Record Populations of Alfalfa Weevils in South Dakota

There are record high populations of alfalfa weevils this year according to data collected in Fall River and Butte Counties in western South Dakota.

Although monitoring for alfalfa weevils has been conducted since last week of April, Anitha Chirumamilla, SDSU Extension Entomology Field Specialist said it was not until the last week of May that significant numbers were observed.

"It appears that the weevil activity lagged approximately three weeks behind average degree day requirements. Almost each and every field in Fall River and Butte Counties has weevil numbers beyond economic thresholds and the damage is significant," she said.

The sampling populations are dominated with young weevil larvae ranging from first to third instars. Instars, Chirumamilla explained refers to the developmental stages of larva between two successive molts. "Larvae at this stage of development have the capacity to cause severe defoliation as they have to go through four stages of development before pupation," she said.

Chirumamilla explained that larval development wraps up about three weeks from hatching to cocoon formation. "With such high populations of early larval instars, fields can turn white from weevil defoliation of the leaves in matter of days," she said.

Recommendations
Early harvest or applications of insecticides are the two recommended management strategies in the current situation.

Early Harvest: Considering the height and growth stage of alfalfa, early harvesting is definitely a low-cost-effort and the least toxic method of reducing the defoliation by weevil larvae. However, the cool and wet conditions in South Dakota are making this difficult.

Larvae and adults have a better chance of hiding under the windrows and delayed baling due to climate might put the regrowth at higher risk. Moreover, adults are actively mating in the fields. Continued egg deposition by weevils in harvested stubble and feeding of the newly hatched larvae will severely affect alfalfa regrowth. Adult weevils feeding on the stubble might predispose the plants to diseases and in case of dry-land alfalfa delayed regrowth might make the plants go dormant.

Chemical Application: The number of weevil larvae in alfalfa fields of South Dakota is beyond economic threshold, which is greater than 75 larvae on 30stems at 16 inches height. This justifies the use of insecticides as a control option. If applied at this level of infestation, insecticides will prevent further damage and will extend the harvesting period window. However, when choosing an insecticide caution should be exercised in observing the pre-harvest intervals.

In case of blooming alfalfa, select insecticides that are least toxic to bees and choose the time of application when they are least active. For information on growing degree day requirements, scouting techniques, and economic thresholds for alfalfa weevils refer to the article: http://igrow.org/agronomy/other-crops/spring-is-here-watch-out-for-alfalfa-weevils/
South Dakota alfalfa growers need to take immediate action to reduce any further losses.

"Considering the record level of infestation this year, alfalfa regrowth should be carefully monitored for weevil activity irrespective of the options chosen for first cutting, early harvesting or insecticide application," said Chirumamilla.

To learn more, visit iGrow.org.