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Alfalfa Weevil Activity Prediction Update: April 20, 2020

During the last week, the colder weather that we experienced across South Dakota resulted in very little degree day accumulation. However, in some areas enough degree days have accumulated to potentially lead to adult activity within alfalfa fields (Table 2).

Scouting for alfalfa weevil should begin in the Rapid City, Hot Springs, Mission and Winner areas by the end of the week when air temperatures will be higher. It is likely that adult activity will begin and will likely increase with the warmer weather that is currently forecasted for next week.

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
144
Newell
160
Rapid City
200
Hot Springs
253
Lemmon
132
Faith
120
Cottonwood
196
Mission
209
Selby
74
Gettysburg
105
Pierre
167
Winner
202
Aberdeen
70
Huron
98
Mitchell
129
Tyndall
156
Sisseton
46
Brookings
73
Vermillion
177

Related Topics

Forage, Crop Management