Alfalfa Weevil Activity Prediction Update: May 4, 2020

Temperatures continue to increase across South Dakota and the degree days are rapidly accumulating. With the exception of areas around Selby, Sisseton and Brookings, alfalfa weevil activity is likely in South Dakota. Alfalfa defoliation is likely to be occurring near the Rapid City, Hot Springs, Cottonwood, Mission, Pierre, Winner and Vermillion areas. The daily high temperatures for the next week are favorable for increased degree day accumulation. For more information regarding alfalfa weevil scouting, please refer to our previous article.

Estimating Activity

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°F. This equation is used for days that the maximum temperature exceeds 48°F. In cases where the minimum temperature is lower than 48°F, the actual temperature is substituted by the 48°F developmental threshold since no development is occurring below it. Degree day accumulation begins on January 1st. 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 1st being the smallest larvae and the 4th 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 1st and 2nd instar growth stages.