Chemical defence of turnip sawfly larvae against Japanese tree frogs

Abstract

Larvae of the turnip sawfly Athalia rosae (Hymenoptera: Tenthredinidae) are known to defend themselves using chemicals against predators such as ants, wasps and birds. However, only a few studies have tested the effectiveness of such chemical defences under field conditions. In a Japanese farm, a tree frog Hyla japonica (Anura: Hylidae) was observed to attack an A. rosae larva, but spit out the larva immediately after taking it into its mouth. To clarify how A. rosae larvae defend themselves against frogs, we quantitatively investigated how H. japonica attacked A. rosae larvae and palatable caterpillars of Pieris rapae (Lepidoptera: Pieridae) under field conditions. We experimentally placed an A. rosae larva or a P. rapae larva in front of tree frogs on the crop leaves. Frogs attacked both A. rosae and P. rapae larvae. However, the rate of predation by frogs was different between A. rosae and P. rapae larvae: 75% of frogs rejected A. rosae larvae, whereas 100% of frogs ate P. rapae larvae. Athalia rosae larvae attacked by frogs released their haemolymph (containing defensive chemicals) from the injured parts of their bodies. These results suggest that A. rosae larvae can chemically defend themselves against frogs in field conditions.