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 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.
Alternatively, you can 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.
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.
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.
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.
Using 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.
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