For the 2011 season, alfalfa weevils are already starting to hatch in Iowa, as of April 11, in the southern third of the state. They will be active in northern Iowa by April 20 to 25. That's the word from Erin Hodgson, Iowa State University Extension entomologist, and Adam Sisson of ISU's Corn and Soybean Initiative. They offer the following observations and scouting and management guidelines.
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 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 5/16th of an inch long. Adults 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.
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: http://mesonet.agron.iastate.edu/data/summary/gdd48_jan1.png
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.