Question: How Do I Stop Squash Vine Borers?
We get a lot of gardening questions from our members via email, instagram, facebook, and our website’s livechat feature. To help out other members (and other gardeners) who might have the same questions, we’ll answer some of the most common questions that we get right here on our blog.
Yes, we provide a lot of pest & disease information in each one of our members-only GrowGuides, but it’s impossible to list out every pest and disease problem our members might encounter, especially considering that we have members in nearly every US state, Canada and Puerto Rico. As you may know, each growing region has its own peculiar pests and diseases, as does each geographical area.
This week, Matt in Greenville, South Carolina (our hometown!) asked us a question that a lot of you might identify with. Matt said he’d lost quite a few squash plants to squash vine borers and to find out if there is anything he can do to prevent future losses without resorting to harmful synthetic chemical poisons.
If this is a problem you’re experiencing or would like to prevent, we hope the info below is helpful!
Photo: A nice pile of summer and winter squash from Tyrant Farms.
Context: What Are Squash Vine Borers?
If you’re a new gardener or are lucky enough to live in an area of the world where you don’t have squash vine borers (like the west coast USA), you might not have any idea what a squash vine borer is.
Squash vine borers (Melittia satyriniformis) are the larvae of an attractive orange and black sesiid moth that is active during the day. Like squash plants themselves, squash vine borers are also native to the Americas.
They’re pollinators, but they have the unfortunate habit of laying their tiny copper-colored eggs on squash plants, which then hatch after 9-11 days, at which point the larvae chew their way inside the plant’s stem and eat it to death. A single adult female borer moth can lay up to 150 eggs in a season.
Due to their similar appearance, many people mistakingly assume the adult squash vine borer moths are in the wasp family.
How To Tell If Your Squash Plant Has Been Killed By Squash Vine Borers
Here’s how you know you’ve got a squash vine borer: one day you have a healthy squash plant and the next day you walk outside to find a plant that is limp and collapsed. You check the ground and the soil is plenty moist. You give the plant a gentle tug at its base to see if something may have eaten its roots (such as a vole) and it’s still firmly in the ground. Then you notice small holes in the leaf stems and/or the main stem of the plant with brown frass around the opening, indicating that something has chewed its way inside.
Usually, once the squash plant has gone limp from damage, there is no way to bring it back. This experience can be heart-breaking, especially if you’re relying on a small number of squash plants for a yield.
6 Methods to Prevent or Stop Squash Vine Borers
Here are six things you can do to prevent or stop squash vine borers from damaging your squash plants:
1. Apply Bacterial Spray(Bt)
Apply an OMRI certified Bacillus thuringiensis (aka “Bt”) spray on the leaf stems and main stem of the plant (not the flowers). Here is a great Bt product you can buy.
Bt is a naturally occurring bacterium found in soils around the world, and research indicates that it is virtually harmless to non-target species (hence its approval for use in organic farming).
If you spray Bt on the stems and leaves of your squash plants once per week (or more if there’s a lot of rain), the squash vine borer’s eggs or larvae will be killed quickly, long before developing into adult moths. Many organic farmers use this method and it may be the most effective way to deal with borers and other squash pests such as squash beetles.
You can even inject Bt into the stems if the larvae have already bored into the plant but the plant is not yet limp.
To further minimize any potential harm to pollinators (just in case the current science proves to be wrong), spray the plants as late in the evening as possible when daytime pollinators begin to go inactive, and avoid directly spraying the flowers.
2. Mulch the Stems
Keep the main stem of the plant covered with mulch as it crawls along the ground. This method helps prevent the adult Squash Vine borer moths from laying eggs on the stems. If the larvae bore into the main stem area of the plant, it doesn’t take them long to kill the plant. This method also helps the vining and crawling squash varieties set new roots along their stems as they grow, so that the plants can continue to support new growth even if they have been infected by a squash vine borer further down the stem.
If you go this route, also be sure to keep a careful eye on the leaves of your squash plants, since borers also lay eggs on the leaf stems or undersides of the leaves. The eggs are small, round, shiny and copper-colored, very similar in appearance to “squash bug” eggs (Anasa tristis). The difference is that squash bugs lay multiple eggs in very tight clusters whereas borers will often only lay a single egg in one spot, or if laying multiple eggs, the eggs won’t be as tightly clustered.
If you see either type of egg on your squash plant, remove them by hand immediately and “smush ’em” (scientific terminology for squishing them between your fingers) or drop them into a cup of soapy water.
Also, if you see one leaf on your plant has gone limp and the others have not, this is a good indication that the limp leaf has a borer inside, especially if you spot an entry hole. Cut the leaf and leaf stem off of the plant immediately as close to the main stem of the plant as possible. Then make sure you bring swift and brutal punishment to the small white squash vine borer larvae munching inside.
3. Grow Resistant Varieties
Most summer squash varieties are very susceptible to vine borers. However, there are other types of squash you can grow whose tough, thick stems are not nearly as susceptible to squash vine borers.
Squash in the Cucurbita moschata family are especially resistant. These include Butternuts and our personal favorite Tromboncinos, a vigorous Italian heirloom that can be eaten early as a summer squash or allowed to grow huge to be stored as a winter squash. If you grow Tromboncinos, be sure you have a lot of gardening space and, ideally, a fence or large, sturdy trellis they can climb on.
Photo: The Tyrant checking out some of her Tromboncino Squash growing over the duck village at Tyrant Farms.
4. Squash Surgery
Lets say you find an obvious entry hole on the stem of your squash plant with frass around it, but your squash plant still appears to be healthy.
What to do?
Break out the surgical tools and play doctor!
A box cutter and a pair of tweezers will do the trick. Make a straight incision down the stem of the plant (not across the stem which would sever it) where the hole is located. Cut as little as you have to until you spot the culprit, or culprits if there is more than one (they’re white grubs with a black head). Remove them with tweezers and dispose of them (our ducks consider squash vine borer larvae and tomato hornworms to be summer delicacies).Then cover the affected area with mulch.
5. Yellow Bowls
As you may have noticed, squash blossoms are yellow. As such, squash vine borer moths are highly attracted to the color yellow.
Many gardeners/farmers have luck trapping and drowning squash vine borers by placing a yellow bowl full of water in the garden. The color attracts the moths to the bowl, they get trapped in the water, soon drowning.
This method isn’t 100% effective but its a good way to control the moth’s populations and determine when and if the moths are out. In our area, we have two waves of moths-one in late spring/early summer and another wave in August.
6. Row Cover
Want to be 100% sure no squash vine borers get to your plants (unless they happened to overwinter in the soil where you’re planting)? Use row cover. It’s a lightweight fabric that lets sun, water, and air in, but keeps insects out. The brand we recommend is Agribon and you can get it here for the price of a few grocery store squash.
The downside of row cover? It also keeps pollinators out which means you’ll need to go outside with a q-tip (or tiny paint brush) in the morning to pollinate your squash plants by hand.
Using your q-tip, collect pollen from your male flowers (the ones that don’t appear to have tiny fruit at the base of the flower) and rub the pollen on the stigma of the female flowers (the ones that have miniature versions of the fruit at the base of the flower). Then cover your plants back up.
Methods to Avoid
A couple of commonly prescribed methods for dealing with squash vine borers that we do NOT recommend:
- Wrapping the stems of your squash plants in aluminum foil. Maybe if you’re only growing a small number of squash plants, this method is worth a try. However, we grow lots of squash plants and our plants have so many leaves and stems, that effectively wrapping each plant would take a week and make our yard look like a metal welder was using it as an abstract art exhibit. Even then, there would still be sections of the plants exposed to borers.
- Spraying your plants with harmful synthetic pesticides. This makes as much sense to us as smoking cigarettes while jogging. If you feel you have to resort to using pesticides that are well-documented to cause harm to a wide range of insect, bird, and amphibian species while also being harmful to humans and pets, then please grow something else or purchase squash from local, organic farmers. It’s just not worth it.
We hope this helps you have a wonderful squash growing season! If you have a potential question of the week, please get in touch to let us know. Thanks for your question, Matt!
Just in case you’re not already a GrowJourney Seeds of the Month Club member, we’d love for you to give us a try (for free) to see if you’d like to start growing with us! And don’t forget: a GrowJourney Gift Membership also makes a unique and special gift.