Crop Talk: How to get rid of Squash Vine Borers (and improve your harvests!)

A friend recently wrote to me with a garden question:

We’ve had great success with our little garden over the past five years now. Always a bounty of tomatoes, beans, peas, and greens and even summer and winter squash now that the bees have found us (I swear there wasn’t a flower in a 1 mile radius before we moved to the neighborhood).  

We’ve done our share of trial and error and gotten our plot mostly figured out. BUT every year no matter what I do we end up with an abbreviated zucchini crop because of squash vine borers! Do you know of anything that can be done before planting or early in the seasons to get rid of these buggers? 

Squash vine borers are buggers, indeed.

Squash vine borer
Squash Vine Borers, photo from UMN Extension

A clearwing moth, it overwinters in the soil as mature larvae, and emerges in June and July, just after many gardeners in the Northeast have planted out their zucchini and other cucurbit crops.

Adults are easy to spot with their bright orange abdomen. They lay their eggs in the soil at the base of plants, and the eggs hatch within 9-11 days. Once the eggs have hatched, the larvae burrow into the hollow stems of zucchini, squash, or cucumbers, and feed on the plant from the inside. This in turn causes the leaf and stem to wilt, and when left unchecked will eventually kill the plant.

Squash vine borer damage
Squash vine borer damage, photo by University of Maryland Extension

SO, what to do?

If you see the adult moths flying around, start checking the base of your zucchini plants. If the larvae have already bored into the plant, you’ll see small holes in the lower stems. Make a slit in the stem with a sharp knife, and using tweezers or your fingers, squish the larvae. Cover the slit with soil for the stem to heal.

This approach is only realistic if you have a few susceptible plants. And while it’s good to know how to react once the pests have arrived, for long-term success you need to take action before and after zucchini season.

To decrease squash vine borer damage, follow these steps:

  • Rotate your crops every year
    • Since mature larvae overwinter in the soil, it’s likely they’re hanging out near the spot your zucchini grew last season. By moving your crops around, you have a better chance starting in a clean, pest-free environment.
  • Cover young transplants with row-cover
    • Crop rotation helps, but once the larvae grow wings, they’ll find your plants. Keep the plants covered with remay, also called agribon, to create a physical barrier that still lets light and rain in. Take the remay off when the plants start to flower.
  • Cultivate before planting
    • Overwintered larvae will be 1-2” deep in the soil. Cultivating the beds with a hoe before planting can help expose and disturb the larvae, giving you a chance to get rid of them before they hatch.
  • Be vigilant in post-crop clean-up
    • Plants left in the ground after they’re past harvest can become a host site for next year’s pests. When the harvest is done, remove the plant from the garden, chop it up with a digging fork, and compost in an active (hot) compost pile/bin.
    • After removing the plants, cultivate thoroughly again to expose larvae. You can then follow up with a cover crop to smother weeds and put nutrients back into the soil.
  • Adjust your planting time, or plan a second succession
    • With a pest like squash vine borers, which emerge in late June and early July, you can decrease their damage by adjusting your planting schedule. Mature plants are less susceptible to extensive damage, so try an earlier planting to get a jump-start on the season and on the pests. Likewise, plant a later succession to keep the harvest going when the first one peters out.
  • Feed your plants
    • Healthy plants are one of the best defenses against pests. Top dress the soil with compost at transplant time, and foliar feed the leaves with a liquid kelp or seaweed solution at the first signs of stress like wilting or yellowing.
  • Bring in the chickens!
    • If you keep backyard chickens, they may be your biggest ally in pest reduction. At the end of the growing season, let them into the garden to clean up the beds. They’re expert grub-eaters, and their manure will add nitrogen to the soil. As always, don’t let them near crops you plan on harvesting soon.  USDA Organic standards require a 120 day wait between the application of raw manure and the harvest of food in direct contact with the soil, and a 90 day wait for the harvest of food that is not in direct contact wit the soil.

zucchini and summer squash harvestAs with any pests, if you have a resident population, it may take a few seasons before they start to decline.  Be vigilant with the pests and gentle with yourself, and the balance (and harvests) will come.

Do you have a garden-related question?  Send me an email and I’ll answer it here on the blog.

1 thought on “Crop Talk: How to get rid of Squash Vine Borers (and improve your harvests!)”

  1. I have a raised bed garden spot, but for the past three years, the squash worms have killed my vegetable plants. Will it help if I dig out and replace the soil in my raised bed?

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