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.

To the Rescue

Although a wilting plant is severely stressed, it's not beyond saving. Sorensen recommends injecting Bacillus thuringiensis or Bt (a naturally occurring bacterium used to control many caterpillar and larval pests) right into the stems of infested plants, using a syringe or small spray bottle.

Removing the larvae by hand by slitting the stem and digging out the grubs with a knife is labor-intensive, but effective. Each stem may harbor several larvae, below as well as above the wound that marks the point of entry, so be sure to look carefully. After surgery, pile moist soil up around the cut stem. It will shortly send out new roots.

Some control methods, such as reducing egg laying by active adult females, are preventive rather than curative. Sorensen says that row covers can help if you are in new ground, that is, if you didn't grow squashes in or near that location the year before. In old ground, "overwintering pupae emerge as adults under the row covers. Routine crop rotation is the solution." But row covers also prevent pollination by other day-flying insects; to ensure fruit set on covered plants, hand pollinate. (Remember that daytime temperatures under row covers can soar to destructive levels in southern gardens.)

Preventive Bt injections given near the base of the stem, starting shortly after the plants begin to flower, may also offer some protection from young grubs.

Destroying all crops at the end of the season will keep still-active larvae from pupating, reducing the number of egg-laying adults the next year; so will two or more shallow passes with the rototiller. Tilling brings overwintering pupae and larvae to the surface where birds and drying winds destroy them.

Parasites? No. Resistant Varieties? Maybe

Although some garden suppliers suggest purchasing and releasing the tiny parasitic Trichogramma wasp to control the egg stage, Sorensen says "egg parasites don't work." Neither, he says, do parasitic nematodes, a larvae control touted by some garden suppliers.

More promising is work by researchers such as Molly Kyle, professor of plant breeding at Cornell University in Ithaca, New York. Kyle is among the few researchers in the country working on developing squash vine borer resistance in vulnerable cucurbits. All butternuts are resistant, so she has introduced this characteristic into a breeding line and now has a yellow squash that looks quite promising. A borer-resistant summer squash is still some years away, however.

Datasheet: Squash Vine Borer

Family: Sesiidae (Clearwing Moth family)

Genus and Species: Melittia satyriniformis (aka M. cucurbitae). In Arizona, M. gloriosa.

Size and Color: Adults, 1- to 1-1/4-inches; larvae, less than 1 inch. Tiny, flat brown eggs are usually laid singly near base of stem. Larvae are white with brown head. Forewings of adults are black; hind wings are transparent with black veins. Abdomen fuzzy, bright orange spotted with black. Hind legs fuzzy, orange and black.

Range: Most of temperate North America, except the Pacific states.

Crops affected: Most members of the cucurbit family.

Shelly Stiles lives and gardens in Buskirk, New York, and has been a contributor to National Gardening.

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