How To Fight Blister Beetles Organically

A black blister beetle on a tomato plant.

If you garden in the southern US, you might be familiar with blister beetles. These garden pests chew your vegetable plants and completely destroy them, and are dangerous to livestock too. Learn how to fight blister beetles organically.


How to Fight Blister Beetles in an Organic Garden

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

My first exposure 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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There isn't much alfalfa hay in central Oklahoma

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. Johnson grass isn't good hay, and it can contain nitrates, especially in drought years. 

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 find more and more beetles, and over time there would be 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.

Many black blister beetles on a chewed-up tomato plant.

What are blister beetles?

Blister beetles come in several varieties and colors; ours are the plain black beetles. 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, one of my friends - 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.

Blister beetles are extremely dangerous to livestock

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.

Blister beetles in the garden

Unfortunately, blister beetles like garden plants and crops as much as they like alfalfa.

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.

A "plant skeleton" -  Blister beetles consumed the leaves on this plant, leaving just twigs and stems.

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.

(I wear these gloves when I garden. They protect my hands from stickers, prickly weed stems, and from blister beetles. I am confident enough to pick a bug off a plant and drop it in a container while wearing these gloves.)

Be aware that the beetles like to simply drop to the ground when disturbed, so knocking them off into a container 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 if necessary.

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.)

Keep the grass cut short and weeds pulled - this 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. 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.

Close-up image of a black blister beetle on a plant.

Update - I've found a great organic deterrent!

Now that I've painted a pretty bleak picture of twice-daily hand-picking these caustic monsters from all of your garden plants, I am overjoyed to share that I've found a solution that, so far at least, is working really well for me! 

Last year was awful, having to fight these things in such a slow and tedious way.

Fast forward a year - to August 2020. My very good blogging friend, Michelle at SoulyRested, shared the recipe for her organic bug spray with me. 

It takes 24 hours to make a batch of this bug-repelling concoction, so I hand-picked beetles until the spray was ready to use, knocking the bugs off my plants and picking off others (while wearing gloves) and dropping them into a coffee can with an inch or so of water in it.

Michelle wasn't sure that the spray would work on hard-shelled beetles. She's used it successfully on juvenile squash bugs and on caterpillars. 

But I had nothing to lose, and I had all the ingredients on hand, so I gave it a try. So I whipped up a batch and when it was ready, I headed out to the garden armed with a spray bottle of her organic bug spray.

Some of the beetles that I'd picked off the plants the day before and dropped into the coffee can of water had managed to get out and fly away! There had been a lot in the can, but now there were only a dozen or so. 

So I poured a bit of the bug spray in the coffee can too. The remaining beetles in the coffee can were dead within ten minutes!

I sprayed the bugs that were on my plants as well as spraying it directly on my tomato plants as a deterrent. I found two places that had an abundance of "nymphs" - groups of young bugs that resembled dirt or coffee grounds on a stem of a tomato plant. I sprayed those liberally as well.

That evening? No bugs.

The next day... one bug.

The following day... just two blister beetles. It had rained lightly overnight, so I reapplied the bug spray.

I've found the best way to fight blister beetles organically in my garden! 

Here is Michelle's organic bug spray recipe

Be sure to wear gloves when you prepare the bug spray since you're working with hot peppers.

Harvesting your produce after using this organic bug spray

If you use this spray, I recommend that you wash your ripe produce well before eating it.

I didn't try tasting a tomato without washing it first, but I imagine it would be a rather "hot tomato" if I did. If that stuff tastes as bad as it smells, well....

But it works! And I can handle the strong smell because it works.

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 blister beetles are good for something!" But now I realize that the point is that I need to get rid of the grasshopper eggs so they don't attract blister beetles in the first place.

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 destroy the rest of your grasshopper population.

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

Nolo bait is available from Amazon, or you can check your local garden center too.

A black blister beetle on a green plant. Text: "Blister beetles: What they are, What they can do, How to fight them organically"

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