Alfalfa Weevil Activity Prediction Update: April 29, 2021

Originally Submitted: April 29, 2021

Things are starting to warm up a little in South Dakota. As a result, we accumulated quite a few degree days throughout the state during the last week. With another warmer week of temperatures in the upcoming forecast, the need to scout for alfalfa weevils is continuing. The low temperatures have still been below the developmental threshold for alfalfa weevils, but weevil activity is going to be more likely with the warmer temperatures during the day.

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 be larval activity likely in the Buffalo, Hot Springs, Lemmon, Cottonwood, Mission, Pierre, Winner, Huron, Mitchell, Tyndall and Vermillion areas. Second instar larvae are likely to be in alfalfa in areas near Hot Springs, Winner and Vermillion.