Here are tips on how to identify, eliminate, and control the squash vine borer.
What are Squash Vine Borers?
Squash vine borers are very difficult to control. If your plants suddenly wilt because larvae may be 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
- Slit the lower stem lengthwise with a fine, sharp knife to remove the larva by hand.
- 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.
- 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.
- 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) is not usually effective. 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.
- Cover crops with floating row covers to prevent egg laying (but only if you are sure there aren't pupae overwintering in the soil).
- Importing parasitic wasps prior to the egg stage can be helpful as these wasps are the borers' natural enemy.
- 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.