Alfalfa Weevil Activity Prediction Update: May 20, 2021

Originally Submitted: May 20, 2021

This week we accumulated degree days for alfalfa weevils at a rapid rate. As a result, we observed many areas of the state potentially move through more than one larval life stage. Scouting for alfalfa weevil should be conducted throughout the state. The upcoming forecast looks favorable for degree day accumulation.

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

Scouting should be occurring throughout the state. Although areas around Winner and Vermillion have accumulated enough degree days for pupation to occur, we still recommend scouting in these areas. It is important to remember that these estimates are to provide a framework for scouting, and there is some error associated between them and actual insect activity. We will continue to monitor for alfalfa weevil activity.