Written collaboratively by Adam Varenhorst, Philip Rozeboom, Patrick Wagner, and Brad McManus.
Originally Submitted: May 9, 2024
Although May hasn’t been extremely warm, the accumulated degree days in South Dakota since January 1, 2024, suggest that alfalfa weevil larvae could be active in several areas of the state. As larvae mature, the defoliation injury that they cause will increase. It’s also important to remember that even though enough degree days have accumulated in previous years, we have observed a lag time between alfalfa weevil larvae and accumulated degree days. In areas with over 300 degree days accumulated, alfalfa should be scouted to ensure that alfalfa weevils don’t cause extensive defoliation (Figure 1). Alfalfa near Caputa, Oral, Cottonwood, White River, Parkston, and Beresford should be closely monitored, as enough degree days have accumulated for second to third instar alfalfa weevil larvae to be present. If no larvae are present, it is likely due to the cooler weather that we have been experiencing. However, early scouting is the best way to prevent alfalfa weevil defoliation.
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 January 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.
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Alfalfa Weevil Activity (Overwintering as Adults) |
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Limited to no activity of alfalfa weevils. |
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Overwintering adult activity is observed in alfalfa fields (begin scouting fields). |
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Spring laid eggs hatch. First instar larvae begin feeding. |
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Second instar larvae feeding. |
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Third instar larvae feeding. |
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Fourth instar larvae feeding. (Period when greatest damage will occur) |
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Pupation. |
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Adult emergence (will overwinter). |
