The olfactory co-receptor IR8a governs larval-frass mediated competition avoidance in a hawkmoth

Finding a suitable oviposition site is a challenging task for a gravid female moth. At the same time, it is of paramount importance considering the limited capability of most caterpillars to relocate to alternative host plants. The hawkmoth, Manduca sexta (Sphingidae), oviposits on solanaceous plants. Larvae hatching on a plant that is already attacked by conspecific caterpillars can face food competition, as well as an increased exposure to predators and induced plant defenses. Here, we show that frass from conspecific caterpillars is sufficient to deter a female M. sexta from ovipositing on a plant and that this deterrence is based on the frass-emitted carboxylic acids 3-methylpentanoic acid and hexanoic acid. Using a combination of genome editing (CRISPR/Cas9), electrophysiological recordings, calcium imaging and behavioral analyses we demonstrate that the ionotropic co-receptor IR8a is essential for acid-mediated frass avoidance in ovipositing hawkmoths.