Multimodal defensive strategies in larvae of two Hemichroa sawfly species

The two European sawfly species in the genus Hemichroa are a contrast in behaviour and appearance, since Hemichroa crocea is gregarious and brightly coloured, whereas Hemichroa australis is solitary and cryptic. Here, their defensive strategies are compared by integrating further components. In both species, ventral glands are minute, and no distinctive volatiles were detected by chemical analysis; hence, these exocrine glands are probably irrelevant in defence. Ethanol extracts of body parts were feeding deterrent to ant workers of Myrmica rubra , especially the integument of Hemichroa australis which was more deterrent than that of Hemichroa crocea . Single, living larvae of Hemichroa crocea were also attacked more frequently by ants. In contrast, single larvae of Hemichroa crocea are reluctantly taken by the bird Parus major that readily feeds on Hemichroa australis . The larvae of both species jerk their abdomen to physically defend themselves and/or to increase their (visual) warning signal (Hemichroa crocea ). The larvae of Hemichroa crocea can scratch the host plant leaf with the tip of their abdomen to produce a sound assumed to convey information in intraspecific communication. However, this behaviour was also elicited from Hemichroa australis , when disturbed, which suggests that it may have another function. The defensive strategy is multimodal in both species. The principal differences are the reliance on gregariousness in Hemichroa crocea , as opposed to the use of integumental chemicals in Hemichroa australis .