Defence strategies of Chrysomela lapponica (Coleoptera: Chrysomelidae) larvae: relative efficacy of secreted and stored defences against insect and avian predators

Larvae of the leaf beetle Chrysomela lapponica defend themselves by release of repellent secretions, but also store potentially toxic compounds in their body tissues. We addressed the role of major groups of predators in the evolution of these two defence strategies by testing effects of these strategies on the behaviour of insect (wood ant, Formica polyctena) and avian (great tit, Parus major) predators. Ants were repelled by larval defensive secretions, but not by larvae devoid of secretions, larval haemolymph or integument. By contrast, birds rejected larvae devoid of secretions after the first attack; this suggests the presence of non-volatile defensive compounds within the larval body. However, survival was three-fold greater for larvae with intact secretions than for larvae with depleted secretions due to (1) irritating effects of secretions, resulting in frequent release of undamaged prey, and (2) faster avoidance learning and better prey memorability based on contact with secretions. Thus, volatile secretions and non-volatile compounds stored in the body act against birds jointly. Secretions sequestered from host plants were more effective than were autogenously produced secretions. We conclude that insect predators could contribute to the evolution of secreted but not of stored defensive chemicals, whereas bird predation could favour the evolution of both lines of defence.