Alfalfa Weevil Activity Prediction Update: May 6, 2021

Originally Submitted: May 6, 2021

With the warmer weather at the end of last week, much of the state accumulated degree days very rapidly. The cooler temperatures the last few days probably slowed down alfalfa weevils again, but everyone should be scouting fields due to the possibility of alfalfa weevil activity. The upcoming forecast looks favorable for alfalfa weevil activity due to warmer temperatures overall, especially because the lows are predicted to be closer to 48 degrees Fahrenheit.

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 throughout all of South Dakota. During the next week there will likely be larval activity throughout the state, except for in the Brookings area. Second instar larvae are likely to be in alfalfa in areas near Hot Springs, Lemmon, Cottonwood, Mission, Pierre, Mitchell and Tyndall. Third instar larvae will likely be in alfalfa near Winner and Vermillion. Defoliation will become more apparent as the larvae continue to grow. As previously mentioned, these estimates may be slightly off due to cool evening temperatures that are not favorable for alfalfa weevils.