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-r elated Myrtaceae, which are known for their high content of essential oils. We describe a novel morphological adaptation of the inner mandibular surfa ce of larval stage Pergagrapta species, which feed on leaves of Melaleuca q uinquenervia (Myrtaceac). This 'scopa mandibularis' forms an extensive mesh of setaccous papillae on what is usually the grinding surface of the mandi ble. Behavioural, chemical and morphological investigations of the sawfly-M elaleuca system suggest that the scopa may function in the physical separat ion of toxic leaf oils from the diet. The oils are stored in a pharyngeal d iverticulum from where they are ejected under two circumstances. Oil from t he diverticulum is voided prior to feeding and/or during feeding at night, which indicates a mechanism to climinate 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 disturb ed. Chemical evidence suggests that 1,8-cineole, the major component in the M. quinquenervia leaves, is selectively metabolized to a more soluble hydr oxycincole. We postulate that the separation and regurgitation of oils is n ot only a defense 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. (C) 2000 The Linnean Society of London.