Agriculture What Has Been Your Experience With Alfalfa Weevil?

By John E. Woodmansee

If you are involved with alfalfa production, you may recall some springtime discussions and news articles in 2025 about alfalfa weevil and the potential issue of resistance. Specifically, Purdue field crops pest management specialist Christian Krupke said there were anecdotal reports that pyrethroids may not be working quite as well as they have in the past.

Before going on, let’s review some facts about alfalfa weevil.

Alfalfa weevil larvae chew on and skeletonize alfalfa leaves. Early damage from larvae appears as pinholes in leaves. Since leaves comprise a significant portion of the forage product we utilize from alfalfa, this is a problem. In addition to yield loss, plant vigor is reduced, and large larval populations can result in defoliation of entire plants and stand loss, giving a field a grayish appearance.

Larvae are slate-colored when small, but turn bright green when full-grown, when they measure about 3/8 inch. They have a white stripe down their back and have a black head.

When larvae are fully grown, they spin silken cocoons on the plant or within the curl of dead leaves. They change into adults in 1-2 weeks. Adults are dark gray to brown with a distinct dark shield-like mark on their back. They are part of the family of snout beetles, characterized by their obvious elongated snouts, which are an elongated proboscis (rostrum) extending from their heads.

Field scouting for alfalfa weevil damage should begin when approximately 250 heat units [base 48°F (8.9°C)] have accumulated from January 1. According to Austin Pearson, climatologist at the Indiana State Climate Office at Purdue University, this target date to begin scouting has varied from April 20 to May 20 since 2016 (data from Fort Wayne, IN).