Moth larvae warning for canola

Growers are being warned to be on the lookout for diamondback moth larvae which have been reported in crops around the Esperance area.

As weather conditions warm, numbers of these pests can blow out and pose a serious challenge to canola crops.

Department of Agriculture and Food WA entomologist Svetlana Micic said DBM larvae had appeared early this year on the eastern south coast and crops in this area should be closely monitored.

DBM larvae grow to 12mm long, are pale yellowish-green and tapered at both ends.

They can usually be found on the underside of leaves and often cause holes that leave the leaf surface intact, creating a window-like effect.

Monitoring of numbers of DBM can be done using a sweepnet at the first signs of damage and at intervals during the growing season from mid-July through to late spring and early summer.

"Two sets of sweep samples - each with 10 sweeps of the net - should be carried out at a minimum of four separate locations within each canola crop to get an estimate of numbers and how they vary across paddocks," Ms Micic said.

"Empty the contents of each sweep onto a white or light coloured surface, such as a vehicle bonnet, and count the number of larvae and presence of other insects.

"If no, or low, numbers of DBM are found, then the crop should be monitored again in two weeks.

"When numbers increase, at least three estimates of larval density across a 12-day period are needed to determine how the population is changing."

DBM larvae can cause extensive damage to canola crops but there is no one size fits all recommendation for control.

Ms Micic said economic thresholds for control varied from 30 to 200 larvae per 10 sweeps and growers needed to consider crop growth stage, grain price and cost of spraying.

She said moisture-stressed crops would be more susceptible to damage.

DBM has developed widespread resistance to many insecticides, including synthetic pyrethroids and organophosphates.

Ms Micic said to reduce the risk of resistance to insecticides developing, it was important DBM was sprayed only when thresholds were exceeded and that insecticide groups were rotated from one season to the next.

It was vital growers check chemical label withholding periods before deciding to spray canola that is nearing windrowing or harvesting and use registered insecticide label rates, she said.

The Grains Research and Development Corporation has issued a new diamondback moth fact sheet.

Visit www.grdc.com.au/GRDC-FS-DBM for more information about prevention.