Fightback against the diamondback

By Helen Olsen

Recent research on diamondback moth (DBM) infestation in brassica crops in the lower south-east of Australia has found that chlorpyrifos is not an effective insecticide against DBM. This has led to the development of a tentative insecticide resistance management strategy.

The study, commissioned by the MacKillop Farm Management Group, also looked at the attractiveness of different brassica crops to DBM, with a view to implementing perimeter trap-cropping of DBM to protect a more valuable main crop.

Initially, the researchers measured the likely efficacy of chlorpyrifos, in response to reports that it was used in the lower south-east region in 2003.

They found that the field concentration of chlorpyrifos of 0.25 percent active ingredient (500 millilitres of product per hectare) gave a ratio of field use to discriminating dose (dose that kills 99 percent of the susceptible population) of 2.8:1.

This is well below the recommended ratio for insecticides of 30 or 40:1. Even doubling or quadrupling the dose will not bring the chemical amount up to an effective level.

In the second part of the study, the researchers compared the effectiveness of seven different treatment regimes (see Table 1) against DBM. The replicated trial at Struan, SA, was conducted in a late flowering, drought-stressed canola crop of 50 centimetres to 65cm height.

Table 2 shows the effect of the different regimes on DBM numbers. Only the emamectin benzoate Affirm® caused a commercially acceptable (86 percent) drop in DBM numbers. The researchers suggest using an oil additive such as Canopy® in conjunction with Affirm®, to reduce cost.

They also say that Canopy® may be used to "rehabilitate" SP insecticides, and improve the efficacy of Bacillus thuringiensis products against DBM.

Canopy® appears to improve both coverage of the plant and penetration into the insect"s body by the insecticide.

The researchers say that the levels of resistance to SP and OP insecticides in field populations of DBM are not enough to greatly reduce control of DBM with these chemicals.

However, resistance combined with insufficient insecticide application and favourable weather conditions (dry and warm) could lead to ineffective DBM control.

Finally, researchers measured DBM larval densities in 10 varieties of brassica sown in a replicated field trial at Frances, SA. There was a correlation found between the development stage of the crop and the level of DBM infestation - that is, the more developed the crop, the higher the infestation (see Table 3).

The results of this study indicate that early-sown or early-maturing turnip and rape varieties planted around the perimeter of more valuable canola and mustard crops may arrest the migration of DBM populations, and reduce infestation of the main crop.

But the researchers point out that much more investigation needs to be done to confirm and fine-tune this as a control strategy.

GRDC Research Code: DAS00011.

For more information: Greg Baker, baker.greg@saugov.sa.gov.au