Some individual insects within a population can be naturally resistant to a particular, or multiple, insecticides. If the same insecticide is used regularly, those resistant individuals survive and go on to breed, possibly passing on the same resistance trait to their offspring. Continued use of that insecticide without adequate rotation can lead to a resistant population over time.
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 Horticulture 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 search the HIA website. Registered users can search the AUSVEG website.
A number of practices can help manage the risk of diamondback moth developing insecticide resistance.
- Always read and follow product labels, including recommended rates and limits on the number of applications.
- Monitor crops regularly and only apply insecticide when the threshold for crop damage is reached and only to areas that need it.
- Rotate insecticides with different modes of action. Do this by using insecticides from different chemical groups. Using chemicals from only one chemical group can lead to rapid development of resistance.
- Encourage beneficial insects through the use soft option sprays such as Bacillus thuringiensis (Bt) that target only the pests not the beneficials.
- Do not use tank mixes of insecticides.
- Practise good crop hygiene. Plant clean seedlings and incorporate crop residue.
- Time applications to the most susceptible life stage of the target pest. For diamondback moth this is the small larvae.
- Ensure the spray rig is properly calibrated and achieving good coverage.
This information focuses on the diamondback moth specific two-window strategy for vegetable brassicas. The two-window strategy helps plan an appropriate insecticide rotation when devising a control program. It is not compulsory to follow this, however you should plan ahead and understand insecticide resistance management.