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
|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.