Record Year for Alfalfa Weevil

June 10, 2021 1:36 PM
Blog Post

Most people are already finished with their first cutting of alfalfa throughout Iowa, but each of our 10 field agronomists reports that alfalfa weevil activity continues and has been much worse than they’ve seen in over a decade. Lance Scott in southwest Iowa took this short video to show significant injury. Alfalfa weevil only has one generation per year. The larvae are the most damaging stage, defoliating leaves and sometimes only leaving the veins behind. Fields may appear frosted, or have a silvery appearance, with heavy alfalfa weevil feeding as leaflets turn brown and die (Photo 1). Based on accumulated degree days, it is likely that larvae are done feeding and pupating throughout much of the southern half of the state. Any adults at this time are going to enter their summer dormancy period (non-feeding) soon.

alfalfa weevils and frosted appearance
Photo 1. Alfalfa weevil larvae in a sweep net sample (left). Heavy alfalfa weevil feeding may result in a frosted appearance (right). Photos by Angela Rieck-Hinz.

Larvae may still be active in some areas and would likely be 3rd or 4th instars that do much of the feeding. Although cutting alfalfa is typically an effective management strategy for alfalfa weevil, larvae can seek shelter under windrows and feed on the crown of regrowth. Now would be an excellent time to scout alfalfa stubble for alfalfa weevil larvae, especially in northern Iowa where larvae may still be feeding. Treatment of regrowth is recommended if eight or more larvae are found per square foot or if regrowth is delayed due to feeding. Regrowth may already be significantly challenged by drought conditions across the state.

Even though alfalfa weevil activity may be wrapping up around the state, this would also be a good time to think about other alfalfa pests. Aphids have been present in alfalfa fields for some time already, but scouting and proper identification can determine if significant injury is occurring and treatment is needed. Typically, very few aphids survive harvesting, so it is possible aphid numbers will be lower in fields that have been mowed. Continue to scout for aphids and their look-alikes, the potato leafhopper. The potato leafhopper is likely beginning to migrate from southern states and is persistent in alfalfa from the second cutting through August.


Ashley Dean Agriculture and Natural Resources Extension Specialist II

Ashley is an education extension specialist for field crop entomology at Iowa State University. She coordinates the Iowa Moth Trapping Network, the Regional Corn Rootworm Monitoring Network, and the Iowa Pest Alert Network. She also develops educational resources for field crop pests in Iowa and ...

Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology, now the Department of Plant Pathology, Entomology, and Microbiology, at Iowa State University in 2009. She is a professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated...