PLANT RESISTANCE AND BIOLOGICAL PEST CONTROL
The IPM approach most compatible with biological control is the development of plant resistance (Kogan 1982). Nevertheless, incompatibilities arise when mechanisms of resistance indiscriminately affect both pests and natural enemies, or when natural enemies are indirectly affected through their hosts or prey. Experimental evidence of incompatibilities is shown in tomato (Duffee & Isman 1981, Duffey & Bloem 1986, Duffey et al. 1986). This may be illustrated with Heliothis zea, Spodoptera exigua and the endoparasitic wasp, Hyposoter exiguae (Vier.). When host larvae ingest a diet with the glycoalkaloid tomatine, the development of the parasitoid is detrimentally affected (Duffey & Bloem 1986). Kogan et al. (1992) warn that such studies demonstrate that depending on the mechanism of resistance, natural enemies may be detrimentally affected; and that when exploiting such mechanisms one should weigh the risk of reducing the natural enemy load versus the benefit of the particular resistance trait.
Obrycki (1986) studying the impact of potato glandular trichomes on Edovum puttleri (Grissell, an egg parasitoid of the Colorado potato beetle, drew similar conclusions. He showed that E. puttleri readily parasitizes L. decemlineata eggs on Solanum tuberosum but that the parasitoid is entrapped in glandular trichomes of Solanum berthaultii. On S. tuberosum, egg mortality is increased not only due to parasitism but probably also to host feeding and superparasitism. But aphid parasitoids that are equally affected by S. berthaultii trichomes in the greenhouse were not greatly affected in the field, showing that moderate levels of trichomes and the biological control of potato aphids are not incompatible. Therefore, it is apparent that both biochemical and physical plant defenses are potentially detrimental to natural enemies. As behavioral adaptations of parasitoids of insects adapted to resistant lines may occur in nature, it would be useful to identify such adapted populations when searching for new sources of natural enemies (Kogan et al. 1992). [For further details, please see <bc-49.htm> ]