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Blister beetles in alfalfa and when to worry

Jared Goplen, Krishona Martinson and Bruce Potter for Progressive Forage Published on 31 March 2022
Black blister beetle

The drought conditions affecting some forage-producing areas of the country have seen an uptick in blister beetle activity. Blister beetles are rarely problematic in the Upper Midwest, but drought conditions increase the concern for blister beetle-infested alfalfa.

If consumed in hay or haylage, blister beetles can cause health problems and death in horses, cattle, sheep and other livestock.

Key points

  • Blister beetles tend to be more problematic in dry, droughty conditions.

  • Blister beetles contain cantharidin, a toxin that causes health problems and death in livestock when consumed in large enough quantities.

  • Horses are more sensitive to cantharidin than cattle, sheep or other classes of livestock.

  • To prevent issues, scout fields one to one-and-a-half weeks prior to harvest, avoid harvesting alfalfa beyond 10% bloom, and watch for beetle “swarms” during harvest.

What are blister beetles?

Adult blister beetles are attracted to and feed on the flowers, pollen and leaves of blooming alfalfa and weeds. 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. Striped and ash gray blister beetles were most common in Minnesota in 2021. Striped blister beetles contain more cantharidin than black or ash gray blister beetles. 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. Blister beetle populations are typically greatest in areas where grasshoppers were present the prior year. Grasshoppers were problematic in many areas in 2021 due to drought, which increases the likelihood of blister beetle outbreaks this year.

Ash gray blister beetle

Blister beetles produce a defensive 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.

Horses are more sensitive to cantharidin than other types of livestock, though they express similar symptoms when great enough quantities of cantharidin are consumed. 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 at harvest

Although the risk of harvesting blister beetles in hay cannot be eliminated, it can be minimized by scouting alfalfa fields prior to harvest and by cutting hay before full bloom, since the beetles feed on the flowers and pollen. In the Midwest, beetle populations are typically low until mid-summer (e.g., July and August), so first and second cuttings tend to have minimal infestations. Beetles are more likely to be found in flowering alfalfa hay and when grasshoppers were present the prior year. Grasshoppers were prevalent in some areas due to the drought in 2021, which increases the risk of blister beetles becoming problematic this year.

Striped blister beetle

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 quality hay. Live beetles that aren’t crushed during the crimping process will typically leave the windrow prior to harvest.

Cutting alfalfa at less than 10% bloom reduces the number of beetles. Farmers should watch for beetle “swarms” during harvest and stop to allow the beetles to disperse. Chemical control is possible if beetles are present – but may not be the best option because dead beetles containing cantharidin may still end up in hay.

What can hay buyers do?

When buying hay, buyers 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 and if it was conditioned. It is nearly impossible to detect blister beetles while feeding contaminated hay. This is because the beetles tend to swarm, meaning only a few bales or even just parts of bales from an entire field may be infested. end mark

PHOTO 1: Black blister beetle. Photo by Bruce Potter.

PHOTO 2: Ash gray blister beetle. Photo by Bruce Potter.

PHOTO 3: Striped blister beetle. Photo by Mike Blaine.

Additional information: Extension University of Missouri: Blister Beetle management in alfalfa

Krishona Martinson is an equine extension specialist with the University of Minnesota. Bruce Potter is an extension IPM specialist with the University of Minnesota.

Jared Goplen is an extension educator with the University of Minnesota. Email Jared Goplen.

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