Plant-based host preference and larval competition of the tachinid parasitoid Epicampocera succincta

We investigated the causes of among-plant variation in the parasitism rate of Pieris melete larvae (Lepidoptera: Pieridae) by the parasitoid fly Epica mpocera succincta (Diptera: Tachinidae). The rate of parasitism by E. succi ncta was much higher on the bitter cress Cardamine appendiculata than on an y other cruciferous plants. Adult female flies were found to be more attrac ted to C. appendiculata than to other cruciferous plants for searching for hosts. The parasitoid appeared to be unable to distinguish P. melete from a n alternative host, Pieris rapae larvae, that coexisted with P. melete on m ost crucifer plants. Similarly, E. succincta failed to avoid P. rapae paras itized by the braconid wasp Cotesia glomerata. C. glomerata is a superior c ompetitor to E. succincta if occupying the same host, killing the host befo re the E succincta larva can grow and depriving the larva of the chance to survive. E. succincta attacked P. rapae larvae on most cruciferous plants, many of which were already occupied by C. glomerata; only on Cardamine appe ndiculata was E succincta free from interspecific competition, because only P. melete was found on this particular plant. However, the strong preferen ce for the hosts on C. appendiculata incurred heavy intraspecific competiti on among larvae on this plant, killing as great a proportion of larvae as i nterspecific competition did on the other plants. The balance between the s trength of intraspecific competition on the preferred plant and of interspe cific competition on the other plants appeared to maintain plant preference by E. succincta.