Scout for Alfalfa Weevil in Southern Iowa

April 12, 2024
ICM News

In addition to checking alfalfa stands for winter injury, it is time to start thinking about scouting for alfalfa weevil. With warm temperatures this year, alfalfa weevil development is well ahead of schedule compared to recent years, and adults have emerged from their overwintering sites to lay eggs in alfalfa stems in southern Iowa. We have received reports from field agronomists in south central and southeast Iowa that alfalfa weevil larvae have been found on plants (and so have pea aphids; Photo 1).

Alfalfa weevil larvae and pea aphids.
Photo 1. Alfalfa weevil larvae and pea aphids found in south central Iowa on April 10, 2024. (Photo by Clarabell Probasco)

Adult alfalfa weevils become active and start laying eggs as soon as temperatures exceed 48°F. Alfalfa weevil egg hatching begins when 200-300 growing degree days (GDD; base 48°F) have accumulated since January 1. Based on accumulated GDD, alfalfa weevils are active throughout southern Iowa and are likely active in other areas as well (Figure 1). You can track GDD for alfalfa weevil by visiting the Pest Maps and Forecasting page and comparing benchmark GDD to the map. Additionally, if you choose your nearest weather station on the right side of the page, you can see forecasted GDD for the next two weeks.

Accumulated growing degree days map.

Figure 1. Accumulated growing degree days (base 48°F) in Iowa from January 1 – April 10, 2024. Map courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy.

Fields south of Interstate 80 in Iowa should be scouted beginning at 200 GDD and fields north of Interstate 80 should be scouted beginning at 250 GDD. If plants are tall enough, use a sweep net to detect larvae (Photo 2), then systematically sample fields to determine larval numbers and plant height. If plants are growing slowly and are too short to sweep, check plants for alfalfa weevil larvae beginning at the terminal leaflets. This encyclopedia article details alfalfa weevil identification and provides a scouting plan as well as information on economic thresholds for making treatment decisions.

Keep in mind that the economic threshold tables were developed based on healthy stands with plants 10-12 inches tall. If an alfalfa field is experiencing signs of stress from recent drought conditions or past pest issues, be on the conservative side of the ET tables when making treatment decisions. Early cutting is preferred to chemical treatments, especially when plants are at least 16 inches tall, but insecticides may be used if early harvest is not an option. The first cutting is at the greatest risk for alfalfa weevil feeding injury (Photo 2).

Alfalfa weevil larvae.
Photo 2. Left: Alfalfa weevil larvae detected using a sweep net. Right: In the same field, heavily defoliated alfalfa appears frosted from a distance. (Photos by Angie Rieck-Hinz, Iowa State University).

Be aware!

  1. Recently, entomologists and agronomists have observed more early-season injury to alfalfa stands and prolonged feeding by alfalfa weevil (sometimes after the first cutting). Typically, adults move to sheltered areas outside of alfalfa fields during the summer and remain there to overwinter. However, during warm autumns and mild winters, alfalfa weevil adults may become active and lay eggs. The overwintering eggs hatch earlier in the spring than our degree day models predict (developed based on adult activity in the spring), thus people may observe earlier and prolonged feeding in alfalfa. Continue to scout after the first cutting!
  2. In the western U.S., pyrethroid insecticides are no longer effective for alfalfa weevil; however, resistance has not yet been confirmed in Iowa or any Midwest state. If alfalfa weevils survive a pyrethroid application, please reach out to us so we can test for resistance.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on April 12, 2024. The information contained within may not be the most current and accurate depending on when it is accessed.


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...