Watch Alfalfa for Blister Beetles

July 14, 2023 11:51 AM
Blog Post

Blister beetles are an occasional late-summer problem in Iowa alfalfa fields. The beetles feed on alfalfa and soybean foliage, but leaf loss is not of economic importance. The bigger concern with blister beetles is their toxicity to livestock when accidentally consumed in feed. The beetles produce cantharidin, an irritant that causes painful blistering when the insects are handled. The cantharidin remains in the beetle’s body even after it dies and can cause health issues in livestock when dead blister beetles are consumed with hay.

Several species of blister beetles are found in Iowa, including gray, striped, margined, and black. Each species has different color patterns, but all have long, narrow, cylindrical, and soft bodies, and a pronounced neck and broad head when viewed from above. They generally range from about 0.75 to 1 inch in length and tend to congregate in the field, a trait that we can use to reduce contamination in finished hay. Because one of the larval stages heavily feeds on grasshopper eggs in the soil, knowledge of grasshoppers and their life cycles can be helpful. Specifically, grasshoppers tend to build up when conditions are dry because rainfall promotes a natural fungus that infects grasshoppers and kills them. Blister beetles tend to also build up when grasshopper populations are high due to an abundant food source for larvae.


ash gray blister beetle on a green soybean leaf
Ash gray blister beetle. Photo by Ashley Dean.

Horses are especially susceptible to blister beetle poisoning. A lethal dose is considered to be approximately 1 milligram per kilogram of body weight for horses. Because horse weight and the dose of cantharidin in different beetles is variable, it is difficult to tell exactly how many beetles will be lethal to horses. A general threshold is that consumption of 25 to 300 beetles can kill a mature horse. Cattle and sheep are much less susceptible, but blister beetles will reduce digestibility of hay. Affected livestock may suffer a variety of symptoms, including colic, tenesmus (straining), elevated temperature, depression, increased heart and respiratory rates, dehydration, sweating, and diarrhea. Horses may also put their muzzles in water without drinking when poisoned. If blister beetle poisoning is suspected, contact a veterinarian immediately.

Furthermore, cantharidin is a stable compound that withstands decomposition even when it is dried or heated, so the hay will retain its toxicity in storage. Crushing or killing the beetles with chemicals does not reduce the toxin. Even the dried juices from crushed beetles or the remains of dead beetles in the hay can cause poisoning in animals.

What can be done to avoid blister beetle problems?

  • Learn how to distinguish blister beetles from other insects.
  • Scout fields prior to harvest to evaluate blister beetle activity. Scout the entire field, because beetles are mobile, but realize that they tend to congregate near field edges with flowering weeds. These large beetles are easy to see, but using a sweep net might help with scouting because they readily drop from plants when disturbed.
  • Eliminate weeds and cut alfalfa before it reaches advanced bloom stages. Flowering plants attract the beetles that feed on alfalfa and weed pollen. 
  • Insecticide treatments are available but must be applied with preharvest intervals in mind. Dead beetles will still poison livestock, so do not plan to feed contaminated hay to animals. Do not treat fields at peak bloom to minimize non-target effects to pollinators and beneficial insects.
  • Crimping or conditioning hay can increase the number of beetles that remain in the swath prior to baling. If possible, try to cut the alfalfa and put in swaths that can be straddled by the tractor to avoid crushing beetles in the windrow.
  • Scout underneath windrows for blister beetles and allow them to move out before baling. 
  • Use or purchase first cutting hay for feeding horses, as blister beetle populations increase later in the summer.
  • Check hay for blister beetle carcasses prior to feeding.
  • Continue scouting subsequent cuttings of alfalfa for re-infestations of blister beetles.


This article was adapted from an ICM News article originally published on pages 165-166 of the IC-480(22) – September 14, 1998 issue.


Ashley Dean Agriculture and Natural Resources Extension Specialist II

Ashley is an education extension specialist for field crop entomology at Iowa State University. She coordinates the Iowa Moth Trapping Network, the Regional Corn Rootworm Monitoring Network, and the Iowa Pest Alert Network. She also develops educational resources for field crop pests in Iowa and ...

Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology, now the Department of Plant Pathology, Entomology, and Microbiology, at Iowa State University in 2009. She is a professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated...

Meaghan Anderson Field Agronomist in Central Iowa

Meaghan Anderson is a field agronomist in central Iowa and an extension field specialist at Iowa State University Extension and Outreach. Educational programming is available for farmers, agribusinesses, pesticide applicators, certified crop advisors, and other individuals interested in...