Chemical defence of Battus philenor larvae against attack by the parasitoid Trogus pennator

1. Parasitoids in the genusTrogus(Hymenoptera: Ichneumonidae) attack the larvae of swallowtail butterflies (Lepidoptera: Papilionidae). Only two of the three major tribes of the subfamily Papilioninae are attacked although species of all three tribes commonly occur together. The tribe Troidini is relatively free of parasitoids of any kind, and it has been proposed that the aristolochic acids sequestered by troidines protect them from parasitism.2. The responses ofT. pennator(Fabricius) to the sympatric troidineBattus philenor(Linnaeus) were examined. Three hypotheses that could explain why this wasp does not parasitiseB. philenorwere considered. (1)Battus philenordoes not produce compounds used by the wasp to locate hosts. (2) The larval integument contains compounds that deter attack. (3) The parasitoid offspring cannot survive inB. philenor.3. The first hypothesis was not supported as the frass ofB. philenorlarvae was found to act as a searching arrestant comparable to the frass of a host species.4. The second hypothesis was supported. TheB. philenorlarvae were rejected when the wasps examined them using their antennae, and ethanolic washes ofB. philenorcuticle deterred attack byT. pennatorwhen applied to otherwise acceptable host larvae. Bioassays of fractions of the ethanolic wash and of pure aristolochic acids established that aristolochic acids were at least partly responsible for the deterrent effect.5. The third hypothesis was also supported. Larvae ofB. philenorattacked by the parasitoids developed into butterflies.6. These results indicate that both behavioural and physiological barriers, the former attributable at least in part to sequestered compounds and the latter of unknown mechanism, preventT. pennatorfrom parasitisingB. philenor.