It was another cool week in South Dakota with not a lot of degree day accumulation occurring. Alfalfa fields should still be monitored for alfalfa weevil activity even though the cool wet weather might have slowed this pest down.

The alfalfa weevil defoliates alfalfa as both adults and larvae, but the larvae cause significantly more defoliation than the adults. In South Dakota, alfalfa weevils overwinter as adults and there are two strains of the weevil that overlap in most of the state. The Eastern strain and the Western strain are nearly identical in appearance, but they do differ in their spring emergence patterns, with the Eastern strain emerging earlier in the season than the Western Strain. As a result, it sometimes will appear that there are two generations of alfalfa weevils during the spring.

One of the ways that we can determine when to start scouting for alfalfa weevils is by calculating degree days. To monitor alfalfa weevil activity, 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 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 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.

For South Dakota, all of the state has now accumulated over 200-degree days and scouting for alfalfa weevils should begin. For alfalfa near Newell, Rapid City, Hot Springs, Cottonwood, Mission, Pierre, Winner, Mitchell, Tyndall, or Vermillion, scouting should be intensified as enough degree days have been accumulated for egg hatch and larval activity between the 1st and 3rd instars. This means that leaf feeding is likely noticeable and weevil larvae could be observed within alfalfa fields. It’s important to remember that degree day estimates are not 100% accurate, but instead provide a guideline for when activity is likely.