The Role of Secondary Plant Substances in the Food Specificity of the Colorado Potato Beetle12

Abstract

An artificial diet without leaf materials, which supports moderate feeding and growth of fourth-instar larvae of the Colorado potato beetle, Leptinotarsa decemlineata (Say), was used to evaluate the chemical basis of plant acceptance and rejection. Additions of powders or certain extracts of potato leaves to this diet enhanced feeding and growth. Eighty-four species of solanaceous and nonsolanaceous plants were studied by testing the effect of the addition of 4% leaf powder to this diet on growth of the fourth-instar larvae. Fifty-seven species showed inhibitory effects and two, Nicotiana rustica and Dryopteris marginalis, were toxic to the larvae. The ether extracts, water extracts, and resulting residues of 27 species also were tested. Among the plants exerting favorable effects as leaf powders, the active substances were not confined to a particular fraction. In some species the water extract and in others the ether extract or residue promoted feeding and growth. In the nonacceptable plants, inhibitory substances were found mostly in the water extract, and rarely in the ether extract, or the residue of leaf powder. This evidence indicates that the resistance of plants to feeding by potato beetle larvae is regulated by chemically and botanically unrelated inhibitors. These findings stress the importance of secondary plant substances as determinants of resistance to feeding by the Colorado potato beetle.