Proximate effects of maternal oviposition preferences on defence efficacy and larval survival in a diet-specialised tortoise beetle. Who knows best: mothers or their progeny?

Abstract

* The fate of an immature herbivorous insect depends on its mother's ability to locate a host that fosters optimal growth. However, immature performance and survival is often decoupled from female host preference. * We used defence manipulation and exclusion experiments to investigate how oviposition choices impact survival in the diet-specialised leaf beetle, Acromis sparsa, whose larvae are defended by host-derived chemical shields, gregariousness and maternal guarding, but can only migrate within a narrow range of leaf options upon the natal host. * Females preferred to oviposit on mature leaves of Merremia umbellate morning glory vines their sole host, that were high above ground where egg parasitism was least, although survival of fully defended larvae was greater on young leaves near to the ground. * Because ant attacks were prevalent in low sites, while bug and wasp attacks were frequent in high sites, we expected a relationship between sequesterable chemicals, defence efficacy and apparency to site-specific predators. * Shields were effective in low but not in high sites. Guarding was effective against ants and bugs in high, but not in low sites. Shields and gregariousness were effective against ants on mature leaves, but ineffective against bugs on young leaves. * Shields derived from young leaves, which were richer in oleic acid, were more effective against wasps then bugs. Larvae in high sites migrated to young leaves that contained less phytol, a bug attractant. * Larval migration may be an adaptation for the exploitation of enemy-free space when maternal oviposition choices are not optimal for survival.