Alfalfa Weevil Activity Prediction Update: April 20, 2020

During the last week, the colder weather that we experienced across South Dakota resulted in very little degree day accumulation. However, in some areas enough degree days have accumulated to potentially lead to adult activity within alfalfa fields (Table 2).

Scouting for alfalfa weevil should begin in the Rapid City, Hot Springs, Mission and Winner areas by the end of the week when air temperatures will be higher. It is likely that adult activity will begin and will likely increase with the warmer weather that is currently forecasted for next week.

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.