Squash Vine Borers

Did your squash plants suddenly wilt? Chances are, squash vine borers are to blame. These larvae burrow in the stems, cutting off the water supply to the plant. If you can find them while they’re still small, you can often remove them and the plant will heal, continuing with production of summer and winter squash.

Squash vine borer eggs are tiny! They are the size of the head of a straight pin, and reddish-brown in color. The eggs aren’t laid in clusters, but singly, on the leaf stems and main stem of the squash plant.

Squash vine borer egg next to a map pin for scale.

Squash vine borer egg next to a map pin for scale.

After the eggs hatch, tiny larvae burrow into the stem. These holes are so small that the plant will often heal over the injury in a few days, leaving only a scar as evidence of intrusion on the stem. 

Cream-colored scar on squash stem. This is created when a squash vine borer burrows into the stem, and the plant heals over the wound.

Cream-colored scar on squash stem, created when a squash vine borer burrowed into the stem, and the plant healed over the wound. Note the tiny brown squash vine borer egg on the right.

Once inside, the larvae feed on the stems, damaging them. When the stems are damaged, the plant has difficulty taking up water, and this is what causes the plant to wilt. Often, just a single leaf will wilt at first, then die. At that point, the larvae will move on to other leaf stems or the main stem to continue to feed. They also grow quite a bit as they do this, from 1/8″ when hatched, to over an inch long.

When they feed, they spit out quite a lot of stem tissue and produce a lot of frass, both of which look like mushy sawdust, oozing out of cracks in the squash stems.

Can you spot the squash vine borer in the leaf stem of zucchini?

Can you spot the squash vine borer and “mushy sawdust” in the leaf stem of zucchini?

Large squash vine borer larva in main stem of zucchini.

Large squash vine borer larva in main stem of zucchini.

After growing in size as a larva, squash vine borers exit the stem of the squash and burrow in the ground to pupate.

To remove squash vine borers from stems, locate an entry scar and make a tiny slit along the stem with a sharp knife. You should see the larva, which looks like a white grub with a black head, nearby. Remove it and seal the hole in the stem with some nylon pantyhose wrapped securely, but not tightly, around it. The plant should heal in a few days.

Young squash vine borer.

Young squash vine borer.

For winter squash with long, vining stems, encourage the stem to root in multiple places, rather than just at one end. Having multiple roots increases the ability of the plant to deliver water, which can help keep it from wilting, even if it is damaged in other areas. Examine the stems daily for eggs, and remove to prevent more from hatching and burrowing into the plant. Keep your squash well-watered and fertilized, so there isn’t additional stress on the plant while repairing borer damage and producing fruit.

If you’re feeling discouraged, you’re not alone. With powdery mildew, cucumber beetles, squash bugs and now- squash vine borers, summer and winter squash plants have certainly struggled to grow and produce this summer. However, there are a few things you can do to help them grow better next year:

1) Plant later in the season, from seeds, not young plants.  In Chicagoland, wait until early-mid June to plant squash from seed. Adult squash vine borer moths tend to lay eggs on the largest, most mature plants when they emerge in July, so planting later means your squash will be smaller, and less attractive for egg-laying. Space your plants according to directions on the seed packet. Crowded plants are more stressed for resources, and more likely to struggle with repairing insect damage than plants spaced further apart.

2) Practice good watering techniques, described here, to keep your plants from being water-stressed. Apply fertilizer two weeks after germination, and again when blossoms form. Healthier plants are better able to repair insect damage than unhealthy plants.

3) You can cover plants with row cover to prevent squash bugs and squash vine borers from laying eggs on the plants. This will also keep bees from getting in to pollinate, so be sure to remove the covers in early morning to give them access to the flowers; or hand-pollinate. You can also try wrapping nylon pantyhose around the stems, so larvae cannot burrow into them once they hatch.  Plants can be left uncovered, but examine often to locate and remove eggs.