A series of recent grower meetings on the management of diamondback moth (DBM) highlighted trap crops, the impact of insecticides on beneficial insects and the influence of brassica variety preference on egg laying DBM.

Department entomologist Françoise Berlandier said there was a range of management tools available including the Insecticide Resistance Management strategy, beneficial insects which can help control DBM and the DBM handbook and ute guide on brassica pests and diseases.

"The national DBM project team is also developing a chart outlining the toxicity of different insecticides on beneficial insects, which will be most useful for growers interested in conserving beneficial insects when applying insecticides to control DBM grubs," Ms Berlandier said.

"Chemicals registered for controlling insect pests of brassica crops and the toxicity of the chemicals against the different groups of beneficial insects are listed on the chart," she said. The chart will be available in mid-2003.

Other DBM research results from around Australia were discussed at the meetings. Results included variety preference experiments conducted in Victoria, which indicate that the variety of cabbage, cauliflower or broccoli did not significantly influence where DBM laid their eggs. However, Ms Berlandier said it should be noted that the experiments only tested some of the commercially available varieties in Australia.

A South Australian spray trial using coloured dyes examined the impact of insecticide application on adult DBM (moths) and DBM parasitic wasps (beneficial insects).

"Spray droplets contacted 68 per cent of wasps but less than half of the moth population was hit by trace amounts of the coloured dye (or, in a commercial situation, by insecticide)," Ms Berlandier said.

"Synthetic pyrethroids (SP) such as Ambush^® and Dominex^®, tend to repel rather than kill DBM. The trial concluded that the majority of moths avoided the spray or received only a little as they moved away from it, allowing them to re-infest the crops afterwards," she said.

"The potential for insecticide resistance also increases as the moths are exposed to a sub-lethal dose of insecticide. In laboratory experiments, the SP permethrin applied onto cocoons of DBM, killed only five per cent of DBM, but killed up to 65 per cent of the beneficial parasitic wasps (Diadegma spp.)."

Ms Berlandier said growers should always target DBM grubs rather than moths, to delay the onset of insecticide resistance and to help preserve beneficial insects.