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Alfalfa Weevil Activity Prediction Update: April 22, 2021

Originally Submitted: April 22, 2021

It was another cool week without a lot of degree day accumulations. With warmer temperatures in the upcoming forecast, the need to scout for alfalfa weevil activity will increase. Most of the state is still having low temperatures that are close to or below 32 degrees Fahrenheit, which will slow down alfalfa weevil activity and development.

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 in areas around Buffalo, Newell, Rapid City, Hot Springs, Lemmon, Faith, Cottonwood, Mission, Selby, Gettysburg, Pierre, Winner, Aberdeen, Huron, Mitchell, Tyndall and Vermillion. During the next week, weevil activity will likely begin in the Sisseton area as well. In the areas around Winner and Vermillion, it is possible that larvae activity and early feeding may be observed in alfalfa fields. Areas around Hot Springs should be monitored for larval activity during the next week.

Table 1. Alfalfa weevil life stages based on degree days.

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

Table 2. Degree day accumulations for South Dakota locations.

Location
Accumulated Degree Days
Since January 1, 2021
Alfalfa Weevil Activity
(Overwintering as Adults)
Buffalo
266
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Newell
223
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Rapid City
246
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Hot Springs
292
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Lemmon
258
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Faith
239
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Cottonwood
261
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Mission
276
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Selby
224
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Gettysburg
215
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Pierre
260
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Winner
323
Spring laid eggs hatch.
First instar larvae begin feeding.
Aberdeen
236
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Huron
250
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Mitchell
275
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Tyndall
242
Overwintering adult activity is observed in alfalfa fields (begin scouting fields).
Sisseton
192
Limited to no activity of alfalfa weevils.
Brookings
159
Limited to no activity of alfalfa weevils.
Vermillion
303
Spring laid eggs hatch.
First instar larvae begin feeding.

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Forage