Alfalfa Weevil Activity Prediction Update: April 8, 2021

Originally Submitted: April 8, 2021

As the temperatures begin to warm up around South Dakota, it is important to remember that some insect pests are accumulating degree days. The warmer the weather gets, the faster the degree days will be accumulated. One of the insect pests that we use degree day models to monitor is the alfalfa weevil.

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 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 Activity

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, Pierre, Winne, Huron, Mitchell and Vermillion. In the area around Winner, it is possible that larvae activity and early feeding may be observed in alfalfa fields next week. Also, by next week, adult activity will be likely in areas around Selby, Gettysburg and Aberdeen.