Behavior and body size modulate the defense of toxin-containing sawfly larvae against ants

The sawfly larvae of most Argidae and Pergidae (Hymenoptera: Symphyta) species contain toxic peptides, and these along with other traits contribute to their defense. However, the effectiveness of their defense strategy, especially against ants, remains poorly quantified. Here, five Arge species, A. berberidis, A. nigripes, A. ochropus, A. pagana, A. pullata, plus three Pergidae species, Lophyrotoma analis, Lophyrotoma zonalis, Philomastix macleaii, were tested in laboratory bioassays on ant workers mainly of Myrmica rubra. The experiments focused on short-term predator–prey interactions, sawfly survival rate after long-term interactions, and feeding deterrence of the sawfly hemolymph. The larvae of Arge species were generally surrounded by few ants, which rarely bit them, whereas larvae of Pergidae, especially P. macleaii, had more ants around with more biting. A detailed behavioral analysis of Arge-ant interactions revealed that larval body size and abdomen raising behavior were two determinants of ant responses. Another determinant may be the emission of a volatile secretion by non-eversible ventro-abdominal glands. The crude hemolymph of all tested species, the five Arge species and L. zonalis, was a strong feeding deterrent and remained active at a ten-fold dilution. Furthermore, the study revealed that the taxon-specific behavior of ants, sting or spray, impacted the survival of A. pagana but not the large body-sized A. pullata. The overall results suggest that the ability of Arge and Pergidae larvae to defend against ants is influenced by the body size and behavior of the larvae, as well as by chemicals.