1 male per 4 females, and eggs were likely to be deposited in that order; differential mortality did not contribute to this precise sex ratio. The sex allocation strategy of P. pyralophagus is likely to conform to strict, i.e. single foundress, local mate competition. Adoption of this strategy is probably influenced by a limited insemination capacity of males; a smaller proportion of females (0.09 vs. 0.21) remained virgin in broods with precise or higher sex ratios (> or = 0.20 males) relative to broods with lower than precise sex ratios (< 0.20 males). Moreover, all females were inseminated in most broods (60%) with precise or higher sex ratios, whereas this did not occur in broods with lower than precise sex ratios. The hypothesized occurrence of strict local mate competition in P. pyralophagus was supported also by observations that: (i) offspring brood sex ratios were independent of maternal brood sex ratios and number of parental females concurrently allocating offspring to a group of hosts, and; (ii) the rate of superparasitism under no-choice conditions was low (approximately 20%), suggesting that rates of outbreeding in the field are low. Other results suggested that fitness in P. pyralophagus was correlated with adult size; longevity and reproductive capacity both increased with adult size in males and females. However, adult size may be more important for females than for males because the differences in reproductive capacity between the largest and smallest individuals was up to 7.3 times greater in females versus < 2 times in males.

Family planning in a stemborer parasitoid: sex ratio, brood size and size-fitness relationships in Parallorhogas pyralophagus (Hymenoptera: Braconidae), and implications for biological control.

Transgenic Bt maize cultivars are widely used in U.S. agriculture. Control of target pests by these cultivars is not complete, and non-target pests are usually tolerant of the toxic activity of these cultivars. Sublethally intoxicated pest individuals may become hosts for parasitoids, but their quality as hosts may be affected as a result of intoxication. This study addresses the effects on various fitness parameters in parasitoids that develop on intoxicated hosts. The parasitoid used in this study was Parallorhogas pyralophagus (Marsh), a gregarious, external idiobiont, and the host was Eoreuma loftini Dyar, a subtropical stemborer. Results showed that ingestion of Bt maize tissue by E. loftini larvae negatively affected some fitness components in P. pyralophagus, whereas other components were not affected. Specifically, immature stage developmental mortality was greater, adult longevity was approximately 1 d shorter in females, and developmental times were approximately 2 d longer in both males and fem...

Impacts of developing on Bt maize-intoxicated hosts on fitness parameters of a stem borer parasitoid

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Family planning in a stemborer parasitoid: sex ratio, brood size and size-fitness relationships in Parallorhogas pyralophagus (Hymenoptera: Braconidae), and implications for biological control.

Impacts of developing on Bt maize-intoxicated hosts on fitness parameters of a stem borer parasitoid