Blister Beetle: A Complete Guide to Identification and Control

These infesting garden and field pests contain a toxin that, if ingested, can seriously harm livestock. Find out about safe, organic solutions for effective blister control here.

Blister beetles are a common field and garden pest best known for the scar-causing secretion they secrete when injured or crushed. The secretion contains a blistering substance known as cantharidin, a substance used to treat lesions caused by the smallpox virus. Cantharidin is so effective at destroying tissue that it is used in wart removers.

Cantharidin is toxic if swallowed. It persists in dead beetles long after the hay they infested has been dried and baled. Horses are especially susceptible to poisoning. Depending on the type of bug, as little as four to six grams of dead insects can be fatal.

People who ingest the pest can expect severe damage to the urinary tract and gastrointestinal lining. Monitor children who put anything in their mouth for possible blister bugs.

In addition to the threat posed by its toxicity, the beetle causes damage to both garden and industrial plants by eating a variety of leaf crops, plant inflorescences, landscape flowers and hay fields.

What are beetles?

Blister beetles (Coleoptera: Meloidae) are named for the cantharidin toxin found in their bodily fluids, which can cause blisters on human skin and life-threatening inflammation in horses and animals.

Cantharidin is also present in Spanish fly, a known aphrodisiac.

Blister beetles are sometimes a problem in field crops such as alfalfa, sweet clover, canola, beans, soybeans, potatoes and sugar beets.

They feed on flowers and leaves of a wide variety of plants, including members of the families Amaranthaceae, Asteraceae, Fabaceae and Solanaceae, and can cause significant defoliation of plants on which they congregate in large numbers.

Blister beetles are gregarious and can be found in large numbers in certain areas of the field. Blister beetles are generally not serious pests and only occasionally cause crop damage under certain conditions.

However, when they appear in alfalfa and other feed crops that horses or other livestock may consume, they can cause severe illness or even death.

How to recognize blister beetles?

Correctly identifying blister beetles from various other species of non-toxic beetles that may resemble them, such as the asparagus beetle, is important for protecting livestock and preventing crop damage.

There are about 250 species of blister beetles, members of the family meloids, distributed throughout the United States. Various species of beetles are common in the East, South, and Midwest.

They can also be found in the grasslands of the West and Southwest, as well as in gardens along the Pacific coast.

Adult blister beetles vary in size and color. Most are an inch and a half to one inch long, with long soft bodies and a broad head with antennae that make up a third of the body length.

The prothorax between the head and body is narrow and resembles a neck. The wings are soft and flexible, the legs are relatively long.

The color can be bright and variegated, striped or flat. Banded blister beetles are shades of gray and brown with yellow stripes running along the wing covers. The ash gray blister beetle is completely gray.

The black blister is completely black. The blister beetle is edged black with a gray to cream stripe along the edge of each wing.

The University of Illinois Department of Plant Sciences has illustrations of these four common types (notice how tightly their wings fit together). Here's an illustrated list of the different blister beetles found in Florida that can also be found elsewhere in the United States.

Biology and life cycle

Adults begin laying clusters of eggs in mid-to-late spring and continue to lay for most of the season. The female lays one to two hundred eggs, less than a millimeter wide and one to two millimeters long, just below the soil surface. The eggs hatch in two weeks or less.

The emerging white larvae have relatively long legs, which they use to search for grasshopper egg pods to eat. Others attach themselves to bees and are also called bee lice.

They feed on eggs and store food in a bee's nest, going through several additional stages of development: from larva to legless larva.

When the pupal stage is completed, the emerging adult falls to the ground after about ten days and begins to feed on cultivated plants.

Adults visit flowers to eat nectar, pollen, and sometimes even the entire flower. Some types of blister beetles even eat leaves.

The larvae, which reach the pupal stage in autumn, can overwinter directly under the soil.

They usually live up to three months. Females are capable of producing multiple clusters of eggs throughout their lives.

How do blister beetles pose a threat to livestock?

The toxic effects of blister beetles on livestock, known as cantharidin toxicosis, are the greatest threat blister beetles pose.

Large numbers of blister beetles and toxic concentrations of cantharidin in alfalfa hay can pose a serious health hazard to horses, cattle and sheep, especially when ingested in large quantities.

Unfortunately, cantharidin is a very stable compound that remains toxic to livestock even when the dried remains of beetles killed during harvest are fed in the feed.

Infestation of alfalfa and other hay can ruin it for sale or feeding.

Hay and forage producers should take every precaution to monitor and prevent infestations. Beetles tend to cluster together when feeding, increasing the likelihood that one or two bales of hay will contain clusters of insects.

You should also be careful when purchasing hay. Find out if blister beetle infestation may be a problem in your area and check what you buy carefully.

How dangerous are blister beetles for horses?

Horses are especially vulnerable to blister beetle poisoning. The horse's digestive tract can become very inflamed, leading to secondary infections and bleeding.

Interestingly, in order to become infected, it is not necessary to swallow a beetle, since hay contaminated with the bodily fluid of crushed beetles can be no less dangerous.

This chemical lowers calcium levels in the blood and irritates the lining of the stomach, small intestine, bladder, and urinary tract.

Horses treated with cantharidin may experience symptoms of colic, including increased drooling, diarrhea, bloody feces, sweating, dehydration, convulsions, and frequent urination.

Animals can die from blister beetle poisoning in as little as 72 hours, so it is important to call your veterinarian as soon as you suspect it.

A lethal dose causes fever, depression, shock and death.

The severity of the reaction depends on the amount of cantharidin ingested and the size and health of the animal and can range from temporary poisoning to decreased digestive function and death.

The concentration of cantharidin in beetles varies depending on the species. Some species may contain up to 50 times more cantharidin than others. Additionally, horses vary in their sensitivity to cantharidin.

Although the toxicity of cantharidin to horses has not been definitively determined, the estimated lethal dose ranges from 0.5 to 1 mg of cantharidin per 2.2 lbs of horse body weight.

How do blister beetles pose a threat to crops?

Blister beetles will feed on almost any leaf growing in your garden. They are found in tomatoes, potatoes, eggplants, peppers and other nightshade vegetables, as well as leafy greens.

In mid to late summer they can arrive in flocks, seemingly overnight, and due to their numbers cause a lot of damage in a short time.

Blister beetles also harm grasshoppers, one of the most destructive pests of farms and gardens. Newly hatched blister beetle larvae use their legs to search for clusters of grasshopper eggs and feed.

In this sense, blister beetles can be considered beneficial insects, but only at the larval stage. When they become adults, all they will be left with is trouble.

Fighting blister beetles

If you suspect a blister infestation or want to take the necessary precautions to prevent it, here are 12 things you can do to effectively control blister:

Check frequently

Frequent and thorough inspection of home gardens can go a long way in controlling beetle damage. Their numbers gradually increase in the first months of the growing season, and an observant gardener can protect them from much damage.

Remove them as soon as you notice them

In July, the garden may suddenly be overrun by hordes of leaf beetles. They usually work in large groups, clustered at the edges of potato beds and lettuce rows.

It is best to remove as many beetles and any plants they damage as possible as soon as they are discovered and before they spread.

Once they are established, they are nearly impossible to completely eradicate, even if chemical or other toxic sprays are used.

Picking by hand with gloves

Hand picking can be effective in home gardens, especially when there are few of them. But never touch blister beetles with your bare hands. Always wear gloves.

Brush the bugs off the plants into a small container of soapy water.

If you shake them off the plants, the beetles will often lie in the dirt and play possum rather than run away. Take advantage and collect them carefully.

Diatomaceous earth distribution

Reduce beetle range and numbers by spreading diatomaceous earth around endangered plants or using it as a barrier in raised beds or other small areas.

Lime from oyster shells can also be used as a barrier, but it is not an effective beetle repellant and acts primarily as a repellent. (This is also useful for maintaining proper soil pH.)

Trim the grass and weeds

Blister beetle infestations often occur around the edges of gardens or in surrounding vegetation. Trim grass, weeds and other growth around the edges of your garden to remove areas where they may begin to grow.

Use row covers

Well-attached row covers can protect your plants from migrating beetles in mid-to-late summer. They will not stop early season adults that overwinter in the soil as late stage larvae.

Use them if you notice clusters of beetles (or expect them) in and around your garden this July.

Avoid growing certain weeds

Pig (Amaranthus species). very attractive to adult beetles. Eliminate it from your landscape completely. Encourage your neighbors to do the same. Iron grass (Vernonia spp.) and ragweed (Ambrosia spp.) I’ll also draw beetles.

Encourage birds to visit

Birds will pick blister beetles directly from your plants. Encourage them to come and feed in your garden. Respect them and do not use sprays or chemicals that may harm them.

Use a biopesticide

If hand picking does not help reduce their numbers, use sprays containing Spinosad, an OMRI-listed biopesticide that kills between 24 and 48 individuals.

Spinosad breaks down into its inert ingredients within two or so days when exposed to sunlight, preventing it from leaching into water.

It is not toxic to birds or fish, but may harm honey bees when first used. Read product label instructions and time application accordingly.

Get rid of grasshoppers

Grasshoppers attract blister beetles, the larvae of which feed on grasshopper eggs. Control grasshoppers by using Semaspore Bait or other natural grasshopper control products.

This is especially important for hay grown for feed. The presence of grasshoppers in forage crops may indicate early stages of blister infestation.

Proceed with caution

Most forage and grass control products, including spraying, cannot completely eliminate insects or the threat they pose. Keep an eye on your fields, especially around the edges, and clear any areas where beetles appear.

When making hay, avoid using tools such as hay tongs, which can crush insects. If you notice beetles in your fields, let the cut hay sit for two days or more to give them time to discard the hay.

Early cuttings taken before the beetles have increased in numbers are usually safer than cuttings taken later in the season.

Be careful when buying hay

When purchasing hay, it is best to find a manufacturer with a good reputation. Even the most careful examination of hay bales and flakes may not reveal hidden dangers.

Know your farmer well enough to trust him when asking about signs of beetles in his crops. The best answer is no.