Bt Crops and the Environment

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COMMENTARY | Bt CORN POLLEN IMPACTS ON NONTARGET LEPIDOPTERA: ASSESSMENT OF EFFECTS IN NATURE Proceedings of the National Academy of Sciences (2000) David S. Pimentel - and Peter H. Raven*

The demonstrations by Losey et al. and Hansen and Obrycki that milkweed leaves dusted with heavy concentrations of Bt corn pollen are toxic to Monarch butterfly larvae (Danaus plexippus) feeding on them were consistent with the known toxicity of Bt endotoxin to Lepidoptera in general and the expression of Bt endotoxins in the pollen of the strains of corn they studied.
Much speculation and some investigations followed, concerning the extent to which the poisoning of Monarch butterflies and other nontarget Lepidoptera might be significant contributors to the mortality of these insects in nature. For example, Shelton and Roush (2) were critical of the two earlier findings, but did not provide any data from nature, despite the fact that Losey et al., in the original report, stated ‘‘it would be inappropriate to draw any conclusions about the risk to Monarch populations in the field based solely on these initial results.’’
In a recent issue of PNAS, Wraight et al. reported their experiments with populations of black swallowtail larvae (Papilio polyxenes) under field conditions. The food plants were located at varying distances from plantings of Bt corn; the authors found no effects on the mortality of the larvae. These results demonstrated that Bt corn pollen from this corn strain is not toxic to this species of butterfly at levels observed in the field no matter how close the larval food plants were to the pollen-shedding corn plants. It has not been demonstrated, however, that Monarch butterflies and black swallowtails are equally susceptible to Bt endotoxins.
Further, the pollen of the corn strain 176 used by Hansen and Obrycki in their experiments with Monarch butterfies was demonstrated by Wraight et al. to be lethal to black swallowtail larvae in the laboratory, whereas the pollen of the strain they used in their field experiments, with 1 y 40 of the Bt endotoxin level of strain 176, was not.
Studies of the effects of corn strain 176 pollen on both black swallowtails and Monarch butterflies in the field are underway in the summer of 2000. For their original report, Losey et al. used still a different corn strain, N4640. At any event, the level of Bt endotoxin in the pollen of the particular corn strain, as expected, has a direct effect on the survival of black swallowtail larvae in the laboratory and presumably on the larvae of other butterfly species as well.
It has not been demonstrated whether different species of butterflies have varying levels of tolerance to Bt toxin, but they probably do, as demonstrated with other toxins. Taking the overall picture into account, the effect on the survival of butterfly populations of Bt corn pollen dusting their larval food plants appears to be relatively insignificant compared with other factors.
. . . Considering the enormous damage caused to human health and to biodiversity through the application of pesticides, it is clear that all efforts should continue to improve crop productivity while reducing the amounts of pesticides applied. In this connection, plants genetically modified to produce Bt endotoxin or other toxins clearly have an important role to play. The environmental effects of Bt endotoxin, freely sprayed or produced by genetically modified corn and other crops, have been assessed for many years, should continue to be evaluated.