Alfalfa Weevil Activity Prediction Update: April 15, 2021

Originally Submitted: April 15, 2021

Due to the cooler temperatures, we didn’t accumulate a lot of degree days for alfalfa weevils during the last week. However, there are some areas in South Dakota where alfalfa weevils may be active. However, the evening low temperatures that are below 30 degrees Fahrenheit are likely keeping the alfalfa weevils from being very active.

Calculating Degree Days for Alfalfa Weevils

To monitor for alfalfa weevils, we can estimate activity based on degree day accumulation. The equation for degree days is:

(Maximum daily temperature + Minimum daily temperature) ÷ 2 - The Developmental Threshold

For the alfalfa weevil, the developmental threshold is 48 degrees Fahrenheit. This equation is used for days during which the maximum temperature exceeds 48 degrees Fahrenheit. In cases where the minimum temperature is lower than 48 degrees Fahrenheit, the actual temperature is substituted by the 48 degrees Fahrenheit developmental threshold since no development is occurring below it. Degree day accumulation begins on Jan. 1. First overwintering adult alfalfa weevil activity is generally observed around 200-degree days and larvae hatch is generally around 300-degree days (Table 1).

The alfalfa weevil has four larval instars, with the first being the smallest larvae and the fourth being the largest larvae. The later instar larvae will consume additional leaf tissue, which results in increased defoliation. As a result, management would ideally be done while larvae are still in the first and second instar growth stages.

Current Estimate on Alfalfa Weevil Activity in South Dakota

Based on current degree day accumulations, overwintering alfalfa weevil adult activity is likely in areas around Buffalo, Newell, Rapid City, Hot Springs, Lemmon, Faith, Cottonwood, Mission, Selby, Gettysburg, Pierre, Winner, Aberdeen, Huron, Mitchell, Tyndall and Vermillion. In the area around Winner and Vermillion, it is possible that larvae activity and early feeding may be observed in alfalfa fields during the next week. Areas around Brookings and Sisseton should be monitored for adult activity during the next week even though the estimates indicate that adults aren’t likely active yet.