Diamondback moths

We have received reports of diamondback moths (DBM) in canola crops in northern Victoria and southern New South Wales. Consultant agronomist, Barry Haskins (Ag Grow), reports DBM in several canola crops around Griffith, in the Riverina district of New South Wales. The numbers are still quite low, although there is increasing feeding to the stems and pods. Barry says the pods are beginning to harden and most crops are 2-3 weeks away from windrowing. Agronomist, Andrew McMahen (Landmark), has reported small DBM larvae in several canola crops north of Manangatang, in the Mallee district of Victoria. Andrew says this is the first time DBM have been noticed in the region for a few years. The caterpillars have been mostly found in irrigated areas and are still in low numbers.

DBM can have a significant impact on canola yields and current control measures are variable in their effectiveness. DBM can develop insecticide resistance readily and so some populations become difficult to control with insecticides. Furthermore, no single insecticide application will completely eliminate the population. In years when populations are large, a two-spray strategy is recommended. As flowering progresses, increasing numbers of larvae will move to the floral buds, flowers and pods. Large caterpillars may feed on small young pods while mature pods are usually only surface grazed.

Increases in DBM populations will be highly variable over the next month, and will differ markedly from region to region. The rate of development from eggs to moths is largely dependent on temperature. At 12°C the life cycle of DBM takes more than 100 days, whereas at 28°C the life cycle takes only 14 days. Before deciding on control, it is important to consider the crop stage, time to windrowing and weather forecast.

Refer to PestFacts Issue No. 9 for further information on DBM.