Alfalfa Weevil Update
Bryan Jensen, IPM Program
The southeast portion of the state is entering that time period when third and fourth instar alfalfa weevil larvae can be found and you may start to see an increase in the amount of defoliation. Fortunately, the cooler weather has given the alfalfa a chance to get caught up and cutting, an excellent form of cultural control, has started in some areas. For those fields where you have noticed elevated first crop defoliation please take the time to revisit those fields 4-5 days after cutting. If environmental conditions are favorable, larvae may survive both cutting and harvest and feed on newly forming crown buds which significantly delays regrowth and lower yields. Check to make sure crown buds are forming. Look for larvae on the soil surface, under leaf litter and at the juncture between soil and the alfalfa crown. During cool, cloudy weather you may find weevils feeding on new alfalfa buds during daylight hours. You may also see increased survival under windrows. It is difficult to make control decisions based on the number of larvae found. A better method is to take another stem sample, as you did with first crop, and treat the field when 50% of the stems have feeding injury. Before deciding to spray, use an insect sweet net to make sure weevils are still present and check for presence of pupae. A sign that feeding may be slowing down.