potato beetles, natural enemies were reduced to undetectable levels. This resulted in aphid outbreaks that could be only marginally controlled by insecticides (253 aphids per meter of row in late July). In the transgenic Bt potatoes where natural enemies were always detectable (1 to 4 per meter), the aphid density was very low (below 10 per meter).
Those two studies compared the impacts of chemically-intensive potato farming and farming with transgenic Bt-potato. No comparison with a no-chemical control treatment was presented, but it might have been inappropriate. Another study provided to the committee indicated that there were no significant differences in beneficial arthropods between plots in which transgenic Bt potatoes or microbial Bt sprays were used (Feldman et al. 1994). Both had similar densities of beneficial arthropods, while plots in which chemical insecticides were used had much lower densities of some of these arthropods (Feldman et al. 1994).
In most of the areas where Bt potatoes will be used commercially, the crop is usually protected against the target Colorado potato beetle with conventional pesticides. In the corn system, most of the acreage has not been treated for the European corn borer, the major target pest of the Bt corn cultivars, because the insect feeds mostly within the plant stem where pesticides are typically not effective. In addition, pesticide treatment is more expensive than the yield losses due to the borer. In this case, the appropriate comparison of environmental impacts of the transgenic Bt cultivars would be with a system where non-Bt corn was not treated with pesticides aimed at the European corn borer.
It is useful to ask what will happen to the long-term biodiversity of agroecosytems if biotechnology provides us with crops that are constitutively toxic to almost all insect herbivores. A study of insecticide effects on quail populations in soybean fields provides interesting insight. Palmer et al. (1998) found no physiologic effects of the pesticide residues on the quail. However, quail in sprayed fields had lower weight gain and lower survival than those in control fields because there were fewer insects to feed on (Palmer 1995). If in the future we continue to commercialize pest-protected crops that constitutively express more diverse insect-specific toxins, we could inadvertantly produce crops that lower general herbivore abundance. This could result in lower biodiversity of species at higher trophic levels that depend on herbivorous insects as food.
Conventional and transgenic pest-protected crops can adversely affect nontarget organisms through direct contact with or ingestion of the plant or pollen by the nontarget organisms and through indirect contact when the pest-protective substances (or their effects) are passed to other