Larvae do the most damage, although adult alfalfa weevils are also injurious. Early season feeding in leaf buds and terminal growing areas may seriously retard normal plant growth. As the larvae grow, they feed on leaves, and eventually skeletonize them. The larvae can easily be seen curled around stems or leaves. As feeding persists, an infested field takes on a distinct grayish appearance. Adult weevils notch main stems, side shoots, and leaf stems. They often sever the latter two from the plant. They also feed on the leaves, causing ragged and torn leaf margins. Females puncture the stem during oviposition. Feeding damage by both adults and larvae greatly reduces the quality and yield of the first cutting of hay, and may reduce the quality of subsequent cuttings.
Adult weevils, 4.0 to 6.5 mm long, oblong, convex, brown to blackish, with 3 clear bands formed of bifid hairs. The legless larvae are about 6-10 mm long when fully grown. They are pale green with a thin white line down the center of the back and have a brown head. Larvae complete their growth in about 3 to 4 weeks. They will then spin a cocoon and pupate either in the leaves of the plant or on the ground, usually by early summer. In most years, there is only one generation.
An integrated pest management program is suggested for alfalfa weevil. The combination of biological, cultural and chemical control practices can effectively reduce weevil populations and damage. It is suggested to review factsheet EPP-2097, “Alfalfa Weevil and Its Management in Oklahoma” for detailed information.
Variety selection: There are no varieties of alfalfa that are adapted to Oklahoma conditions and have high resistance to the alfalfa weevil. However, some varieties show tolerance to this pest.
Cultural practices: Egg numbers and subsequent larval damage can be reduced if the crop is grazed during the winter by cattle.
Biological control: There are both insect parasites and fungal diseases that affect alfalfa weevils. These organisms occur naturally in Oklahoma.
Chemical control: Insecticide applications may be used to reduce weevil populations, but they should only be applied when the crop has been scouted and damaging levels of larvae are found. Review the Current Report CR-7177, “Scouting for the Alfalfa Weevil In Oklahoma” for guidance on scouting.
Please contact your local county extension office for current information.