Alfalfa Weevil Activity Prediction Update: May 15, 2023

Map showing the current degree day accumulations of alfalfa weevils for several South Dakota locations. For a detailed description of this map, please call SDSU Extension at 605-688-6729.
Figure 1. Alfalfa weevil degree day accumulations as of May 15, 2023, 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.

Originally Submitted: May 12, 2023

During the last week, the entire state reached enough degree days to have active alfalfa larvae. In some locations, enough degree days have been accumulated to have third instar larvae feeding. As larvae mature, they will cause increasing defoliation to alfalfa. It’s also important to remember that even though enough degree days have accumulated, in previous years, we have observed a lag time between alfalfa weevil larvae and accumulated degree days. In areas with the highest accumulation of degree days, alfalfa should be scouted to ensure that alfalfa weevils don’t cause extensive defoliation.

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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