Some individual insects within a population can be naturally resistant to a particular, or multiple, insecticide modes of action. Insecticides that belong to the same group, have the same mode of action. If the same mode of action insecticide is used regularly, those resistant individuals survive and go on to breed, possibly passing on the resistance trait to their offspring. Continued use of that mode of action without adequate rotation can lead to a resistant population over time and those insecticdes in that activity group will fail to control the pest.
Resistant populations of diamondback moth already exist. Strains with moderate to high resistance to the synthetic pyrethroid permethrin have been found in Queensland, Victoria, New South Wales, Tasmania and Western Australia.
Some Australian populations have resistance to the group 6 insecticide, emamectin, and group 28 insecticides, chlorantraniliprole and flubendiamide.
In Thailand and the Philippines resistance has also developed to flubendiamide and chlorantraniliprole. In a Hort Innovation Australia (HIA) project (VG08062) that identified resistant populations, the authors predicted “declining efficacy with a number of important synthetic insecticides” in the near future. For more information on this project search the Horticulture Innovation Australia website. Registered users can search the AusVeg website.
A number of practices can help manage the risk of diamondback moth developing insecticide resistance. The CropLife Australia Insecticide Resistance Management Review Group has devised guidelines for a number of pests and crops, including diamonback moth in vegetable brassicas. The following is based on those guidelines:
- Incoporate IPM techniques into the overall pest management program and monitor insect populations for loss of field efficacy of insecticides.
- Ensure spray rig is properly calibrated and achieving good coverage with appropriate sized spray droplets.
- Monitor crops regularly and only apply insecticide when the threshold for crop damage is reached and only to areas that need it.
- Time applications to the most susceptible life stage of the target pest. For diamondback moth this is the small larvae.
- Be aware of insecticide mode of actions used in the nursery phase of the crop and ensure a one generation break exists before re-use of that same mode of action in the field phase of the crop.
- Always read and follow product labels. Some products place a limit on the number of times they can be applied per crop and when they can be applied.
- To encourage beneficial insects, use Bacillus thuringiensis (Bt) sprays and avoid broad spectrum sprays, particularly early to mid-crop cycle.
- Be cautious of using tank mixes where both active ingredients control DBM as this strategy is generally not considered best practice for resistance management.
- DO NOT re-treat a spray failure with a product from the same chemical group.
- Practise good crop hygiene to reduce DBM pressure — plant clean seedlings and incorporate crop residue as soon as practical after harvest.
- Select insecticides in accordance with the current insecticide resistance management strategy for your region.
The insecticide resistance management strategy for Western Australia is the two-window strategy.