The few data sets on nontarget impacts of transgenic pest-protected plants come mostly from crops that express Bt toxins (Hoy et al. 1998). However, a considerable number of studies have examined direct and indirect effects of conventional pest-protected crops and of wild host plants that differ in their pesticidal properties. Some of these studies are reviewed in the following sections (2.6.2 and 2.6.3).
A number of studies have shown that the leaf hairs and leaf hair exudates found in resistant cultivars can kill predators and parasitoids of insect pests directly (Bottrell and Barbosa 1998; Price et al. 1980). This can result in a loss of biocontrol of the target pest. A classic study by Rabb and Bradley (1968) demonstrated that the sticky trichomes on tobacco leaves significantly decreased parasitism of the tobacco budworm (Heliothis virescens F.). A study by Kauffman and Kennedy (1989) demonstrated that allelochemicals in the trichomes of resistant tomatoes were toxic to a parasitoid (Campoletis sonorensis) of the corn earworm (Helicoverpa zea). The trichomes in these tomato lines also had an adverse effect on other parasites and predators (for example, Farrar and Kennedy 1993; Kashyap et al. 1991). A study of trichomes of potato plants showed that the impact of increased trichome density on natural enemies seen in the greenhouse was decreased under some field conditions (Obrycki and Tauber 1984); the study underscored the importance of creating experimental conditions similar to those experienced by insects in typical agro-ecosystems.
Changes in glossiness of leaves (for example, Eigenbrode et al. 1995) and general plant architecture (e.g., Kareiva and Sahakian 1990) can also affect the efficacy of natural enemies. Because plant-produced volatile substances attract many natural enemies, a new cultivar with an altered profile of insecticidal volatile chemicals could have reduced attraction to parasites or predators resulting in decreased biological control of the pest (Bottrell and Barbosa 1998; Dicke 1996).
The chemical composition of forage plants can have a significant effect on livestock and bees (Keeler et al. 1978). Some chemicals can be passed on in the milk of cows and goats (Dickson and King 1978) and the honey of bees (Bull et al. 1968). Plant breeders have long recognized that some cultivars of forage crops can adversely affect livestock growth and