Alfalfa weevil hatch is occurring earlier than normal this year. The insect is active now in Iowa and you should be scouting fields. That warning comes from Erin Hodgson, Iowa State University Extension entomologist, and Adam Sisson, ISU integrated pest management specialist. Following are their recommendations and scouting 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. Adults eat along the leaf margin, leaving irregular notches. 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 active now. To follow accumulating degree days (base 48°F) throughout the year, visit the ISU Mesonet website.
To initially detect alfalfa weevil larvae in the spring, use a sweep net to sample the field. 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 the plant height to determine if a foliar insecticide is warranted. Remember, cutting alfalfa for harvest is an effective management tool for alfalfa weevil larvae, and an insecticide application may be avoided if harvesting the alfalfa within a few days.