The characteristics and bhaviour of the Saw-Fly Perga affinis affinis (Hymenoptera).

Abstract

Perga affinis affinis is a member of a complex of saw-flies, widely distributed in south-eastern Australia, which are responsible for defoliation of several valuable woodland eucalypts. Reproduction is parthenogenetic, and the females insert their eggs into the leaf tissues. The larvae are highly gregarious and are voracious feeders, especially in the last two of their six instars. Individual colonies readily amalgamate, and enormous aggregations of larvae leave the trees in spring and form compact cocoon masses in the soil beneath their host trees. The prepupal stage in the cocoon enters a state of diapause, which may persist for upwards of 4 years; most individuals complete their life cycle in 12 months, but a significant proportion spend an additional year in diapause. Especial attention has been paid to behavioural characteristics of the insect. Oviposition involves a complex series of operations; the female spends several hours abrading the leaf surface before inserting her eggs. The larval colonies form resting clusters on branches during the day and at night move out to the foliage edge to feed; the reassembly of colonies is ensured by physical contact between individual larvae as they move, and by communication between the colonies and stray individuals. The larvae store oils derived from the leaves on which they feed in a large oesophageal diverticulum; this liquid is regurgitated in defence display, and is used, together with silk, to form a cocoon. In studies of frass production by larvae, individual pellet and total frass weights were found to be highly correlated, as were also the rate of frass production and air temperature.