Alfalfa Weevils Active throughout Southern Iowa

April 12, 2021
ICM News

Adult alfalfa weevils become active and start laying eggs as soon as temperatures exceed 48°F. Like other insects, the development of alfalfa weevil depends on temperature, and we can use accumulation of growing degree days (GDD) to predict activity. Alfalfa weevil egg hatching begins when 200-300 GDD (base 48°F) have accumulated since January 1.

Based on accumulated temperatures since January, alfalfa weevils may be active in the southern half of the state (Figure 1). 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 degree days. Areas in northern Iowa have lower GDD accumulation and may not see activity yet, but with forecasted temperatures we could see activity by the end of April.


Figure 1. Accumulated growing degree days (base 48°F) in Iowa from January 1 – April 8, 2021. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.

Biology

Alfalfa weevil is an important defoliating pest in alfalfa. Heavy infestations can reduce tonnage and forage quality. Alfalfa weevil larvae typically cause the majority of plant injury. Newly hatched larvae can be found feeding on terminal leaves, leaving newly expanded leaves skeletonized. Maturing larvae (Photo 1) move down the plant and begin feeding between leaf veins. Peak larval activity occurs around 575 GDD. Silken pupal cases are often attached to leaves in the lower canopy or in leaf litter.


Photo 1. Mature alfalfa weevil larvae have a dark head and pale green body with a white stripe down the back. Fully-grown larvae are about 5/16 inches long. Photo by John Obermeyer, Purdue University Extension.

The time it takes to reach the adult stage is dependent on temperature but is usually around eight weeks. Adults (Photo 2) cause less plant injury than larvae. They feed along the leaf margin, leaving irregular notches. Female alfalfa weevils can lay 800-4,000 eggs in a lifetime and insert 5-20 eggs at a time into alfalfa stems. A heavily infested field will look frosted or silver (Photo 3).


Photo 2. Alfalfa weevil adults have an elongated snout and elbowed antennae. Their wings and body are mottled or brown. Photo by Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org.



Photo 3. Heavily-defoliated alfalfa fields appear frosted from a distance. Photo by Whitney Cranshaw, Colorado State University, ipmimages.org.

Scouting and Management

After reaching benchmark degree days (200 in southern Iowa and 250 in northern Iowa), use a sweep net to sample for adults and larvae. South-facing slopes warm up faster and may be a place to start sampling. Once the first larvae are collected in your sweep net, you need to know four pieces of information to decide if the economic threshold has been reached in that field:

  1. Market value of the hay ($/ton).
  2. Control costs ($/acre).
  3. Plant height (inches).
  4. Number of larvae.

The last two pieces of the economic threshold determination can be gathered by scouting the field. Collect six alfalfa stems from 5 random locations throughout the field (total of 30 stems) by breaking them off at the base, making sure to be gentle so as not to lose larvae during the process. Measure the height of these plants. Most of the larvae can be dislodged by vigorously shaking the stems into a bucket. Small larvae can be difficult to separate from the plant, so the plants should also be carefully inspected after shaking. Take the average plant height and count the total number of larvae per 30 stems, then use Table 1 to determine if an insecticide application is warranted.

Cutting alfalfa is an effective management tool for alfalfa weevil larvae, and an insecticide application may be avoided if harvesting occurs within a few days of reaching the economic threshold. Harvesting is preferred to chemical treatments once plants are 16 inches tall.


Table 1. Economic threshold of alfalfa weevil, based on the total number of larvae in a 30-stem sample (Originally published by John Tooker, Penn State Extension).

 

For more information on how to interpret the table, click here.

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, 2021. The information contained within may not be the most current and accurate depending on when it is accessed.

Authors: 

Ashley Dean Education Extension Specialist I

Ashley is an education extension specialist for field crop entomology at Iowa State University. She coordinates the Iowa Moth Trapping Network, develops educational resources for field crop pests in Iowa, and aids in the research efforts of the

Erin Hodgson 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...