Alfalfa Weevil

Bryan Jensen, IPM Program

Sometimes the occasional pests are the hardest to manage. Not because they are more complex or cause more damage, but because we forget to anticipate and are surprised by their damage. Alfalfa Weevil is certainly one of those pests. Statewide, alfalfa weevil populations have been low if compared to populations encountered in the 1970’s and early 80’s. However, over the last two years we have received more damage reports than usual. I don’t know if this is a trend that will continues in 2009 or not, but each year we get reports of someone, somewhere, in the state that had unexpected economic damage. One of the underlying factors for this unexpected damage is “failure to anticipate”. We are all busy and spring is arguably the busiest time of the year. We are focused on immediate problems and don’t always take time to look for those occasional pests.  

Southern Wisconsin has now accumulated 300 weevil degree days (or soon will have) which is when eggs are starting to hatch and scouting is recommended. 

Please keep in mind that egg hatch will occur over a period of time and that early instars will do little feeding compared to mature larvae. Therefore, incidence and severity of damage is likely to increase over time. I bring this up because it is common to hear statements like “Weevils must have moved in over the weekend” as people refer to a noticeable change in damage if viewed from a distance. Well, that doesn’t happen. Weevil larvae are not very mobile and do not move from field to field. What does happen is the rate of defoliation increases as larvae grow and damage rapidly becomes apparent. Timely scouting would have identified those fields much sooner. You could have noticed the tiny pinhole feeding of young larvae and avoided the defoliation of the mature larvae by possible taking an early harvest.

To get a feel for what may be happening in your area, spot check alfalfa fields for weevil damage in areas that warm up quicker (eg. south facing slopes, sandy areas, etc.). Look for tiny pinhole feeding in the upper leaves. Sometimes you may have to separate the folded leaflets to observe early damage and larvae. Young larvae tend to be more of a slate color than the lime green color of more mature larvae. But all instars will have a black head. Don’t be confused is you notice very large larvae which appear to alfalfa weevil. If they have a tan head they are clover leaf weevil and rarely cause significant damage in Wisconsin.

Photo 1: Alfalfa Weevil instars will have a black head.

The economic threshold for first crop alfalfa is when 40% or more of the stems have leaf feeding. If a field is within 7-10 days of harvest an early cut will likely control alfalfa weevil. If an insecticide application is needed don’t forget to look at the harvest restrictions when choosing a product. Some labels require a 14-21 day harvest interval. Fields which have significant first crop damage should be scouted soon after second crop regrowth develops to determine if there is a risk of damage from surviving larvae.