Alfalfa Weevil Activity Prediction Update: May 22, 2025

Map showing the current degree day accumulations of alfalfa weevils for several South Dakota locations. For a detailed description, please call SDSU Extension at 605-688-4792.
Figure 1. Alfalfa weevil degree day accumulations as of May 22, 2025, for selected South Dakota locations. Weather data was provided by the South Dakota Mesonet.

Written collaboratively by Adam Varenhorst, Philip Rozeboom, Patrick Wagner, and Brad McManus.

Degree day accumulations for alfalfa weevil larvae indicate that third to fourth instar larvae should be active in fields throughout South Dakota. 

Confirmation of alfalfa weevils in western South Dakota indicate that scouting should be occurring. However, while scouting fields in eastern South Dakota, no alfalfa weevil larvae were observed. 

Alfalfa should be scouted statewide. One of the possible reasons for the lag between degree day estimates and field observations is the cool nighttime temperatures and precipitation that we have been experiencing.

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 January 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.

Table 1. Alfalfa weevil life stages based on degree days.
Accumulated Degree Days
 Alfalfa Weevil Activity (Overwintering as Adults)
0 to 199
 Limited to no activity of alfalfa weevils.
200 to 299
 Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
300 to 371
 Spring laid eggs hatch. First instar larvae begin feeding.
372 to 438
 Second instar larvae feeding.
439 to 504
 Third instar larvae feeding.
505 to 595
 Fourth instar larvae feeding. (Period when greatest damage will occur)
596 to 814
 Pupation.
815 to greater
 Adult emergence (will overwinter).

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