Effects of genetic structure of Lupinus arboreus and previous herbivory onPlatyprepia virginalis caterpillars

Two leaf-feeding caterpillars, western tussock moth (Orgyia vetusta) and ra nchman's tiger moth (Platyprepia virginalis) are abundant on Lupinus arbore us along the California coast. Previous experiments and observations sugges ted that feeding caused by either of these two folivores could reduce the p erformance and possibly the abundance and distribution of the other species . Previous common garden experiments also indicated that genetically determ ined characteristics of the host plants were important for O. vetusta. Here we examined the effects of familial origin of the host plant, and previous damage caused by O. vetusta on the abundance of P. virginalis. Plants with parents from one of three locations had higher numbers of P. virginalis th an plants with parents from the other two locations. However, this effect o f plant origin depended on the statistical analysis and was not as strong a s the effect of prior damage by O. vetusta on numbers of P. virginalis. Cou nter to our expectation. bushes that supported higher levels of damage by O . vetusta in the previous summer had more P. virginalis caterpillars. This strong effect could result by both moth species selecting bushes with the s ame traits or as the result of herbivory by O. vetusta enhancing the suscep tibility of bushes for P. virginalis.