The ash grey blister beetle, a member of the family Meloidae, has recently been discovered on properties all over Colorado. This discovery raises significant concerns due to the toxic chemical cantharidin produced by blister beetles, which poses a severe threat to livestock, especially horses. Understanding the implications and management of blister beetles is crucial for hay producers and livestock owners in the region.

Blister Beetles and Cantharidin

Blister beetles, also known as oil beetles, are a diverse family with over 300 species in the United States and more than 2,500 species worldwide. They produce cantharidin, a potent toxin that can irritate the gastrointestinal and urinary tracts of animals, leading to severe illness or death. The primary concern is cantharidiasis, or blister beetle poisoning, which occurs when livestock consume hay contaminated with cantharidin, but if the horses or livestock accidentally ingest the beetles, it can also cause serious illness and death.

Life Cycle and Habitats

Blister beetle larvae are predators of subterranean grasshopper and bee eggs. Their presence is often linked to grasshopper outbreaks, which are currently prevalent in Colorado. The adult beetles are particularly attracted to alfalfa and certain weeds, such as dandelions, during their bloom. Therefore, managing these host plants and timing hay harvests to avoid peak beetle activity are essential strategies for reducing the risk of contamination.

Symptoms of Blister Beetle Poisoning

Blister beetle poisoning can manifest in several distressing symptoms, including:

  • Blisters on the tongue and in the mouth
  • Colic
  • Diarrhea
  • Discharge of blood or intestinal lining in stools
  • Urinary problems, including bloody discharge
  • Death

If any of these symptoms are observed, it is crucial to contact a veterinarian immediately.

Management and Prevention

Effective management of blister beetle contamination involves several key practices:

  • Reducing weedy host plants in hay fields and pastures.
    • Mowing before they can seed, it is ideal to keep all vegetation mowed under 4-6″ to help with this.
    • Utilizing weed preventative/pre-emergent and weed killers to eradicate weeds from pastures.
  • Harvesting hay before the bloom stage to minimize beetle attraction.
  • Opting for first-cutting hay and later cuttings (fourth or after the first frost) to avoid peak beetle activity periods.

If you think you may have a blister beetle outbreak, you can reach out to your local CSU Ag Extension office to confirm through this link:

Impact on Livestock

While horses are most commonly affected by cantharidin poisoning, cattle and sheep are also susceptible. The accidental or inadvertent ingestion of as few as 40 beetles can be fatal for horses depending on the species, highlighting the importance of vigilant management and prevention practices.


The discovery of ash grey blister beetles in Elbert, Colorado, underscores the urgent need for awareness and proactive management among hay producers and livestock owners. By understanding the life cycle of blister beetles and implementing effective control measures, the risk of cantharidiasis can be significantly reduced. For more detailed information and guidance, refer to the comprehensive resources provided by Colorado State University Extension at Blister Beetles in Forage Crops.


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