Alfalfa Weevil Potential Activity Update Back »

Written collaboratively by Adam Varenhorst, Amanda Bachmann, Laura Edwards, Philip Rozeboom, and Patrick Wagner.


During the past week most of South Dakota has accumulated enough degree days (300-595 accumulated degree days), (Table 1) to now have alfalfa weevil larvae present (Table 2). Scouting should be intensified in areas where larvae may be in later instars (439-595 accumulated degree days) as they have a greater potential to cause significant defoliation.

As a reminder, the equation for degree days is:

(Maximum daily temperature - Minimum daily temperature) ÷ 2 - The Developmental Threshold

For alfalfa weevils, 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. Initial overwintering adult alfalfa weevil activity is generally observed around 200-degree days and egg hatch is generally around 300-degree days (Table 1).

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

Accumulated Degree Days Alfalfa Weevil Activity (Overwintering as Adults) Recommendation
0-199 SLimited to no activity of alfalfa weevils. Scouting is not necessary.
200-299 Overwintering adult activity is observed in alfalfa fields (begin scouting fields). Begin scouting. for adult populations.
300-371 Spring laid eggs hatch. 1st instar larvae begin feeding. Begin scouting for larvae populations.
372-438 2nd instar larvae. Continue scouting for larvae populations and defoliation.
439-504 3rd instar larvae. Continue scouting for larvae populations and defoliation.
505-595 4th instar larvae. Continue scouting for larvae populations and defoliation.
596-814 Pupation. Scout for remaining larvae.
815-greater Adult Emergence (will overwinter). No scouting necessary.

 

Table 2. Degree day accumulations for several South Dakota locations.

Location Accumulated Degree Days
Since January 1, 2018
Recommendation
Buffalo 347 Scout for adults.
Newell 318.5 Scout for adults.
Rapid City 422.5 Scout for adults and larvae.
Hot Springs 520.5 Scout for adults and larvae.
Lemmon 366.5 Scout for adults.
Faith 322 Scout for adults.
Cottonwood 383.5 Scout for adults.
Mission 382 Scout for adults.
Selby 330.5 Scout for adults.
Gettysburg 322 Scout for adults.
Pierre 390.5 Scout for adults.
Winner 435 Scout for adults and larvae.
Aberdeen 374 Scout for adults.
Huron 381.5 Scout for adults.
Tyndall 402.5 Scout for adults.
Sisseton 354.5 Scout for adults.
Brookings 300 No scouting necessary.
Vermillion 458.5 Scout for adults and larvae.

 

For more information regarding alfalfa weevil scouting and thresholds, please refer to 2018 Alfalfa Weevil Activity Prediction and Scouting Recommendations.

If thresholds are exceeded, there are several insecticides available for management. Please refer to the 2018 Alfalfa and Oilseeds Pest Management Guide for rates and labeled insecticides.

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