Here are tips on how to identify, eliminate, and control the squash vine borer.
What are Squash Vine Borers?
It can be so disappointing when your squash is thriving and suddenly starts to wilt. This is the squash vine borer.
The plants wilt because larvae is boring within the lower part of the stems. Look for holes and green to orange-yellow sawdust-like frass (droppings).
They attack squash, zucchini, pumpkins, and gourds. They prefer Hubbard squash and are not as fond of butternut squash. Cucumbers and melons are not usually an issue.
The borers overwinter in soil as pupae in cocoons. When the adult clearwing moths emerge in early to midsummer, they lay eggs singly or in small groups at the base of stems. The eggs will hatch within 1 to 2 weeks after being laid. The larvae will then bore into stems to feed for about 2 to 4 weeks; sometimes they may also bore into the fruit. In northern areas, there is one generation per year; in southern areas, there may be 2 generations.
If caught early, it's possible to save the plant. If caught after the eggs hatch, it may be too late. Controlling the squash vine borer is mainly about prevention.
How to Identify Squash Vine Borers
The eggs are tiny, flat, oval, and brown. If you slit open a stem lengthwise with a fine, sharp knife, you will see the borer larva, which has a fat, white, wrinkled body and brown head; it can grow to about an inch long.
The adult is a moth about 1/2 inch long that looks a bit like a wasp with a black body, marked with orange-red. The hind wings are transparent and the front wings are metallic green.
How to Control Squash Vine Borers
- Sometimes the best solution is to start your squash as early as possible. This way you'll be harvesting before the summer time when vine borers become active. If you plant early, you may need to be prepared to cover your plants in case of frosts.
- Do not plant squash in the same bed two years in a row. Squash vine borers overwinter in cocoons in the soil. Also, clean up ALL debris and clean up your soil in the fall.
- If you catch them VERY early, you can manually remove the squash vine borer. Slit the lower stem lengthwise with a fine, sharp knife to remove the larva by hand. Gardeners can make a vertical slit in the stem with a sharp knife and manually remove the larvae. One plant can house several. Then cover the slit stem section with moist soil about the point of injury to promote formation of secondary roots. Also, extra rich soil near the vines helps rerooting.
- Or, if you spot entrace holes and "sawdust," try inserting a wire and thread through the stem for some distance to kill the inside larvae.
- If possible, catch and destroy the moths at twilight or in early morning when they are resting on the upper side of leaf bases.
- There are insecticides, too, but timing is critical; they are effective when applied at the time that eggs are hatching. The biological insecticide B.t. (Dipel) can be injected into the squash stems, however, it is not usually effective because the larvae are protected inside the plant. Many find carbaryl (Sevin) to work. See your local garden store for advice on appropriate chemical controls.
- Preventative measures include covering the stems with a barrier, such as strips of nylon stockings, to prevent egg laying.
- Trap the adult orange moths with yellow sticky traps and yellow-colored bowls of soapy water.
- Importing parasitic wasps prior to the egg stage can be helpful as these wasps are the borers' natural enemy.
- One of the better solutions is to cover crops with floating row covers to prevent egg laying (but only if you are sure there aren't pupae overwintering in the soil). You can also drape these row covers over frames. This will not only give you a head start over pests but protect your plants from strong heat and frost.
- A trap crop of very early-planted Hubbard squash can be used to alleviate pest pressure on other squash.
- As soon as the squash is harvested, get rid of the vines. Till the soil in the fall and spring to get rid of overwintering pupae.
- If you give up on zucchini, which is susceptive to vine borers, try growing a squash that's less resistant such as Cucuzzi (Lagenaria siceraria), also known variously as the snake gourd; it's pale green and twists and spirals like a snake.
- Plant extra squash for the pests! They are around for 6 to 8 weeks and can only eat so much (or, so we think!).
See our Squash & Zucchini page for more plant care tips.