Scout for Alfalfa Weevil in Southern Iowa

April 15, 2022
ICM News

In addition to checking alfalfa stands for winter injury, it is time to start thinking about scouting for alfalfa weevil. Even with recent cool temperatures, overwintering alfalfa weevil adults have become active, made their way to alfalfa fields, and have likely begun laying eggs in stems. Alfalfa weevil is a cool-season pest and is able to survive less than ideal temperatures by moving under residue or near the crown.

Adult alfalfa weevils become active and start laying eggs as soon as temperatures exceed 48°F. Alfalfa weevil egg hatching begins when 200-300 GDD (base 48°F) have accumulated since January 1. Based on accumulated GDD, alfalfa weevils may be active in the southern and western parts of the state (Figure 1). Areas in northern Iowa have lower GDD accumulation and may not see activity yet. Forecasted cooler temperatures are likely to slow down development of both the plant and pest across the state.

Growing degree days map.
Figure 1. Accumulated growing degree days (base 48°F) in Iowa from January 1 – April 13, 2021. Map courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy.

In Iowa, fields south of Interstate 80 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 1), then systematically sample fields to determine larval numbers and plant height. If plants are still short due to slow development this spring, 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. 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 1).

alfalfa weevil.
Photo 1. 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.

Keep in mind that the economic threshold tables were developed based on healthy stands with plants 10-12 inches tall. Recent conditions (in 2022 we had drought, high alfalfa weevil populations, and a fall armyworm outbreak) may have increased stress on alfalfa stands, and those fields will be impacted greater than healthy stands. If this is the case, be on the conservative side of the ET tables when making treatment decisions.

Be aware! 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 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! 

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 15, 2022. 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...