Blister beetles: What hay producers, farmers need to know

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By Rick Greenwell

Several species of blister beetles occur in Kentucky but large populations are not common. These mid- to late summer insects are active from about mid-July through early August. They are especially attracted to and feed on flowers. An aggregation pheromone released by the insects as they feed results in accumulations of beetles or “hot spots”, often along field margins.

Tips for hay producers
• Learn to recognize blister beetles and understand their behavior. A proactive preventive program will reduce the chances of infested hay.
• Blister beetles are not active when the first cutting is made in Kentucky; harvest at the late bud stage or when the first flowers open for high quality hay for horses.
• Blister beetles are attracted to blooms. Manage harvest intervals to minimize flowering of alfalfa or weeds in grass hay. Practice good broadleaf weed management. If flowering plants are present at harvest, inspect them carefully for blister beetles.
• Avoid crimping hay during harvest. Straddle cut swaths to avoid crushing beetles with tractor tires.
• There is no efficient way to inspect baled hay carefully enough to be sure that it is free of blister beetles.

Tips for horse owners
• Buy first cutting hay; blister beetles are not active then.
• Develop a relationship your hay producer so that you know their production practices and hay quality. Buy from local sources.

Blister beetles have long (3/4” to 1-1/4”), narrow bodies, broad heads, and a1/3 the length of the entire body. The segment behind the head is narrow, so the beetle appears to have a “neck”. The front wings are soft and flexible in contrast to the hard front wings of most beetles. The black blister beetle (jet black) and the margined blister beetle (black with thin gray stripe around wing covers) are common species in Kentucky.

Female blister beetles lay clusters of eggs in the soil in late summer. The small, active larvae that hatch from these eggs crawl over the soil surface entering cracks in search for grasshopper egg pods which are deposited in the soil. After finding the egg mass, blister beetle larvae become immobile and spend the rest of their developmental time as legless grubs. Blister beetles will not lay eggs in hay and the larvae do not feed on or develop in hay bales.