What is a Blister Beetle and What Does it Look Like?

Blister Beetles are members of a family of plant-feeding insects (Meloidae) that contain a toxic defensive chemical known as cantharidin.  This chemical protects them from predators.  Accidentally crushing a Beetle against the skin can result in a painful blister, the source of the insect’s common name. 

Blister Beetles have long (3/4 to 1-1/4 inch) narrow bodies, broad heads, and antennae that are about 1/3 the length of their entire bodies. The front wings are soft and flexible in contrast to the hard front wings of most Beetles.


Blister Beetles have a head that is broad and vertical. The section of the body between the head and the wings (prothorax) is narrower than the wings, and narrower than the head. It appears that the insect has a neck.

Striped Blister Beetles are about 5/8 inch long and one-fourth as wide. They are gray to brown with yellow stripes running lengthwise of the wing covers. The ash-gray Blister Beetle is about 1/2 inch long and is completely gray. The Black Blister is about 1/2 inch long and is solid black.

Where are Blister Beetles Found?

Historically, Blister Beetles have been found in arid regions of the US where grasshoppers are abundant most every year.

The adults feed on leaves in the tops of a plant but are especially attracted to flowers where they feed on nectar and pollen. They gather in groups, so large numbers can occur in concentrated clusters in a field. 

Blister Beetles have a wide host range including alfalfa, clover, soybean, potato, tomato, melon, cotton, and eggplant. These Beetles are mid to late summer insects, active in mid-July and early August which translates to the third or fourth cutting of hay.

Why Should a Horse Owner Care About Blister Beetles?

In sufficient quantity, the cantharidin in the bodies of living or dead Blister Beetles can be toxic, and in some cases lethal, to horses. The lethal dose is estimated to be between 0.5 and 1.0 mg of cantharidin per kilogram (2.2 lbs) of body weight. Cantharidin is very stable and remains toxic in dead Beetles for a long time.

Cantharidin can cause sever inflammation and the formation of blisters on the skin.  If absorbed through the intestines, symptoms can include inflammation, blisters in the mouth, straining, temperature, increased heart rate and respiration, sweating, diarrhea and frequent urination within the first 24 hours.

Happy Horse Healthy Planet _colic 3his is accompanied by secondary infection and bleeding.  Calcium levels in the horses may be drastically lowered and heart muscle tissues destroyed.  Animals can die within 24 to 72 hours, so it is imperative to contact a veterinarian as soon as a Blister Beetle poisoning is suspected.

Simply touching a Blister Beetle, either dead or alive, is enough to cause inflammation and blistering of a horse’s skin within hours of contact.

Animals may be poisoned by eating crushed beetles in cured hay. The severity of the reaction, ranging from temporary poisoning, to reduced digestive ability, to death, depends upon the amount of cantharidin ingested and the size and health of the animal. 

If an owner suspects a horse has ingested Blister Beetles, they would need to transport the horse to the nearest equine hospital for treatment.

How Can I Reduce My Horse’s Risk?

You can also reduce the risk of feeding blister beetles to your horse by understanding some Blister Beetle basics and by taking the following precautions:

  1. If practical and possible, grow your own alfalfa so that you can use proper preventive management practices.
  2. Buy from local sources. Develop a relationship your hay producer so that you know their production practices and hay quality.
  3. Buy first cutting hay. Blister Beetles are not active then.
  4. You should examine the hay carefully for the presence of Blister Beetles before feeding your horse hay.

What Should a Horse Owner Do if a Horse Eats A Blister Beetle?

If a horse ingests even a few Beetles, the insects’ cantharidin can cause ulceration and inflammation of the mouth, stomach, and intestines. Clinical signs including decreased appetite, frequent drinking and urination, colic, and depression can be apparent within hours.

Treatment is aimed at reducing absorption of the toxin through administering activated charcoal and mineral oil. Intravenous fluids, gastrointestinal protectants, and broad-spectrum antibiotics are also used.  All of these should be given by a veterinarian.

In the event that one horse in a barn suffers from Blister Beetle toxicity, veterinarians recommend that owners stop feeding the hay to other horses in the barn and keep a close eye for signs of toxicity in other horses.

~Peace and Good Feed,

The Nerd

Leave a Reply

Please log in using one of these methods to post your comment: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s