Searching for particular traits of sawfly (Hymenoptera: Tenthredinidae) larvae that emit hemolymph as a defence against predators.

Easy bleeding is a defence strategy that allows the larvae of some Tenthredinidae sawfly species to emit deterrent hemolymph when attacked by a predator. However, a drawback of this defence is that hemolymph is frequently in contact with the exterior, thus potentially subjected to multiple microbial infections at any body's integumental spot. Here we aimed to identify physiological traits that are linked to easy bleeding. First, larvae of several sawfly species were subjected to daily experimental losses of hemolymph equivalent to 10% of their body weight, and changes in body weight and survival were recorded. Easy bleeders' survival rates were better compared to non-easy bleeders. Second, testing hemolymph melanisation revealed that nearly all sawfly hemolymph samples did not melanise over a 24h period. Third, inhibition zone tests against live Escherichia coli were conducted using hemolymph collected 24-48h after a sterile wounding and an infection with Micrococcus luteus, as well as from control, untouched individuals. Sterile wounding induced similar antibacterial activities compared to those detected in the control group. However, the activity was significantly enhanced upon infection in some species, similarly to other insects. Thus, easy bleeders have a tendency to compensate for hemolymph loss resulting from predator-prey interactions, whereas a non-melanising hemolymph is probably a characteristic of sawflies, and the antimicrobial activity can be high but is comparable in easy bleeders versus other insects.