Multimodal defensive strategies in larvae of two Hemichroa sawfly species

Abstract

The two European sawfly species in the genus Hemichroa are a contrast in behaviour and appearance, since H. crocea is gregarious and brightly coloured, whereas H. 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 H. australis which was more deterrent than that of H. crocea. Single, living larvae of H. crocea were also attacked more frequently by ants. In contrast, single larvae of H. crocea are reluctantly taken by the bird Parus major that readily feeds on H. australis. The larvae of both species jerk their abdomen to physically defend themselves and/or to increase their (visual) warning signal (H. crocea). The larvae of H. 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 H. 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 H. crocea, as opposed to the use of integumental chemicals in H. australis.