based on the balance between adverse effects on beneficial and detrimental species. For example, Kauffman and Flanders (1985) found an adverse effect of pest-protected soybean on a parasitoid of the Mexican bean beetle, but the effect on the beetle pest was greater than on the parasitoid; it was therefore concluded that overall the cultivar would be useful. Hare (1992) reviewed the published data on overall effects of pest-protected host plants on biological control. Of the 16 case studies he reviewed, there were antagonistic interactions in six, positive synergistic interactions in two, neutral effects in five, and a concentration-sensitive relationship in three.
There have been a few published studies of the indirect effect of transgenic Bt-producing crops on natural enemies. Depending on the species involved some studies reported no significant effects (for example, Hough-Goldstein and Keil 1991), but others have found adverse effects. Giroux et al. (1994) reported that the ladybug predator of the Colorado potato beetle consumed fewer potato beetle eggs when potato Bt levels were high. Hillbeck et al. (1998a, b) found that when chrysopid larvae were reared on prey that were fed Bt-producing corn, they suffered 62% mortality. When they were reared on prey that were fed non-Bt corn, mortality was only 37%. Those results were found with two prey species, one sensitive to Bt and the other insensitive. The results indicate that it was the Bt toxin in the bodies of the prey, and not simply unhealthy prey, that caused the heightened mortality.
It is important to ask whether such indirect effects will have a harmful effect on the agroecosystem. In some cases, the use of conventional pest-protected plants has lead to decreased foraging efficiency of predators and parasitoids (Boethel and Eikenbary 1986). A few field studies have attempted to measure the effect of transgenic Bt potatoes on the diversity of insects in crop fields (Hoy et al. 1998). In one 4-year study at five locations in the Wisconsin potato-production system, it was found that populations of predators were, on the average, 63.8% lower in fields where non-Bt potatoes were managed with typical insecticides than in fields of transgenic Bt potatoes. Parasitoid populations in the chemically treated potato fields were, on the average, 58.4% lower than in Bt potato fields. Insecticidal treatment of potatoes to control Colorado potato beetles with conventional insecticides often results in secondary outbreaks of aphids because the aphids are released from biological control. In three of the four years of the study, aphid populations (which are not affected by Bt toxins) were lower in the transgenic Bt potato fields than in chemically treated potato fields, presumably because of greater biological control.
A study of potatoes in Ohio reported similar results (Hoy et al. 1998). In potato fields treated with pyrethroid insecticide to manage Colorado