Attack by the squash vine borer is characterized by sudden wilt of the plant. Larvae bore within stems, usually in the lower one meter (three feet) of the stem. Stems can be girdled by borers, preventing water and nutrients from circulating in the plant. The point where a borer enters a stem is marked by a hole exuding yellow granular or sawdust-like frass. Injured vines often decay and become wet and shiny. Infested plants may be weakened or they can die; the ultimate effect on the plant depends on the number of borers and their location. Over 100 larvae have been found in a single plant. If a plant wilts but there is no evidence of borers, other possible causes include root feeding by larval cucumber beetles and bacterial wilt infection.

Hosts:

Squash, zucchini, pumpkins and gourds are attacked by the squash vine borer. Hubbard squash is preferred over other hosts; butternut squash tends to be less susceptible than others. Cucumbers and melons are usually not attacked.

Description:

Eggs are oval, flattened, dull-red in color, and 1 mm (1/25 inch) in diameter. The larva is a fat, grub-like caterpillar with a white wrinkled body and a brown head. A fully-grown larva is 25 mm (1 inch) long. The pupa is brown and 16 mm (5/8 inch) long, and contained inside a cocoon that is made of earth-covered black silk and is 19 mm (3/4 inch) long.

Squash vine borer larva. Photo from the University of Connecticut Cooperative Extension.

The adult is a moth that looks like a wasp; the body is black with orangish-red markings, and the hind legs are feathery with black and orange hairs. The front wings are metallic green, and the hind wings are transparent; the wingspan is 25 to 37 mm (1 to 1.5 inch). Male and female moths are similar, although the male is more colorful, smaller, has a narrower abdomen, and more feathery antennae.

Life Cycle:

The squash vine borer overwinters as a fully-grown larva in cocoons in the soil, 2 to 15 cm (1 to 6 inches) deep. It pupates in the spring and the adult (a moth) emerges in June. These moths are distinct in that they are active during the daytime, resting on leaves in the evening; most moths are active at night. The moths fly slowly in zig-zags around plants and lay eggs singly on stems; eggs are usually found on the main stem near the base, but are also found on leafstalks or on the undersides of leaves. Moths are active for about one month. Eggs hatch in 9 to 14 days. Larvae enter the stem at the plant base within a few hours after hatching from the eggs. Larvae feed inside the stem for four to six weeks. Fully-grown larvae leave the stems and crawl into the soil to pupate. There is usually one generation per year, but a partial or complete second generation is possible.

Control:

The squash vine borer is most susceptible to natural enemies in the egg stage. The eggs are commonly attacked by parasitic wasps. Larval and adult ground beetles can attack the larvae of the squash vine borer, but do not appear to cause significant mortality.

Cultural:

• Destroy vines soon after harvest to destroy any larvae still inside stems.

• Disk or plow the soil in fall or spring to destroy overwintering cocoons.

• Cover vines at leaf joints with moist soil, to promote formation of secondary roots that will support the plant if the main root and stem are injured.

• A trap crop of very early-planted Hubbard squash can be used to alleviate pest pressure from other cucurbits.

Physical:

For small gardens:
• Borers can be removed from vines if detected before much damage is done. Examine stems in early summer; once holes are detected, slit the stem longitudinally with a fine sharp knife, remove the borer, then cover the wounded stem with moist soil above the point of injury to promote additional root formation.

• Stems can be covered with a barrier, such as strips of nylon stockings, to prevent egg laying.

• Catch and destroy the moths, especially at twilight or in early morning when they are resting on the upper side of leaf bases.

• Hand-pick the eggs before they hatch.

Chemical: The squash vine borer can be killed by chemicals, but appropriate timing is crucial. An insecticide is effective when applied as eggs are hatching. A preventive treatment regime consists of applying an insecticide when vines begin to run, and re-applying every seven to ten days for three to five weeks. The application should be directed to the base of plants, at crowns and runners. Chemicals used for borer control in gardens include pyrethrum, malathion or carbaryl (Sevin), applied as sprays or dusts. The biological insecticide B.t., in the forms currently available, is not effective because it cannot be applied to the plant parts that are eaten by the borer.

Adapted from the Ohio State University Extension, 1999