Analysis of a chemical defense in sawfly larvae: easy bleeding targets predatory wasps in late summer.

Contact of certain sawfly larvae with predators frequently elicits an easy bleeding behavior involving local disruption of the integument and release of hemolymph droplets. The efficacy of this putative antipredator defense was investigated in the turnip sawfly, Athalia rosae, by using common garden experiments in which particular guilds of predators were excluded, and through manipulations of the chemical defense of larval prey. After 3 d of exposure to walking and/or flying arthropod predators, the proportion of surviving sawfly larvae remained quite high at 71.3% in midsummer, but dropped to 37.5% in late summer. However, Pieris rapae caterpillars without the bleeding defense were killed within hours by predatory wasps (Vespula vulgaris). Topical treatment of P. rapae caterpillars with sawfly hemolymph or sinalbin, a glucosinolate sequestered by the sawflies, revealed that the easy bleeding of A. rosae is an efficient defense against wasps, partially due to this compound. These experiments are the first to demonstrate the efficacy of this chemical defense mechanism in a natural setting.