Gardening Articles: Edibles :: Vegetables

Squash Vine Borer

by Shelly Stiles


Holes in the base of squash plants are signs of the squash borer

When a squash stem suddenly wilts, and you find punctures or cracks near the base of the plant, you know that squash vine borer is present. A pest of summer squash and other plants of the squash family, it is most common east of the Rocky Mountains and somewhat in the desert Southwest. It doesn't occur in the Pacific states.

Although it's seldom present in large enough numbers to threaten commercial harvests, in home gardens the borer's feeding activities can mean no squash or pumpkins at harvest time. To defend your plants all summer, arm yourself now with some cultural techniques, a couple of natural pesticides, and most important, an attitude of vigilance.

A Borer's Life

The squash vine borer belongs to the clearwing moth family (so named for their one or two sets of finely etched, transparent wings without scales), a group that also includes such well-known garden pests as the raspberry crown borer, peach tree borer, and lilac borer moths. Most clearwing moths look wasp- or hornet-like, but the adult squash vine borer looks like a large, glassy-winged, bright orange and black fly-fishing lure. In the South, says Ken Sorensen, an entomologist at North Carolina State University in Raleigh, adults are active in late May or early June. A second brood appears in late July and early August. In zones 6 and cooler, only one brood a season is common.

"After mating," says Sorensen, "the female moth zeroes in on the base of the plant, generally laying eggs on the lower six inches of the stem. They hatch within a week, and the larvae -- dirty white grubs with brown heads -- immediately bore into the stem." There they spend the next four to six weeks eating their way toward the tips of the plant. At the end of the feeding period, the larvae, now an inch or more long, move to the soil. They will overwinter at a depth of an inch or two as silk-enclosed pupae, or, in the South, occasionally in the larval stage.

Signs of Trouble

Few of us notice the day-flying adult or the tiny, dull-colored eggs it lays. It's only after the larvae start to feed that most of us know we have a problem.

Wilting is usually the first symptom, often appearing suddenly and worsening quickly. On closer inspection, obvious discolored wounds can often be seen at the base of the plant and stems and at the wounds, masses of a yellow-green grainy substance known as frass -- the hygienic German word for caterpillar excrement.

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