Spotted wing drosophila (SWD) is active across the Commonwealth. Many producers have identified SDW on their farms and are applying weekly sprays during the harvest period to manage this insect. These repeated sprays raise concern over the development of insecticide resistance. Frequently spraying with the same types of materials increases the possibility of SWD developing resistance.
Insecticides are grouped according to their modes of action. Some people think of these as chemical classes. Many pesticide labels display Insecticide Resistance Action Committee (IRAC) codes to identify insecticides according to their modes of action. Often when an insect develops resistance to a chemical, it develops resistance to other chemicals that share the same mode of action. There are instances where resistance to one insecticide confers cross-resistance, meaning resistance to chemicals within two or more modes of action. Often when resistance occurs in a population, it becomes fixed, and the resistance is maintained into the future even if the insecticide is no longer used.
The best way to manage insecticide resistance is to prevent it from occurring in the first place. For producers, this means considering resistance as a possibility anytime they intend to use insecticides repeatedly. One general rule of thumb that I try to follow is to switch modes of action with each new generation of an insect pest. So, if it takes a month to complete a generation with a particular pest, I would use a different insecticide with a different mode of action (different IRAC group) each month. With SWD, it takes just over a week to complete its life cycle, which means I would need to switch modes of action each week. An effective rotation would include at least three modes of action. Switching to different insecticides within the same IRAC group does not help to manage resistance at all.
The SWD management fact sheet (EntFacts-230) lists the available insecticides and provides their IRAC grouping codes. Among the ‘Effective’ and ‘Very Effective’ insecticides, we have three modes of action represented: IRAC groups 3A, 1B, and 5. Using this strategy is more expensive in the short term, but avoiding insecticide resistance is cheaper in the long run.
By Ric Bessin, Extension Entomologist