Insecticide trials for control of the steelblue sawfly ( Perga affinis affinis ), a primary defoliator in young commercial eucalypt plantations of south-eastern Australia

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Abstract
The contact insecticides carbaryl, chlorpyrifos, fenthion, malathion and the Pyrethrins, the systemic insecticides dimethoate and omethoate, and the microbial insecticide dipel (Bacillus thuringiensis), were tested in an irrigated 2.9 year-old eucalypt plantation in north-central Victoria during late May 1996 for effectiveness in controlling the steelblue sawfly (Perga affinis affinis) at concentrations recommended by the manufacturers against a range of target invertebrate pests, and at half and twice those concentrations. The steelblue sawfly, potentially a highly destructive ‘primary’ defoliator in young plantations up to several metres high, is known to attack 16 species of eucalypt, including E. camaldulensis and E. globulus, which are among the favoured species for commercial hardwood plantation programs in Australia. Upper crown foliage is particularly prone to damage between mid-winter and late-spring, although whole trees and entire stands can be stripped of foliage. Significant loss of height and diameter growth, stunting and occasional tree mortality can result, especially after repeated annual attacks. Large adult populations tend to build up where cool moist conditions prevail during spring and summer and where woodlands of susceptible species are nearby. The tests were based on 4 758 larvae, of which 33.3% were at the instar I stage, 64.8% at instar II and 1.9% at instar III on E. camaldulensis hosts. Biological control was not evident, and the experimental procedures used for handling, transport and storage had no adverse effects on the health of the larvae. Aqueous suspensions of 0.02% fenthion, 0.1% malathion and 0.01% Pyrethrins, applied to infested foliage as a fine mist, together with 0.1% chemspray wetting agent, caused a highly satisfactory 90.7 to 100.0% mortality among larvae within two days. They scored well against at least seven of nine desirable selection criteria, but malathion has the lowest hazard rating to human health; the Pyrethrins appear to be the least stable in the environment, and fenthion is the cheapest to spray. All three insecticides are presently not registered for use against the steelblue sawfly. Registration by the National Registration Authority for Agricultural and Veterinary Chemicals would be necessary if they were used in broadscale spraying during late autumn or early winter against early larval instars I to III. Omethoate at 0.12%, 0.06% and 0.03% concentrations, chlorpyrifos at 1.0%, 0.5% and 0.25%, and the high (0.06%) and recommended (0.03%) concentrations of dimethoate also induced very high larval mortalities (98.9% to 100.0%), but were rejected for use mainly because of their comparatively low LD50 (rats) values and other environmental concerns. The low concentrations of dimethoate, malathion and the Pyrethrins (0.015%, 0.05% and 0.005% respectively), and also the low to high concentrations of carbaryl (0.2%, 0.1%, 0.05%) and dipel (0.4%, 0.2%, 0.1%), caused insufficient mortality among larvae to be acceptable for application.