Alfalfa Weevil and Insecticide Resistance

This past week, I received a report of high levels of alfalfa weevil larvae following an insecticide application. The producer was concerned that the insecticide he was using was not effective. I visited with the grower and collected several hundred larvae from one of the affected fields for a ‘quick and dirty’ laboratory bioassay. 

Figure 1. Alfalfa weevil is an early-season pest with one generation per year

The bioassay involved dipping untreated alfalfa stems in a 10% spray solution based on 15 gallons of finished spray per acre, shaking the excess moisture off, air-drying the stems, then placing the stems in petri dishes with 10 larvae for 24 hours. I evaluated the larvae against four insecticides at maximum strength and an untreated check. The insecticides were from four different modes of action groups (MOA) that are available for use on alfalfa. Below is a table with the results.

The grower indicated that a pyrethroid was applied for alfalfa weevil control and used for about 20 years, so the results in the table are not surprising considering the long use of that MOA.  The other modes of action worked well, but the larvae were tolerant to the pyrethroid. The message here is that if we over-use one MOA, pests may have the ability to adapt to tolerate that MOA.

Insecticide Group24 hr Mortality (%)*
Carbamate (MOA 1A)  70.8  AB
Organophosphate (MOA 1B)92.6  A
Pyrethroid (MOA 3A)  7.7  C
Oxadiazine (MOA 22)86.8  B
Untreated check  3.7  C
*Means followed by the same letter are not significantly different.

Strategies to Avoid Pesticide Resistance

So, what is the strategy to avoid insecticide resistance by pests? First, we need to recognize what the MOAs are of the pesticides we use. These are displayed as a number on the front of the pesticide label.  For example, Lorsban has the following box on the label to signify its MOA:


All Insecticides in the group 1B have the same MOA. To reduce the likelihood of resistance, we need to rotate the MOA of the pesticides we use regularly. Sometimes this may mean using a product that is slightly more expensive or has marginally less efficacy. However, the purpose is to help maintain the effectiveness of all insecticide MOAs.  I consider ‘regular’ rotation to mean, switching MOA with each new generation of a particular pest. For example with alfalfa weevil, it has one generation per year, which means that each year we would need to change our MOA by selecting an insecticide from a different MOA group. Changing to an insecticide from the same MOA does not help manage resistance as the insecticide attacks the pests in the same way as the previous insecticide. Ideally, we would like to use three or more MOAs in rotation to prevent or delay development of resistance.

Besides rotating MOAs, we also need to be using other IPM tactics to help manage resistance. This would include monitoring for pests and using economic injury levels to determine whether an insecticide is justified. This can help to reduce the frequency of applications and help to manage costs as well. For alfalfa weevil, there are widely used scouting and decision guidelines available in ENT-17. By scouting and using threshold to determine, the need for control as also help to promote natural enemies of pests.

By Ric Bessin, Entomology Extension Specialist

Posted in Forages, Pesticide Topics
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