Alfalfa Weevil Hatch Happening Now

April 8, 2011
ICM News

By Erin Hodgson, Department of Entomology and Adam Sisson, Corn and Soybean Initiative

Alfalfa weevil is an important defoliating pest in alfalfa. Heavy infestations can reduce tonnage and forage quality. Adults can feed on plants, but the larvae typically cause the majority of damage.

Newly hatched larvae can be found feeding on terminal leaves, leaving newly expanded leaves skeletonized. Gradually maturing larvae (Fig. 1) move down the plant and begin feeding between leaf veins. Alfalfa weevil larvae have a dark head and pale green body with a white stripe down the back. Alfalfa larvae are about 5/16 inches long. Adults (Fig. 2) eat along the leaf margin, leaving irregular notches. Alfalfa weevil adults have an elongated snout and elbowed antennae. Their wings and body are mottled or brown in color. A heavily infested field will look frosted or silver (Fig. 3).

alfalfa weevil larvae      alfalfa weevil adult
Figure 1. Alfalfa larvae.
                         Figure 2. Alfalfa weevil adult.     
Fig. 1 Photo by Clemson Cooperative Extension Slide Series, Fig. 2 Photo by Joseph Berger,

alfalfa weevil damage
Figure 3. Heavily defoliated alfalfa fields appear frosted from a distance. Photo by Whitney Cranshaw, Colorado State University,

Alfalfa weevils develop based on temperature or accumulating degree days. Scouting in fields should begin at approximately 200 degree days for areas south of Interstate 80, and 250 degree days north of Interstate 80. Based on accumulated temperatures since January, weevils are likely active now, or will become active in the next several days, in the southern half of the state (Fig. 4). We would expect weevils to become active in northern Iowa by April 20 - 25. To follow accumulating degree days throughout the year, visit the ISU Mesonet website.  The base 48 F degree day map is updated daily at:

Figure 4. Accumulated growing degree days (base 48°F) in Iowa from January 1 — April 7, 2011.
Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.

To initially detect alfalfa weevil larvae in the spring, use a sweep net to sample. After finding larvae, collect six alfalfa stems from five locations throughout the field. Take each stem and shake into a bucket to dislodge larvae from the plant. Average the number of larvae per stem and plant height to determine if a foliar insecticide is warranted (Table 1). Remember cutting alfalfa is an effective management tool for alfalfa weevil larvae, and an insecticide application may be avoided if harvesting within a few days.

Table 1. Economic threshold of alfalfa weevil, based on the average number of a 30-stem sample



Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. Hodgson can be contacted at or phone 515-294-2847. Adam Sisson is a program assistant with responsibilities with the Corn and Soybean Initiative. Sisson can be contacted by email at or by calling 515-294-5899.

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 8, 2011. The information contained within may not be the most current and accurate depending on when it is accessed.


Erin Hodgson Associate Professor

Dr. Erin Hodgson started working in the Department of Entomology at Iowa State University in 2009. She is an associate professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated pest management (IPM) for field crops. Dr. Hodgson's curre...

Adam Sisson Extension Program Specialist III

Adam Sisson is an extension specialist with the Iowa State University Integrated Pest Management (IPM) program. Sisson focuses on the development of publications and other educational resources for farmers, agribusiness, and students. He receive...