2019 Alfalfa Weevil Activity Prediction

 

Despite a warm week, the 2019 growing season is behind schedule due to the cool, wet conditions experienced over much of the state this spring. One of the major insect pests of alfalfa in South Dakota is the alfalfa weevil. Before 2018, this insect was reported as having large populations throughout much of South Dakota. However, in 2018, we received fewer reports of alfalfa weevils, which may have been a result of the cooler and wetter spring conditions that were observed last spring. It’s possible that populations may also be lower in 2019 due to the similar spring conditions that we are currently experiencing.

 

 

The alfalfa weevil defoliates alfalfa as both adults and larvae. However, the larvae cause a lot more defoliation than the adults. One of the ways to determine when to start scouting for alfalfa weevils is by calculating degree days. In South Dakota, alfalfa weevils overwinter as adults. There are two strains of the alfalfa weevil that are present in South Dakota and overlap in most of the state. They are the Eastern strain and Western strain. Although the two strains are nearly identical in appearance 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.

 

 

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. The 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.

 

 

For South Dakota, most of the state has now accumulated over 200-degree days and scouting for alfalfa weevils should begin. For alfalfa near Hot Springs, Winner, Tyndall or Vermillion scouting should be intensified as enough degree days have been accumulated for larvae hatch and larvae activity between the 1st and 2nd instars. This means that leaf feeding is likely noticeable and weevil larvae should be observed within alfalfa fields. For information on how to scout for alfalfa weevils and threshold recommendations please refer to the article, “Scouting for Alfalfa Weevils.”