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Alfalfa Weevil Activity Prediction Update: May 4, 2020

Updated May 04, 2020
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Adam Varenhorst

Assistant Professor & SDSU Extension Field Crop Entomologist

Temperatures continue to increase across South Dakota and the degree days are rapidly accumulating. With the exception of areas around Selby, Sisseton and Brookings, alfalfa weevil activity is likely in South Dakota. Alfalfa defoliation is likely to be occurring near the Rapid City, Hot Springs, Cottonwood, Mission, Pierre, Winner and Vermillion areas. The daily high temperatures for the next week are favorable for increased degree day accumulation. For more information regarding alfalfa weevil scouting, please refer to our previous article.

Estimating Activity

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°F. This equation is used for days that the maximum temperature exceeds 48°F. In cases where the minimum temperature is lower than 48°F, the actual temperature is substituted by the 48°F developmental threshold since no development is occurring below it. Degree day accumulation begins on January 1st. 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 1st being the smallest larvae and the 4th 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 1st and 2nd instar growth stages.

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 several S.D. locations.

Location
Accumulated Degree Days
Since January 1, 2020
Buffalo
269
Newell
277
Rapid City
331
Hot Springs
402
Lemmon
256
Faith
240
Cottonwood
311
Mission
330
Selby
186
Gettysburg
227
Pierre
314
Winner
353
Aberdeen
205
Huron
232
Mitchell
277
Tyndall
289
Sisseton
149
Brookings
173
Vermillion
337

Related Topics

Forage, Crop Management