Alfalfa weevil larvae can do a lot of damage to an alfalfa field. If populations reach the economic threshold level, you need to pull the trigger and spray an insecticide. The adult stage of the alfalfa weevil is already active this spring, with the warm weather. Erin Hodgson, an Iowa State University Extension entomologist, provides the following information and guidelines for scouting and managing alfalfa fields to protect against this insect pest.
Adult alfalfa weevils begin moving as soon as temperatures exceed 48 degrees F and begin laying eggs in alfalfa. Alfalfa weevil eggs develop based on temperature, or accumulating degree days, and hatching can start around 200-300 degree days. Start scouting alfalfa fields south of Interstate 80 at 200 degree days and fields north of Interstate 80 at 250 degree days. Based on accumulated temperatures since January, weevils could be active throughout Iowa (Fig. 1).
Figure 1. Accumulated growing degree days (base 48 degrees F) in Iowa from January 1 to April 8, 2014. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.
This dynamic degree day map is updated daily and is available online. To more accurately time scouting efforts in April and May, use this website to generate up-to-date information for your area.
Click on the "View Degree Day Map" button in the left corner of the page and then set the parameters for degree days to create a new map. Make sure to set the start date to January 1 of the current year and the end date to today; set base temperature to 48 degrees F and ceiling temperature to 86 degrees F.
Peak larval activity occurs around 575 degree days. Often silken pupal cases are attached to leaves in the lower canopy or in leaf litter. The time it takes to reach the adult stage is dependent on temperature, but can take about eight weeks. Adults (Photo 2) eat along the leaf margin, leaving irregular notches. A heavily infested field will look frosted or silver (Photo 3).
Photo 1. 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 Clemson Cooperative Extension Slide Series, www.ipmimages.org.
Alfalfa weevil adults have an elongated snout and elbowed antennae. Their wings and body are mottled or brown in color.
Photo 3. Heavily-defoliated alfalfa fields appear frosted from a distance. Photo by Whitney Cranshaw, Colorado State University, ipmimages.org.
Management guidelines: how to scout, when to treat
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. After larvae are first collected in sweep nets, collect six alfalfa stems from five locations throughout the field. Take each stem and vigorously shake into a bucket to dislodge larvae from the plant. Small larvae can be difficult to separate from the plant and therefore careful plant inspection is also needed.
Average the number of larvae per 30 stems and plant height to determine if the economic threshold is approaching (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 of reaching the economic threshold. For more information on how to interpret the table, go to http://iowabuglife.blogspot.com
Table 1. Economic threshold of alfalfa weevil, based on the average number of larvae in a 30-stem sample (Originally published by John Tooker, Penn State Extension.
NOTE: Erin Hodgson is an ISU associate professor of entomology with Extension and research responsibilities; contact her at [email protected] or by calling 515-294-2847.