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Squash Vine Borer Control and Prevention

The squash vine borer is not common along the west coast, but for the majority of the United States it’s a common pest. The larvae burrow deep into the soft vines of zucchini or summer squash and kill off the plant.

Today I’ll tell you everything you need to know about the lifecycle of these red and black moths and their offspring, and how to get rid of them when they appear!

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 Squash Vine Borer
A day-flying clearwing moth, the vine borer adult. Source: Mean and Pinchy
Common Name(s) Squash vine borers, vine borers, squash borers, squash stem borer, etc.
Scientific Name(s) Melittia cucurbitae
Family Sesiidae
Origin North America, parts of South America & Mexico, southeastern Canada
Plants Affected Cucurbitae species of squash/gourd/pumpkin
Common Remedies Diatomaceous earth, aluminum foil wraps, neem oil to kill eggs, etc.

The squash vine borer is the larvae of the clearwing moth known as Melittia cucurbitae. The moth itself is often confused with bees or wasps as it looks similar, and has distinctly red or red-orange coloring on a black body.

While the clearwing moths themselves don’t really harm our plants, the larvae feed ravenously. They will burrow right into the stems and devour the soft tissues inside, causing wilting and eventual plant death. Nobody wants to lose their prize squash plants, especially during the summer!

Before we dive into this, it should be noted that borers are not the same as the squash bug (Anasa tristis). I’ve got a great piece on how to handle squash bugs, and while some treatments are similar, they aren’t identical!

Squash Vine Borer Life Cycle

Squash vine borer eggs
Eggs laid on the underside of leaf stalks. Source: Raleigh City Farm

Sometime in June or July, the adult vine borer moth will emerge from the cocoon in the ground where it spent its winter. These moths are day fliers rather than nighttime moths, and move much like a wasp does.

Within days, the moths will mate and then lay eggs. A moth will lay a cluster of eggs on a specific plant, then move on to the next one. A single moth can, in this way, become destructive to an entire garden.

The small brown eggs can be found on low leaf stems or on the main stem of the plant, and occasionally on the underside of leaves. However, most often I find them on the main stem very close to the soil level.

A week after the eggs are placed, they hatch and larvae quickly bore into the stem of their host plant to feed. Inside, it will eat the tender plant tissues and will block water that flows along the stem, causing the plant to wilt and eventually die.

Larvae are fat and white with a dark brown or black head. These look like some weird grub or worm, and are sometimes called squash worms because of that.

The larvae will feed inside their host plant for 4-6 weeks, then exit the stem and burrow deep into the soil to pupate. Those cocoons will hide within the soil until the following year, when the next batch of squash vine borers will emerge.

There’s only a single generation each year.

However, a field that’s been attacked in the past is likely to have problems in subsequent years, as those pupae are hiding in the soil below. Tilling the soil can turn the cocoons up to the surface and allow birds access to juicy morsels, but often they’re not all discovered.

Common Habitats of Squash Vine Borers

Front view of squash vine borer
A frontal view of the adult squash vine borer. Source: mwms1916

Once hatched, the larvae immediately live inside their host plant while they grow. Once they’re ready to pupate, they move into the soil beneath the plant. Because of this, they’re primarily found in or around squash plants or in soil where squash plants have been grown.

What Plants Do They Like?

Most larvae bore through the inner tissue of Cucurbita maxima as their first choice. This species includes varieties such as Hubbard squash, buttercup, Lakota squash, Jarrahdale pumpkin, and a few others. These are mostly winter squash varieties.

If there aren’t Cucurbita maxima plants nearby, the second choice will be Cucurbita pepo. These are most commonly referred to as summer squash, and these are quite popular. Included in this species are most pumpkins, zucchini, acorn squash, straightneck or scallop squashes and more.

The least susceptible species of squash is Cucurbita moschata. As this species tends to have more rigid vines, while the borers can attack them, they’ll usually head towards easier targets. In this species, the most popular squash is the butternut, but there’s many more including some varieties of crookneck squash.

Where the larvae bore into the stem, it suffers from rot. You may also see a hole or multiple holes in the stem’s exterior, accompanied by frass. Frass is a sawdust-like debris that results from the borer chewing into the vine.

How To Get Rid Of Squash Vine Borers

It can be tricky to eliminate these pests, because usually once you see the vine borer adult, it’s already laying eggs. You can kill off individual moths (and should!), but once the vine borer gets into the plant, most sprays won’t work.

Don’t panic, though! There are ways to handle these little devious pests. Here’s recommendations for squash vine borer control, and tips on how to prevent them in the first place!

Organic Controls

Squash vine borer larva in plant
Example of larval feeding on the interior of a stem. Source: NYSIPM

The first line of defense is always prevention. While I’m going to cover other prevention methods in a segment below, there’s two things which can be done from the moment you put your plant in the garden, so let’s cover those here!

Diatomaceous earth can help dissuade vine borer moths from placing eggs on your plants. If the egg never gets laid on your plant, it’ll never hatch into a vine borer!

However, diatomaceous earth no longer works when it gets wet, so while you can dust it on all surfaces of the plant, you’ll want to be sure the plant doesn’t get moist. You’ll also need to reapply this after rainy weather, once the plant has dried again.

Wrap the stem base in aluminum foil when you plant. Don’t extend down below the soil line, but cover the stem itself with foil. This will prevent borers from getting into the stem in the first place, but you may be going out and wrapping new growth for a while.

If a borer does get inside your vine, you can conduct surgery to kill it off. Find the hole, and look closely at it to determine the direction the borer went. With a sterile razor blade, make an incision no deeper than halfway into the stem. Find the borer and use a toothpick to kill it.

Surgical methods can be slightly risky for the plant, so if you do opt to do this, be sure to bury the wounded stem beneath the soil’s surface. It may send out new roots from that area, and covering it should help protect against some bacterial or fungal diseases (but not all).

Neem oil can smother the eggs. Since it protects against a number of other pests and diseases, this is a good thing to spray regularly anyway!

If you’re seeing adult borer moths, you can use bacillus thurigiensis to eliminate them and their eggs. Using a sprayed form such as Monterey BT, or a powdered form like Garden Dust, will protect your plant against infestation.

A spinosad spray can also be effective at killing off adult moths and their eggs. I’m partial to Monterey Garden Insect Spray.

However, be aware that neither bacillus thurigiensis or spinosad sprays to the exterior of the plant will not have any effect on borers already inside the vine!

Finally, while I hate to recommend them for any reason, weekly applications of a chemical spray do work. However, these can also be risky to your pollinator insects, so you’ll have to determine if this choice is right for you.

If you do opt to use chemicals, common ones used include carbaryl, permethrin, bifenthrin, and esfenvalerate.

Injecting Your Squash Vines

Remember how I said that neither BT nor spinosad sprays would have an effect if there was a borer in the vine? There is another way to use liquid BT to deal with borers: by injecting the vine directly.

Get a 3cc hypodermic needle from a medical supply store, and sterilize the needle’s tip in a mix of half bleach and half water. Then fill the syringe with 1cc of liquid BT.

About an inch and a half above the soil line, you will carefully insert the needle into the stem, going in about halfway. Then slowly press down the plunger to fill the lower stem with BT.

Repeat these injections every 7-10 days to keep plants borer-free.

If you can already see a hole where a vine borer has entered, inject the stem about one inch above the hole. The BT will drip down onto the borer and kill it off.

Be sure that after every injection you sterilize the needle again. To do this, place the needle into your bleach/water mix and draw bleach and water into the plunger, then press it back out. Repeat that 2-3 times, being sure to completely submerge the needle. Sterilization prevents the spread of plant diseases.

Environmental Controls

Squash vine borer top view
A close-up look at the top of one of the female moths. Source: Keith Roragen

Environmental controls are necessary as well to combat these tricky little bugs.

One of the best options is to add beneficial nematodes to your beds. These microscopic soil-dwellers will attack and destroy the pupae of borers along with a wide variety of other insects. Be sure that you get Steinernema-species nematodes, especially Steinernema feltiae, as these are the most effective!

There are two types of traps which are also effective: colored traps and pheromone traps.

Colored traps include yellow sticky traps and yellow water traps. The color yellow draws adult moths in as they normally look for yellow squash blossoms. The sticky traps will cause them to get stuck and die. A yellow bowl full of water can cause them to drown.

Pheromone traps lure vine borer adults by scent as well as their color, enticing them to get close. And once they do, they get stuck on the sticky surface of the trap and die there.

Till the soil after the season ends, once you’ve removed the plants. Vine borers do not go too deep in the soil to pupate, and are usually only in the first few inches. Tilling will bring them to the surface and make them susceptible to the elements or to hungry birds.

Finally, it’s important that even if your plants do get infested, you practice good garden maintenance. Remove dead plants before the borers reach maturity. You should not compost those plants. Destroy them entirely to kill the larvae!

Prevention Methods

Squash vine borer moth with wings extended
An adult moth landing on heather. Source: Distant Hill Gardens

As I said earlier, prevention is key for defeating the squash borers. Here’s a short list of options that help prevent them overstaying their welcome!

Floating row covers are effective, but only if you didn’t have squash in those beds the year immediately before. Placing a floating row cover over a new bed which isn’t colonized with overwinterint pupae can help keep the adults from laying eggs on your plants.

Crop rotation is essential for combatting these pests. Since the borers lay eggs in the soil beneath the plants, moving your plants to a new bed every year can help prevent their return.

Since they only have one generation per year, you can simply outwait them and replant. A new plant placed sometime in late July will be safe from the pests, as they will have already done their business and disappeared.

If you’ll be planting multiple types of squash, you can consider doing trap crops. If you’ll be planting Cucurbita pepo plants like zucchini, consider planting a couple Cucurbita maxima plants nearby. The squash borers will go after the C. maxima plants first.

Finally, planting resistant species is one of the easiest ways to avoid dealing with borers. Species like Cucurbita moschata (mentioned above) tend to be much more resistant to these pests than other squash plants.

Frequently Asked Questions

Q: Are other plants susceptible?

A: … yes and no. While they have been known to attack cucumbers, melons, and even celery if they have no squash at hand, they really tend to avoid other plant types. For some reason, the tasty innards of summer or winter squash and pumpkins are the most appealing!

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