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False wireworms

Consultant, Glen Smith (3-D Ag), has reported damage to canola plants at the 4-6 leaf stage in a paddock near Henty, in the Riverina district of New South Wales. False wireworm beetles have been found in reasonable numbers and are believed to be causing the observed damage and plant loss. There are several different species of false wireworms in south-eastern Australia, however it is likely that vegetable beetles (Gonocephalum spp.) are the culprit in this instance. Glen says plants have been chewed off at the ground level, which is typical of vegetable beetle damage.

Vegetable beetles have one generation per year, with adults emerging in spring. Common species are 6 – 10 mm long, and have a dark grey-black coloured body, which is oval-shaped and often covered in soil. Larvae are elongated, hard-bodied, cream, yellow or tan in colour, with three pairs of legs just behind the head. The body is cylindrical and up to 20 mm long. Unlike many other species of false wireworms in which the larvae are the most detrimental life-stage, it is the adult stage of vegetable beetles that are most damaging to canola plants.

The affected paddock was sown into a large amount of cereal stubble, which favours the build-up of false wireworm populations. False wireworms are common in fine textured soils, with high levels of organic matter. To monitor for false wireworms check under stubble prior to sowing, particularly if the paddock is coming out of cereal or pasture. Early identification and detection is important; typically once damage has become obvious it is too late to implement effective control.

Glen says the paddock was sprayed with a dimethoate and alpha-cypermethrin mix to control several pests post-emergence, but it appears these beetles have continued to survive. Vegetable beetles are difficult to control with foliar applications of insecticides. In experiments conducted in Western Australia, it has been found that vegetable beetles are not killed using chemical sprays aimed at other species, such as vegetable weevils. Removing excess stubble and trash is an effective strategy where this pest is a problem in successive years. For further information on false wireworms, click here.

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