A few precautions can help prevent blister beetle contamination of hay and avoid sickness or death in horses that eat it, says Bruce Anderson, University of Nebraska Extension forage specialist.

Adult blister beetles rarely are found in first-cut or October-cut alfalfa, but they can be numerous in late summer, says Anderson. They congregate, or swarm, in small areas of a field and feed on blossoms, but they’ll eat leaves if blossoms aren't available.

Fortunately, blister beetles don’t travel far; they usually aren’t found beyond the first 50 yards of the field margin, he says. So avoid feeding alfalfa from near field margins to horses.

One of the best ways to lessen the danger from blister beetles is to mow hay without mechanically conditioning. Since blister beetles often swarm, a large group can be crushed by the rollers and killed. As a horse eats the hay, it may consume over 100 beetles in a single flake. But if a conditioner isn’t used, the beetles will crawl out of the swath and fly away.

Insecticides like Sevin or a synthetic pyrethroid also can be used, particularly around field margins, but that adds to your costs and often kills beneficial insects like honeybees as well, says Anderson.