While monitoring alfalfa plots at the Cimarron Valley Research Station (CVRS) Perkins, OK over the weekend I observed some early 1st instar alfalfa weevil (AW) larvae on several plants, in addition to some minor tip feeding (Fig. 1).

Figure 1. Alfalfa Weevil larva tip feeding on new growth. University of Kentucky.

Degree Day totals at the Perkins research station are currently at 264, with many areas of the southern part of the state at or above 300. Therefore, we are in the timeframe for potential feeding by young larvae to begin. Based on information from the Alfalfa Weevil Model as degree day numbers accumulate from January 1 each year, 150 - degree days serves as guide for monitoring to begin looking for larval activity. However, this is just a starting point. Several variables can come into play after 150 - degree days are achieved and before threshold occurs. The cold weather experienced back in January and recently will slow the development of any early developing larvae. Also, temperatures in single digits or below such as we had in late January over several days can increase mortality, not only in larvae, but also eggs that may have already been laid. 

As we move forward, and consistent warm weather starts to occur, adult activity will increase, and additional oviposition (egg laying) will take place. In warmer conditions (>70°), larval development can happen quickly. Alfalfa weevil larvae go through four instars (developmental stages) (Fig. 2), with the later instars generally doing the most (>80%) of the damage (Fig. 3). 

Figure 2. Alfalfa Weevil larva developmental stages (1-4 instars). Cornell University

After oviposition, with optimum weather conditions, development from egg to final instar can occur in approximately two weeks. Monitoring and decision making must be made quickly during this timeframe, as defoliation of alfalfa fields can happen in as little as 3 days with heavy infestations.

Figure 3. Larval damage. Oklahoma State University.

The past two seasons, based on mild winter weather and early insect activity, AW threshold applications in the Stillwater, OK (Payne County) area were made in late February.

These applications represented a timeline of almost three weeks ahead of historical first threshold applications. In addition, with earlier insect activity and thresholds being reached sooner, plant height just coming out of winter dormancy was much shorter. This resulted in application being made with 1.0 – 2.0 inches on new growth, with a threshold level per stem at around 0.25. Keep in mind the shorter the plant the lower the threshold and there is an increased chance of damage.  With degree days approaching or exceeding 300 in many southern areas of the state, threshold could arrive early this year as well. More information on scouting can be found in CR- 7177, Scouting for the Alfalfa Weevil in Oklahoma.

Weather is unpredictable. After a warm period, we could still be in for some cool/wet temperatures into March. Continued monitoring is essential for keeping up with this perennial pest.

***Keep an eye out for future news releases for seasonal updates.

For more information on seasonal insect pests in alfalfa throughout the year, visit the Oklahoma Alfalfa Management Calendar for Insects and Diseases.