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Blister beetles in alfalfa hay

Krishona Martinson, Equine Extension specialist;  Jared Goplen, Extension educator; Bill Hutchison, Extension entomologist; and Roger Moon, Professor emeritus

Swarm of blister beetles. Photo: Doug Danforth
The shortage of horse quality hay in Minnesota has many owners purchasing hay from outside Minnesota and the Midwest. Along with this comes the risk of purchasing alfalfa hay infested with blister beetles. While rare, beetle-infested hay can cause health problems and death in horses and other livestock.

Blister beetles are black elongated beetles that are attracted to and feed on the flowers, pollen, and leaves of blooming alfalfa and weeds. Although not uncommon in Minnesota, blister beetles tend to be more numerous in alfalfa produced in arid southern states and plains states where grasshoppers are problematic because developing beetles feed on grasshopper eggs.

Pay attention to production details

When buying hay, owners should learn as much as possible about its production, including who produced it, where it was produced, and what the cutting and maturity was at harvest. If details aren’t available, owners should pay close attention to alfalfa hay that was mature or flowering when harvested.

Unfortunately, detecting blister beetles in hay is practically impossible because the beetles tend to swarm. This means that only a few bales, or even just parts of bales from an entire field may be infested. When feeding small square bales, an observant owner may find some dead, crushed beetles and discard that bale; however, beetles are very difficult to spot.

What to look for

If owners see black elongated beetles, it is best to dispose of the hay. Adult blister beetles are long, narrow beetles (0.5 to 1.25 inches long) with a broad head and antennae that are straight and about one third their total body length. Several species exist, including black, ash gray or striped beetles, each having unique color patterns.

Blister beetles naturally produce a compound called cantharidin. The level of cantharidin produced is highly variable and released when the beetle is crushed during hay making. Cantharidin remains toxic in dead beetles and does not decrease during storage.

When eaten, specific symptoms in horses include sores or blisters on the tongue and in the mouth, colic, straining, increased temperature, depression, increased heart and respiratory rates, dehydration, sweating, diarrhea, bloody feces, and frequent urination. If cantharidin poisoning is suspected, owners should contact their veterinarian immediately. There is no specific antidote beyond supportive care, which includes mineral oil, intravenous fluid therapy, activated charcoal, and analgesics.

Minimize risk

Although the risk of blister beetles in hay cannot be eliminated, it can be minimized during harvest. In the Midwest, beetle populations are relatively low until mid-summer (e.g. August), so first and second cutting tend to have minimal infestations. Beetles are more likely to be found in flowering alfalfa hay and when grasshoppers are present.

Hay that is simultaneously cut and crimped with a mower conditioner is more likely to kill beetles, keeping them in the windrow. However, crimping speeds drying, which is important for harvesting high quality hay.

Cutting alfalfa at less than 10% bloom reduces the number of beetles; however, hay cut at this maturity is often too nutrient dense for most horses. Farmers should watch for beetle "swarms” during harvest, and stop to allow the beetles to disperse. Chemical control is not the best option because dead beetles may still end up in hay. Instead, farmers should practice integrated pest management which includes scouting (for beetles and grasshoppers) and controlling weeds.
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  1. Might there be blister beetles in Eastern WA. third cutting alfalfa hay? I see solid black beetles around the pallets of my 3rd cuttlin alfalfa hay.

    1. Contacting Washington State University Extension would likely be your best bet. They would have a better feel for local populations, etc. You can reach them here:


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