Insect stings and spider bites can be a cause for concern in horses, sometimes leading to skin irritations, poisoning, allergies and infections.

Several species of pests and external parasites can cause severe health problems in horses. Flies are the most common and persistent pests for horses. Blood-sucking species like horse flies and mosquitoes are even more serious as some are vectors of severe illnesses that can affect horses and humans.

There are also many types of non-parasitic insects and arachnids that can cause irritation, poisoning, and allergies in horses.

The good news is there are steps you can take to protect your horses from all of the world’s most annoying pests. Read on to learn everything you need to know about the most common bugs, flies, spiders, and other nuisances to keep away from your equine friends.

Common Equine Pests

Horses are susceptible to a variety of external pests and parasites. Most equine pests are either insects or arachnids, some of which can cause irritation, discomfort, and even life-threatening reactions.

Insects and arachnids are members of the largest animal phylum on Earth, the Arthropods, which comprise 85% of all species in the animal kingdom. [1]

While insects are important for ecological functions, such as pollination and decomposition, some species of insects can cause issues for horses. [2] The four largest orders of insects are: [3]

  • Coleoptera (beetles)
  • Diptera (flies and mosquitoes)
  • Hymenoptera (ants, bees, and wasps)
  • Lepidoptera (moths and butterflies)

Arachnids belong to the class Arachnida; they include spiders, mites, and ticks. As adults, these pests have a two-part segmented body, eight legs, and do not have wings and antennae. [4]

By understanding the threats posed by these insects and arachnids, horse owners can better protect their animals from stings, bites, and the potential diseases they carry.


Coleoptera, generally referred to as beetles, make up 40% of known insect species; they can be found in almost every habitat on Earth. [5]

Beetles vary widely in size, ranging from one millimeter to ten centimeters in length.

In equine toxicology, the most concerning insect belonging to the Coleoptera order is the blister beetle (Epicauta spp. and Pyrota spp.). [4]

Blister beetles

blister-beetle | Mad Barn Canada

Blister beetles, or oil beetles, are pollen- and flower-feeding beetles that consume alfalfa blossoms. Over 2,500 known species of blister beetles are named for their cantharidin secretion, a potent blistering agent. [4]

In the United States, the most common types of blister beetles belong to the Epicauta spp. Adult beetles belonging to these species vary in size from 0.16 to 0.70 in (4 to 18 mm). [4]

Adult blister beetles feed on leaves and flowers. In North America, Epicauta spp. have a propensity for consuming flowering alfalfa legumes that are commonly used to produce hay. [6]

Blister beetles are attracted in great numbers to feed on the pollen and nectar of alfalfa blossoms. When horses consume hay infested by the beetle, the toxin enters their system through the gastrointestinal tract, where it is rapidly absorbed. The resulting condition is called cantharidin toxicosis or blister beetle poisoning. [6]

While rare, cantharidin toxicosis is a very severe condition as the ingestion of as little as five grams of beetles is enough to cause the death of a 1,100 lb (500 kg) horse. [6]

Horses that ingest sufficient quantities of the toxin can experience blistering of the entire gastrointestinal tract (from mouth to anus). Additionally, horses with blister beetle poisoning present with rapid heart rate, stiff gait, appetite loss, frequent urination (pollakiuria), renal (kidney) failure, and fever. [6]

No antidote is available for blister beetle poisoning. Treatment focuses on detoxification and supportive care. Horses with blister beetle toxicosis carry a guarded prognosis. [6]


Given the absence of a cure, prevention is the key to ensuring the well-being of horses at risk of cantharidin toxicosis.

Due to the secretion of cantharidin, blister beetles have few natural predators, making biological control (i.e. introducing the pest’s natural predator to reduce their population) a non-viable option.

Chemical control (i.e. using pesticide) is also not advised, as the application of pesticides directly to their food can harm horses. With these limitations for pest control, the most effective way of preventing cantharidin toxicosis is hay management. [4][6]

If alfalfa hay is produced on-farm, flowering plants should be checked for the presence of beetles while the alfalfa is flowering. Whether alfalfa is offered in pasture or processed into hay on-site, it is important to confirm there is not a blister beetle infestation before providing home-grown alfalfa to your horses.

Modern farming practices such as crimping during baling are to be avoided as they do not allow blister beetles and other potential pests to flee the pasture. [6]

Utilizing hay cut outside of blister beetle season, whether produced in-farm or bought, can significantly decrease the risk of cantharidin toxicosis. Leaving hay to dry before baling is considered the best practice, as it allows the beetles to leave the legume and can increase the chances of predators eating the beetle carcasses instead of domestic animals in the pasture. [6]