Squash Vine Borer

The squash vine borer (Melittia cucurbitae) is the caterpillar of a day-flying moth. It tunnels inside the stems of squash and pumpkins, and its first sign is often a healthy-looking zucchini plant wilting badly on a hot afternoon. By then the larva is already inside the stem, so prevention matters more than cure.

Identification and life cycle

Adult. A clearwing moth that looks and flies like a wasp. It has a bright orange-red abdomen with black dots, metallic greenish-black front wings, clear hind wings, and hind legs fringed with orange and black hairs. The wingspan is about 1 to 1½ inches. The moths fly during the day, usually starting in early summer when squash vines begin to run: often June in the South and late June to July farther north.

Eggs. Tiny, flat, reddish-brown eggs laid one at a time, mostly on the stems near the base of the plant and on the leaf stalks. They hatch in about 7 to 10 days.

Larva. On hatching, each larva bores straight into the stem, often near the soil line, and feeds inside for 4 to 6 weeks. Grown larvae are cream-white with a brown head and about 1 inch long. They then leave the stem and burrow into the soil to spin a cocoon, where they spend the winter. Northern gardens see one generation a year; the Deep South can see two.

Signs of attack

• Sudden wilting of a runner or the whole plant in the heat of the day, at first recovering overnight.
• Small holes near the base of the stem with moist, sawdust-like frass (larval droppings), green at first and later yellow-brown, pushed out of them.
• A stem that is soft, split or rotting near the base. Slit it open and you'll find the larvae inside.

Plants attacked

Zucchini and other summer squash, pumpkins, acorn squash and most other Cucurbita pepo types, and Hubbard and other C. maxima winter squash are most often attacked. Hubbard is so attractive to the moths that it is sometimes used as a trap crop. Butternut and other C. moschata squash have solid, fibrous stems that the larvae can't easily tunnel into, so they resist the borer. Cucumbers and melons are rarely attacked.

Prevention

• Grow resistant squash. Butternut is the obvious choice for winter squash. For a zucchini substitute, 'Tromboncino', a C. moschata summer squash eaten young, rarely suffers borer damage.
• Row cover until bloom. Cover plants with floating row cover from planting until female flowers open, sealing the edges with soil. Don't cover a bed where squash grew last year, because moths emerging from the soil would be trapped inside with the plants. You can also keep plants covered and hand-pollinate.
• Time your planting. Where there is one generation a year, a planting of summer squash sown in late June or early July often misses the main egg-laying period and still has time to bear.
• Watch for the moths. Starting when vines begin to run, set out a yellow bowl filled with soapy water near the plants. The moths are drawn to yellow, and catching them tells you egg-laying has begun. Then check stems for eggs every few days and scrape them off.
• Encourage backup roots. Cover a few leaf joints along each vine with soil. They root, so a plant whose base is damaged can keep going.
• Wrap the stem base. Wrapping the bottom few inches of the stem with aluminum foil or a strip of row cover can make egg-laying harder, though results vary.
• Rotate and till. Plant squash in a different part of the garden each year, and work the soil in fall or spring to disturb the overwintering cocoons.

Treatment

Remove the larvae. As soon as you see frass, use a sharp knife or razor blade to slit the stem lengthwise near the entry hole, cutting along the stem rather than across it. Remove or kill the larvae inside, then cover the cut section with moist soil and keep it watered so it can root. This works best when caught early.

Inject Bt. Bacillus thuringiensis (Bt) kills caterpillars that eat it, and the borer is a caterpillar, but larvae inside the stem never touch a spray on the outside. Some gardeners inject a Bt solution into the stem with a syringe at the entry hole instead.

Protect stems during egg-laying. An insecticide labeled for squash vine borer, such as some pyrethrin products, can kill newly hatched larvae only if it is already on the lower stems when the eggs hatch. That means reapplying to the stem bases about once a week while the moths are flying, following the label. Keep sprays off open flowers to protect bees.

Clean up. Pull and destroy badly infested plants, larvae and all, before the larvae leave to pupate in the soil. Don't compost them whole.

Squash plants also face squash bugs and cucumber beetles. For growing advice, see zucchini and butternut squash.