Blister Beetles: How To Fight These Pests In The Organic Garden

Blister beetles are toxic to horses and sheep, and cause blisters, swelling and irritation when they come in contact with human flesh. Here's how to fight them in your garden.

I'd never even heard of blister beetles before we moved to Oklahoma.

My first exposure to them was the sign in the local feed store that said they cannot guarantee that the alfalfa hay they sell is free of blister beetles.

Where we had lived in Michigan, right near the Ohio border, when farmers sold "hay" or fed "hay" to their own livestock, it was alfalfa hay. Beautiful alfalfa hay. Inexpensive alfalfa hay. In fact, we'd brought a few bales with us when we moved so the goats would have hay they were used to, and we could take some time to find a new source.

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We soon learned that "hay" in Oklahoma usually means native grass hay - in other words, whatever grows out there in the field. Yes, there are some who grow and sell nice Bermuda grass hay. Others sell very nice prairie grass hay. There is a science to growing good hay. (And I've found a wonderful friend who grows the most wonderful hay!)

But some sell hay that is mostly Johnson grass. Yes, we learned a lot about hay that first year, including who NOT to buy hay from.

Alfalfa hay? There wasn't much of it here, and evidently it might have blister beetles in it, and evidently blister beetles were bad.

We also learned that our goats preferred good native grass hay anyway, especially if it had some of their favorite weeds in it. Goats can be picky.

So blister beetles weren't really on my radar.

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The invasion of the blister beetles

Then two years ago I found black beetles on my tomato plants... beetles I'd never seen before. They ate the leaves voraciously, and if you looked closely, you'd just see more and more beetles, and fewer and fewer leaves.

So I did some research and discovered they are - you guessed it - blister beetles. And in my research I learned just how dangerous blister beetles can be.

And I understood what the sign in the feed store meant too.

How to fight blister beetles organically.

What are blister beetles?

Blister beetles come in several varieties; ours are the plain black bugs. They're about half an inch long, with a large head, long antennae, and a long, narrow body.

When blister beetles are startled or disturbed, they secrete a chemical called cantharidin. This bodily fluid, when it comes into contact with human flesh, causes swelling, irritation and blisters.

The medical world says that the blisters "aren't very serious" and will disappear in a week to ten days' time. However, a woman I know - who innocently collided with a flying blister beetle - said the resulting blisters on her neck were painful and annoying and that it was a very long week until the welts finally healed.

Cantharidin, however, can be extremely toxic to livestock, especially horses and sheep. When hay is cut and baled, the insects are often crushed and excrete the caustic chemical. Even dead blister beetles are toxic, and consuming just half a dozen of the dead beetles in hay can result in a horse's death due to the irritation they cause to the horse's digestive system.

And blister beetles like alfalfa plants. Fortunately, they aren't as prevalent in grass hay, and grass hay is the only kind we buy these days.

Aside from the danger to livestock, blister beetles cause a lot of damage to crops and gardens. They seem to arrive overnight in droves, and can decimate the leaves on plants in a very short time.

Potato plants, tomatoes, peppers, eggplants... they are soon just skeletons of leafless twigs and stems.

Blister beetles will eat your garden plants, leaving them leafless. Here's how to fight them organically.

Fighting blister beetles organically

So how can you get rid of blister beetles safely and organically?

Safety first! Wear gloves when gardening to lessen the chances of coming in contact with their caustic secretions. Long sleeves, long pants and a hat will also help protect you - if you can stand to wear that many clothes in the hottest part of summer.

The most effective way to fight these bugs is to hand-pick them from your plants, but again, don't use your bare hands! Wear gloves, and try to knock the insects off the plants and into a container of soapy water.

Be aware that they like to simply drop to the ground when disturbed, so this can be difficult. It's also time-consuming, but I've found it to be the best way to keep ahead of the infestation. Start as soon as you see the first insect, and diligently hand-pick them daily or even twice a day.

Using diatomaceous earth is also pretty effective. Sprinkle the DE on the soil around the plants and on the plants themselves to help keep insects away. (DE can affect honey bees and other pollinators, however.)

Keeping the grass cut short and weeds pulled seems to help slow the movement of blister beetles from one area to another. Don't allow grass to grow tall next to your raised beds, or let the vegetable plants sprawl to the ground and create a "bridge" from the bed to the ground.

Use floating row covers, held up with hoops over your plants, to keep the adult beetles off of your plants. Knowing that the beetles appear here in mid-July or so, covering my plants before the beetles arrive will help protect my garden. However, any immature beetles, known as nymphs, that might hatch in the soil around your plants would be inside that protective netting.

Also, any vegetable plants with flowers that need to be pollinated would be isolated under the netting. If the beetles can't get though the netting, bees can't either.

Wild birds love to feast on the beetles, so encourage songbirds to visit your garden. Add a nearby feeder, a water source, and places to perch in the garden so the birds can sit and survey your plants.

How to fight blister beetles in the organic garden.

Preventing blister beetles might start with preventing grasshoppers

Nymphs, the young blister beetles, feed on grasshopper eggs. At first I thought, "oh good, at least they're good for something!" But now I realize that the point of that information is that I need to get rid of the grasshopper eggs so they don't attract blister beetles.

And since grasshoppers are also munching on my garden plants, controlling grasshoppers isn't a bad thing to do anyway. However, what you really want to do is prevent the grasshoppers from laying eggs.

Female grasshoppers lay eggs in the soil in the fall, and they hatch in the spring. They prefer loose, undisturbed soil to lay their eggs in. An easy way to prevent the eggs from hatching is to till your garden soil in early spring. If you have raised beds like I do, rake the soil deeply in early spring with a garden rake for the same effect.

Neem oil has been shown to disrupt grasshoppers' ability to lay eggs.

Although this method relies on timing, using nolo bait will disrupt the grasshopper life cycle throughout the summer, which means the females won't be able to lay eggs in the fall. This single-cell organism will infect and destroy grasshoppers.

Nolo bait must be applied when the grasshoppers are young and are about a quarter of an inch in length. The young grasshoppers consume the bait, are infected, and die from blood poisoning. Older grasshoppers then eat the dead ones, and are killed by the infection. As long as at least one grasshopper is continually infected, it will continue to affect others.

Nolo bait should be applied to a large area so that enough grasshoppers are infected throughout the summer.

Amazon carries nolo bait; you might check your local garden center too.

How to fight blister beetles in the organic garden

Have you dealt with blister beetles? How do you fight them?

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  1. yep!yep! and yep! Very informative article! I wish I had had this when I was fighting them! We got ours from a load of cheap hay I had bought for garden mulching! EEEK! They are horrible! I might add one thing, the soapy water is great, but a hand vacuum is wonderful!especially for escapes! It can get in little crevices where your fingers can do damage. Easier to feed to the chickens that way, too! :)

    1. Oh, the vacuum is a great idea!

  2. The vacuum is brilliant!! We had them so bad the first year we moved to our homestead! The most miserable bugs ever! They come in droves and do so much damage in such a short time! Ours are the striped variety. They just seemed to laugh at me when I dusted with DE. It may have worked a bit, but not like I had hoped. A friend is dealing with them now, and is trying neem. Thanks for the tips!

  3. Oh my goodness...they certainly are nasty little bugs. :(
    I'm glad I don't have a big problem with them here...yet! Thanks for sharing your experience dealing with these pests on Farm Fresh Tuesdays!

  4. Thanks, Lisa. Yes, they are really destructive garden pests! I hope these organic methods of fighting and preventing blister beetles are helpful to someone.


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