Alfalfa Weevil Potential Activity Update

By Adam J. Varenhorst

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.

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