Insecticide trials for control of the Autumn Gum Moth (Mnesampela privata), a primary defoliator in commercial eucalypt plantations prior to canopy closure

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Abstract
The contact insecticides carbaryl, chlorpyrifos, fenthion, malathion and pyrethrins, the systemic insecticides dimethoate and omethoate, and the microbial insecticide dipel (Bacillus thuringiensis), were tested in an irrigated 2.8-year-old eucalypt plantation in north-central Victoria during mid-April 1996, at concentrations recommended by the manufacturers, and at twice and half those concentrations, for effectiveness in controlling the autumn gum moth Mnesampela private on foliage of Eucalyptus globules. The autumn gum moth is potentially a highly destructive feeder on juvenile leaves in up to 4-year-old plantations prior to canopy closure. Significant loss of growth, stunting and occasional tree mortality can be caused by severe attacks. Twenty-seven species of eucalypt are known to be variably susceptible. The tests were based on 25 treated trees plus an untreated ‘control’ tree, and on an examination of 234 infested leaves bearing 4832larvae, of which 98.7% were at the instar I to III stage, and the remainder at instar IV. Biological control caused only 3.6% mortality among the larval population. Aqueous sprays of 0.10% malathion, 0.0 I% pyrethrins or 0.25% chlorpyrifos, applied to foliage as a fine mist, caused a highly satisfactory 94.7 to 100.0% mortality among the larvae within two days. The concentrations for malathion and the pyrethrins conformed to recommendations by the manufacturers for controlling caterpillars (though not specifically the autumn gum moth), whereas the chlorpyrifos concentration was half of the concentration recommended against ants, cockroaches, silverfish and spiders. It appeared that the use of malathion is satisfactory at half the recommended concentration (i.e. at 0.05%) in areas where natural mortality of larval instars I-III of the autumn gum moth exceeds 10.0%. Chlorpyrifos was judged the least preferred option after malathion and the pyrethrins, on account of its higher mammalian toxicity, slow rate of degradation in soils and need to be registered for use against either caterpillars or the autumn gum moth in particular. Fenthion at 0.08, 0.04 and 0.02% concentration, omethoate at O.12, 0.06 and 0.03%, carbaryl at 0.2, O.I and 0.05%, dipel at 0.4, 0.2 and O.1% and dimethoate at 0.06, 0.03 and 0.015% caused insufficient mortality(< 66.9%) among larvae within 2 days of spraying to be recommended for use. Future research should examine higher concentrations of these insecticides (except of the highly hazardous omethoate) and different strains of B. thuringiensis (e.g. var. kurstaki (Foray 48B)), for effectiveness in controlling larval instars I to III of the autumn gum moth during mid-autumn, so that the choices for chemical control can be expanded.