Impact on insects and animals

The first successful example of using a foreign plant gene to confer resistance to insects was reported in 1987 (Hilder et al. 1987) and involved transformation of tobacco (Nicotianum tabacum) with the cow pea trypsin inhibitor (CpTi) gene. Since then there have been many reports of success in insect management using transgenic crop varieties.

The bacterial endotoxins isolated from Bacillus thuringiensis (Bt), comprise one of several groups of proteins which have been shown to have insecticidal properties to a range of economically important insects. Transgenic crop varieties engineered with Bt resistance are already in commercial use in the USA and China, while a number of plant proteins, such as inhibitors of proteases, lectins and other digestive enzymes, are being evaluated for their efficacy as insect-resistance mechanisms (Gatehouse et al. 1998).

It is important that genes selected for the control of insect pests have acceptably little effect on non-target insects including predators of the target pest insects, in order to maintain insect diversity in GM crops. Clearly, if there is an effect upon predators that is comparable with current control practices then little benefit will accrue from the deployment of GM crops, a point made strongly by the Royal Society for the Protection of Birds in their submissions on GMOs (1997). Impact assessments are therefore required to examine the effects on non-target organisms in the crop environment.

non-target species have provided, at best, equivocal and often controversial results which have served only to fuel the GM debate rather than provide hard scientific facts on which to base a thorough impact assessment.

Research into the impact of potato plants expressing the snowdrop lectin GNA upon 2-spot ladybirds which feed on the aphidy Msus persicae demonstrated that the ladybirds were affected adversely in terms of fecundity, egg viability and longevity (Birch et al. 1998). However, the authors point out that the effects may either be a direct result of ladybirds preying on aphids which have digested transgenic plant material containing the lectin, or may also be due to poor nutritional quality of the aphids themselves as a food source. Other studies involving the parasitic was Epulophus pennicornis and the tomato moth Lacanobiua oleracea demonstrated that the parasitic wasp was not affected when it parasitised moth larvae reared on transgenic potato plants expressing the snowdrop lectin GNA (Gatehouse et al. 1997).

More recently Losey et al. (1999) published a report indicating that pollen from transgenic Bt-resistant maize plants had a detrimental effect on the larvae of the non-target Monarch butterfly (Danaus plexippus), which is considered to be a sensitive indicator of environmental disturbance in the USA. Larvae, which normally feed on the leaves of the milkweed (Asclepias curassavica) plant were fed on leaves that had been dusted with unquantified amounts of pollen from the transgenic Bt maize plants. Results indicated that larval survival rate was only 56% compared to 100% survival for larvae fed on leaves dusted with untransformed pollen. Superficially these results indicate an unacceptable environmental impact from Bt maize. However, closer analyses have revealed a number of serious criticisms of the research, including the use of laboratory studies only, no-choice feeding regimes, lack of stringency, lack of quantification and the use of inappropriate controls (Hodgson 1999). The experiments were not conducted in the field sono in vivo data were available to confirm that (a) milkweeds occur in maize fields, and (b) that Monarch butterflies occur on these milkweeds bearing in mind the insecticide programme received by conventional maize. This once again reiterates the requirement for comprehensive risk assessments based on thorough science.

Schuler et al. (1999) have conducted research concerning the environmental effects of Bt-resistant GM oilseed rape on a non-target insect. The results demonstrated that the behaviour of non-target insects can also play a part in determining how Bt plants will affect their populations and should be considered when trying to evaluate the environmental impact of GM crops. Their laboratory-based experiments evaluated the ecological impact of the GM crop on the diamondback moth (Plutella xylostella), a pest that damages the oilseed rape crop, as well as the natural bio-control agent of the diamondback moth, a parasitic wasp (Cotesia plutellae), which kills the moths’ caterpillars by laying its eggs in them. Results demonstrated that parasitoid wasp larvae that were oviposited in Bt-susceptible moth larvae not surprisingly died with their hosts. In contrast wasp larvae that had been oviposited in Bt-resistant moth larvae feeding on transgenic plants survived and demonstrated no adverse effects of exposure to the Bt toxins either as adults or in the development of their own larvae.

The research group then examined the behaviour of the female parasitic wasps in the presence of GM and non-GM leaves. It is known that the female wasps locate the host diamondback moth larvae using herbivore-induced volatiles released from the damaged plants. A wind-tunnel was used to compare the flight response of the wasp towards Bt-susceptible and Bt-resistant diamondback larvae which were allowed to feed on Bt leaves. The flight and feeding behaviour of each wasp was then measured. In this test, 79% of the parasitoids flew to Bt the leaves damaged by resistant moth larvae, with only 21% choosing Bt leaves damaged by susceptible larvae. The apparent lack of effect on the survival or host-seeking ability of the parasitic wasp suggested that Bt plants may have an environmental advantage over broad-spectrum insecticides.