Ontogenetic change in effectiveness of chemical defence against different predators in Oxycarenus true bugs

Many prey species change their antipredator defence during ontogeny, which may be connected to different potential predators over the life cycle of the prey. To test this hypothesis, we compared reactions of two predator taxa – spiders and birds – to larvae and adults of two invasive true bug species, Oxycarenus hyalinipennis and Oxycarenus lavaterae (Heteroptera: Oxycarenidae) with life‐stage‐specific chemical defence mechanisms. The reactions to larvae and adults of both true bug species strikingly differed between the two predator taxa. The spiders were deterred by the defences of adult bugs, but the larval defences were ineffective against them. By contrast, birds attacked the larvae considerably less often than the adult bugs. The results indicate a predator‐specific ontogenetic change in defence effectiveness of both Oxycarenus species. The change in defence is likely linked to the life‐stage‐specific composition of secretions in both species: whereas secretions of larvae are dominated by unsaturated aldehydes, secretions of adults are rich in terpenoids, which probably serve dual function of defensive chemicals and pheromones. Our results highlight the variation in defence between different life stages and the importance of testing responses of different types of predators. To assess the inter‐ and intraspecific differences in defence of a model prey, the seed bugs (Oxycarenus spp.), we presented the bugs to two predator taxa, jumping spiders and birds (great and blue tits). The experiments revealed a predator‐specific ontogenetic change in defence efficacy in the tested prey: while the larval defences were effective only against birds, the adult defences were more effective against spiders.