A few years ago, a bioassay of alfalfa weevil larvae collected in a central Kentucky alfalfa field showed low levels of control by a pyrethroid compared to other insecticide modes of action. In this particular instance, pyrethroids were used exclusively for alfalfa weevil control for well over a dozen years. For alfalfa weevil, there are only 4 different modes of action registered. When pyrethroids lose their effectiveness, only three modes of action are left to select from for this pest. So, growers must be careful to not overuse one mode of action such that the pest population in an area becomes tolerant to that insecticide, or even resistant. Once a population becomes resistant to an insecticide or a group of insecticides, the population may stay resistant for a long period of time, even if the insecticide is not used.
Key IPM Strategy
IPM strategies can be used to prevent or delay the development of resistance. One key IPM strategy is to not use an insecticide unless the pest population exceeds the economic injury level on average across an entire proposed treatment area. This means the area needs to be monitored regularly (weekly) such that samples are taken to represent the entire field. Often with alfalfa weevil, there are pockets within the field that exceed economic thresholds, but the entire field is not above the threshold. In this instance, either the person should hold off, wait, and resample to determine if the threshold is crossed at a later time, or spot spray those ‘hot’ spot areas. By delaying sprays or only spraying a portion of the field, natural enemies are preserved and the opportunity for natural control is increased.
Rotate Modes of Action
Proper pesticide management can also help prevent or delay resistance. If and when insecticide sprays are needed, it is important to rotate among modes of action. Repeated consecutive use of the same mode of action favors development of resistance to that mode of action. Rotating among different products within the same mode of action does not help and will also favor resistance. It is recommended to rotate modes of action with each new generation of the target pest. Since alfalfa weevil has one generation per year, this means that each year growers should rotate to a different mode of action from what was used the previous year. It is best to use 3 or more modes of action in rotation to fight the development of resistance.
By Ric Bessin, Entomology Extension Specialist