Removal of lichen secondary metabolites affects food choice and survival of lichenivorous moth larvae

Host use and selection by herbivores are often determined by host chemistry. Lichen secondary chemicals frequently have been assumed to have a defensive role against herbivores similar to that of higher plants, but thus far there is only circumstantial evidence of the adverse effect of lichen secondary chemicals on specialized lichen-feeders. We studied the impact of lichen secondary metabolites on performance and host preference of lich-enivorous larvae of the moth Eilema depressum using a recently developed manipulation method that allows the removal of a major part of lichen secondary metabolites from the extracellular space of the lichen thallus without harming their primary metabolism. All larvae died on intact thalli of Vulpicida pinastri and Hypogymnia physodes, whereas, after extraction of most of the lichens' secondary chemicals (e.g., pinastric and physodic acids, respectively), survival of neonate larvae ranged between 75% and 85%. In turn, atranorin, the major secondary metabolite in the cortical layer of Parmelia sulcata, merely retarded the growth of larvae during their first days, but had no long-term impact on survival or performance of larvae. In preference experiments, treated thalli with lowered concentrations of lichen secondary metabolites, with the exception of Xanthoria parietina, were preferred to intact thalli containing secondary. chemicals. Our results show that lichen secondary metabolites may act, at natural concentrations, as strong antiherbivore compounds against E. depressum larvae and may play an important role in their host selection.