Response of the lichen-eating moth <Emphasis Type="Italic">Cleorodes lichenaria</Emphasis> larvae to varying amounts of usnic acid in the lichens

Lichens are characterized by a great variety of secondary metabolites. The function of these substances remains partly unknown. In this study, we propose that some of these metabolites may expel insect herbivores. To test this hypothesis, we reared larvae of the lichenivorous moth Cleorodes lichenaria on three selected lichens, Cladonia arbuscula subsp. mitis, Usnea hirta, and Usnea dasypoga. In experimental setup, the secondary metabolite usnic acid was removed from the lichens with acetone prior to feeding, whereas a control was left untreated. On all three lichens, removal of usnic acid from the lichens using acetone significantly prolonged survival of larvae and increased their viability. Larvae reared on control lichens contained significantly more usnic acid than those reared on treated lichens, both in their biomass and their faeces. These results support the hypothesis that usnic acid serves as a repellent against insect feeding, besides its well established functions of UV protection and antimicrobial properties.