Squash vine borer (Melitta curcurbitae) is a garden pest that destroys squash plants and is almost impossible to stop once it has gotten inside a squash vine, but there are smart, organic gardening practices you can adopt to prevent this pest from ever becoming a problem.
The name squash vine borer applies to both the larvae that feed on squash plants and the species’ adult stage, a day-flying black and orange clearwing moth. The moth lays flat, oval-shaped brown eggs at the base of plants on the stems or leaves. When the eggs hatch a week to 10 days later, the larvae bore into stems and continue to work their way through the plants, hollowing out stems and vines as they eat. They affect both bush-type and sprawling squash plants, so vine-less plants are not safe from the vine borer.
If your squash plants are yellowing and wilting, they may be affected by squash vine borer.
Check the stems near the base of the plant for small holes and frass, which looks like sawdust. These are signs that squash vine borer larvae — white caterpillars, up to an inch long, with legs and black heads — are already inside the plant.
Squash Vine Borer Prevention Through Plant Choice
Not all squash plants are equally susceptible to squash vine borer. Squash vine borers like to target Cucurbita maxima (like Hubbard squash) and Cucurbita pepo (like acorn squash, zucchini and pumpkins) but rarely are found on Cucurbita moschata (like butternut squash.)
Cucurbita moschata is less affected because it has dense vines that borers have trouble penetrating. If you make a point of choosing to plant squash varieties that are less attractive to squash vine borer, your garden is likely to stay borer-free. But you don’t need to stop planting the other squash that you love to grow and eat — you just need to take precautions.
Squash vine borers can also affect melons and cucumbers — these are both in the Cucurbitaceae family, like squash — but this is rare.
Squash Vine Borer Prevention Through Timing
Knowing when squash vine borers are active in your region is the first step. In the North, squash vine borer moths are laying eggs in late June and early July. In the South, they emerge in May and there can be up to two generations per year and moths may continue to lay eggs through mid-August. The West Coast is least impacted by squash vine borers, where they are uncommon.
In the South where winters are mild, plant squash as early as possible (as soon as the soil warms to 60° Fahrenheit) so you may harvest before squash vine borer is active — or at least before borers kill the plants. Northerners can wait until late July to plant squash, after the borers have come and gone.
Squash Vine Borer Prevention Through Crop Rotation
If your garden had a squash vine borer problem the season before, chances are that there are cocoons in your soil that will emerge as vine borer moths just as your squash plants are beginning to take off (unless you timed your planting differently).
Planting a different type of crop where you planted squash the year before will deprive vine borers of their host plant, and they will move on from your garden. The following year, squash vine borers may still find your squash plants, but at least you haven’t made it easy for them.
Squash Vine Borer Prevention Through Barriers
A floating row cover placed over your squash seedlings will prevent squash vine borers from laying their eggs in your garden. This is a simple, inexpensive fix, but be aware that row cover will also prevent pollinators from reaching the plants. (If the plants are not pollinated, they will not produce fruit.) Be sure to remove the row cover once the plants begin to flower to allow access to pollinating insects.
Alternatively, you can replicate the pollination process by hand-pollinating. First, identify a female flower to pollinate. It will have a small embryonic fruit between the flower and the plant stem. This embryonic fruit needs pollen in order to grow and mature. Next, pick a male flower — you can tell it is male because there will be nothing between the flower and the stem — and peel back the petals to reveal the pollen-covered anther. Brush the anther around the stigma of the female flower, and then close the flower with a clothespin to allow the pollination process to complete.
Another method is to take a small, soft painter’s brush and dab the brush onto the anther to collect some of the pollen, and then lightly “paint” the pollen onto the stigma of the female flower.
Another barrier method is wrapping the stems of emerging squash plants with aluminum foil. Only the first inch or two above the soil line needs to be wrapped. This will stop the adults from laying eggs on the plant or, at least, stop the hatched larvae from entering the plant. Every 10 days, remove the foil and wrap the stems again so the stems have room for continued growth.
Covering the main stem of the plant with mulch will also deter the moths from laying their eggs.
Squash Vine Borer Prevention Through Trap Crops
One more strategy you can employ is to plant trap crops: plants that you’re willing to sacrifice that serve as decoys. Trap crops draw in pests — diverting them away from the plants that you’re trying to protect.
To pick an effective trap crop, find out which plants a pest loves best. In the case of squash vine borers, Blue Hubbard squash, Magda squash and acorn squash are good options. Sow or transplant one of these varieties into your garden two weeks before planting the crops that you intend to grow for yourself.
Space the trap crop and the true crop about 10 feet apart from each other, give or take two feet. If all goes according to plan, the pests will target the trap crops before they seek out the true crops, which in a best-case scenario are squash varieties that they find less appealing for egg laying.
Trap crops are also effective tools for monitoring and detection. When you find eggs or damage on your trap crops, you know to raise up your garden’s defenses and be extra vigilant. You can destroy eggs on your trap crops as you find them, but should you miss any, it’s not a big deal if they hatch and the larvae destroy the trap crop. That’s what the trap crop is there for.
Detect Squash Vine Borer Moths with Pheromone Traps
Squash vine borer moth pheromone traps are good tools to help detect — but not control — the presence of squash vine borers. The traps use a pheromone lure that attracts male squash vine borer moths but not the females. This is good because gardeners certainly don’t want to attract egg-laying females to their gardens. However, when the males are present, it’s a sign that the females must be in town too.
Hang a trap just before squash vine borers are expected to arrive in your area, and check them daily for any catches. Once you start seeing moths caught in the trap, you know it’s time to inspect your plants for eggs and, if you haven’t done so already, this would be a great time to put up row cover as an exclusion method of control.
The first time you find a trapped moth, record the date. When the next growing season comes around, you’ll have an accurate idea of when squash vine borers will show up, give or take a week. Your own recorded date will be more reliable than what you can find in books or online because due to climate change, some pests are emerging earlier than ever before.
The lures have a lifespan of a month or a month and a half, depending on the brand, so when you put out a trap, set a reminder in your phone to take the trap down or replace the lure once the time has elapsed. You may need just one lure a year for early detection, though if you live in a warm climate where two generations of squash vine borers occur annually, it’s a good idea to replace an expired lure so the trap remains effective all summer.
Simple, effective paper traps combine lures with sticky paper to ensnare moths. If the sticky paper becomes covered in insects or dirt, it will no longer be effective, so the whole trap should be replaced.
There are plastic, bucket-type traps available that use a lure and an insecticide rather than sticky paper. These cost more than sticky traps but may be worth the extra expense in a dusty garden where sticky paper quickly becomes too dirty to be effective.
Remember that these traps are not a replacement for control methods. The traps may kill many male moths in your garden, but all it takes is one female moth to lay eggs for a squash plant to meet its end.
What to Do to Save a Plant Already Affected by Squash Vine Borer
If you detect squash vine borer early on, you may still be able to save the plant.
Unlike other pests that lay eggs in tight clusters, squash vine borer moths spread out eggs, making them harder for you to spot and remove. Inspect the stems and leaf stalks near the base of the plants for brown oval eggs that are “glued” on, pick them off and dispose of them.
Another good piece of advice is to wipe down the stems with a damp cloth a couple of times a week to remove any eggs you couldn’t easily spot. This is reported to be effective as any chemical control method.
Because it takes squash vine borers some time to make their way through a stem or vine (eating plant tissue as they go) you may be able to kill borers before they have gotten too far. A thin wire with a sharpened tip can be inserted from the base of the plant up the stems to kill borers. Then, mound soil over the base of the plant to cover up the damage caused by both the borers’ entry points and the wire.
Instead of blindly using a wire inside a stem, you can use a single-edged razor blade or a utility or Exacto knife, slice into the stem a couple of inches above the entry hole to look for and remove the white larvae.
Alternatively, you can use Bacillus thuringiensis, or Bt, a biological control used in organic gardening. Bt is a bacteria that will only harm caterpillars (moth and butterfly larvae) so it is safe to use around people, pets and other insects — but take care when applying near milkweed or other butterfly host plants. However, Bt sprayed on the outside of the plant won’t stop borers on the inside.
Use a syringe to inject liquid Bt inside the plant stems between 1 and 2 inches above the soil line. Follow the manufacturer’s instructions for diluting Bt concentrate, then use just 1cc per stem or vine.
Injecting Bt is also an effective preventative measure for squash vine borer. Repeat the process every week until the threat of squash vine borer has passed.
Once a squash vine borer infestation has gotten very far — borers, given enough time, can move several feet through vines — the borers cannot be killed without killing the plant in the process. If the plant has stopped producing, it’s time to remove the vines and throw them in the trash, borers and all.
Control Squash Vine Borers with Beneficial Nematodes
If you grew squash in your garden during the last growing season, overwintering squash vine borer pupae may be in the soil, waiting to emerge as moths that will soon breed and produce the next generation. The good news is you can enlist beneficial nematodes to destroy the pupae before they can mature. Like using Bt, applying beneficial nematodes is a form of biological control that is approved in organic gardening.
There are more than 25,000 known species of nematodes, some of which are bad for human health, livestock or crops, though there are a few nematode species that are marketed for sale because of their ability to control pests. For squash vine borer control, look for a product that includes Heterorhabditis bacteriophora, an entomopathogenic nematode species. (At this time, NemaSeek Hb by Arbico Organics is the only source of Heterorhabditis bacteriophora that is readily available to consumers.)
Beneficial nematodes in soil seek out pupae and find their way inside the pupae’s bodies. Once inside a pupa, a nematode releases a bacteria that kills its host. The nematode then reproduces, and its offspring subsists off the dead pupa until there is nothing left to eat, at which point they seek out more pupae. The nematodes will repeat this process until there are no pupae left.
NemaSeek Hb has a shelf life of 14 days when refrigerated, though it should be applied right away because nematodes are living creatures that thrive in soil, not the fridge. And the entire container should be used the same day it is first opened. The manufacturer also advises only applying early in the morning or predusk when soil temperatures are above 42°F.
Use a hose-end sprayer or pump sprayer to apply the nematodes directly to moistened soil, though if you have neither, you can use a watering can. Continue to water the area every three to four days. Two nematode applications are recommended, a week to 10 days apart, and even more applications may be required for a severe infestation.
If borers are already present inside squash plants that you want to rescue, nematodes can be injected into the vines using a syringe, much like how Bt can be injected into vines. The nematodes will seek out and destroy the borer larvae the same way they destroy borer pupae.
As a side benefit, Heterorhabditis bacteriophora will also control other borers as well as asparagus beetles, berry root weevils, billbugs, black vine weevils, carrot weevils, chafers, Colorado potato beetles, corn rootworms, spotted cucumber beetles, flea beetles, Japanese beetles, leafminers, May/June bugs, sweet potato weevils, ticks and more pests.
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Links & Resources
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NemaSeek Hb by Arbico Organics
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