health (Reitz and Caldwell 1974). For example, reed canary grass varies in palatability as a forage crop: the least palatable cultivars have the highest alkaloid concentrations (Williams et al. 1971). Cultivars of alfalfa vary in content of saponin which can depress the growth of chickens (Hanson et al. 1973; Reitz and Caldwell 1974). The terpenoid gossypol makes cotton resistant to caterpillar pests (Shaver and Lukefahr 1969), but cottonseed meal from high gossypol cultivars is poisonous to swine and causes darkened yolks in eggs of chickens (Reitz and Caldwell 1974).
Some plant toxins similar to those in pest-protected plants are relatively stable and can be found in decaying plant tissues (Horner et al. 1988). Only a few studies have examined the effects of such compounds on detritivores (Horner et al. 1988; Paavolainen et al. 1998). One study of balsam poplar showed that the tannins and phenolic chemicals in fallen leaves could inhibit mineralization through two interactions with detritivores. The result can be reduced soil-nitrogen availability (Schimel et al. 1996).
Bt crops have not had an impact on honeybees in tests conducted for EPA approval of Bt crops (EPA 1998c). However, at high concentrations the Cry1Ab toxins engineered into crops have been shown to be toxic to Collembola that are part of the detritus food chain (EPA 1997a). Another Bt toxin, Cry9C, was not found to be toxic to the same Collembola species (EPA 1998c). Cry1Ab but not Cry9C was found to have some toxicity to Daphnia (EPA 1997a and 1998c).
Few studies have focused on measuring the impact of pest-protected plants on the population dynamics of insects (reviewed in Gould 1998). Likewise, few have examined the direct physiological effects of Bt on nontarget herbivores, but some nontarget lepidopterans feeding on crops that contain Cry1A Bt toxins are likely to be affected. These nontarget insects would include lepidopterans that are not pests (that is, do not provide significant damage to the crop) and lepidopteran pests that are not sufficiently affected enough by a pest-protected plant for pest-protection of this kind to be economically useful.
Pollen from wind-pollinated pest-protected plants can be deposited on nearby vegetation (figure 2.2) and inadvertently ingested by nontarget leaf-eating insects. For example, a small fraction of corn pollen is known to disperse up to180 ft from the edge of the crop (Raynor et al. 1972) and can be deposited on milkweed (Asclepias sp.), which are common in and along edges of corn fields in the midwestern United States where about half the population of US monarchs spend some of the summer (Wassenaar and Hobson 1998). Milkweed is the only food of monarch