Host plant adaptations in myrtaceous-feeding Pergid sawflies: essential oils and the morphology and behaviour of Pergagrapta larvae (Hymenoptera, Symphyta, Pergidae)

Australian pergine sawflies typically feed on eucalypts and other closely-related Myrtaceae, which are known for their high content of essential oils. We describe a novel morphological adaptation of the inner mandibular surface of larval stage Pergagrapta species, which feed on leaves of Melaleuca quinquenervia (Myrtaceae). This 'scopa mandibularis' forms an extensive mesh of setaceous papillae on what is usually the grinding surface of the mandible. Behavioural, chemical and morphological investigations of the sawfly- Melaleuca system suggest that the scopa may function in the physical separation of toxic leaf oils from the diet. The oils are stored in a pharyngeal diverticulum from where they are ejected under two circumstances. Oil from the diverticulum is voided prior to feeding and/or during feeding at night, which indicates a mechanism to eliminate host-associated oils. Larvae rest in close aggregations during the day, when they retain a full diverticulum, but the oils may be emitted for defensive purposes when larvae are disturbed. Chemical evidence suggests that 1,8-cineole, the major component in the M. quinquenervia leaves, is selectively metabolized to a more soluble hydroxycineole. We postulate that the separation and regurgitation of oils is not only a defence mechanism against predators, as usually stated, but also a mechanism by which pergid larvae eliminate oils from their diet, to reduce the toxicity of their food plants.