CUTTING HAY FOR HORSES – WATCH OUT FOR POISONOUS BLISTER BEETLES
Several blister beetle species feed on alfalfa, including the ash-gray blister beetle, black blister beetle and striped blister beetle. Adult blister beetles are easy to recognize due to their large size (½ to 1½ inch), soft bodies, short wing covers, long legs and a neck-like appearance between head and prothorax. Most blister beetle species have one generation per year. Adults become active in early to mid-summer and females lay eggs in the soil. Eggs hatch in about two weeks into larvae. Larva are beneficial insects and are effective predators of grasshopper eggs. Blister beetles overwinter in soil as larvae. Field reports indicate that blister beetles are high again in 2022 (like 2021) in most areas of hay production due to high grasshopper populations. Adult blister beetles are attracted to blooming alfalfa fields and weeds (goldenrods, dandelions). Adults are active June through September and will feed on nectar, pollen, leaves, stems and flowers.
The most severe threat from blister beetles is their poison called cantharidin, which is toxic to people and livestock, especially horses. Cantharidin oil is released when beetles are crushed and even dead beetles have high levels of the toxin that does not weaken. Cantharidin is highly toxic and irritates the gastrointestinal and urinary tracts, which can lead to death in horses. It is estimated that about 30 to 50 striped blister beetles could be potentially lethal to horses. Fewer beetles cause sores or blisters on tongue and mouth, colic, diarrhea, bloody feces, depression, elevated temperatures, increased heart rate, increased breathing rate and dehydration.
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Poisoned horses often place their muzzle in water without drinking. Hay infested with blister beetles is a big concern for hay producers and livestock owners, especially horse owners. Cattle and sheep can also be poisoned. Levels of toxicity to beef cattle has not been identified.
Cantharidin poisoning has no cure except supportive care including mineral oil, intravenous fluid therapy, activated charcoal, and anesthetics. Blister beetles feed for a short period of time and then migrate to other areas of the field or to new fields. After the alfalfa is cut, they often move out into blooming canola and other blooming field crops. Below are a few management recommendations.
• Raking may dislodge dead beetles from hay.
• Scout harvested hay and underneath windrows closely for blister beetles and allow blister beetles to move out of drying hay before baling. Turning the windrow may be helpful to get blister beetles to move out.
• If blister beetles are suspected in harvested hay, either don’t feed the hay or provide alternative feeds/hay for livestock. Don’t force livestock to eat hay contaminated with blister beetles.
• Chemical control is generally not recommended at harvest, because dead beetles could be incorporated into cured hay instead of falling onto the ground. Remember, dead blister beetles still have a level of cantharidin toxic to livestock.
• Grinding hay only dilutes the cantharidin toxin when the hay is mixed into a final ration.
Karl Hoppe Extension Livestock Specialist, Carrington Research Extension Center and Janet J. Knodel NDSU Extension Entomologist wrote this article. The complete article with thresholds is available at bit.ly/3OH5FrX.
Montana State University, U.S. Department of Agriculture and Montana Counties Cooperating. MSU Extension is an equal opportunity/affirmative action provider of educational outreach.
