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Squash Vine Borer

Garden pest · Melittia cucurbitae

Description​

The squash vine borer is a clearwing moth whose caterpillars tunnel inside squash stems. It primarily attacks summer squash, zucchini, pumpkins, and other squashes, especially those in the species Cucurbita pepo. Adult moths fly during the day and lay small eggs on stems near the plant’s base. After hatching, the caterpillars bore into the stems and feed within them, interrupting the movement of water and nutrients. Affected vines may wilt suddenly despite moist soil, and heavily damaged plants can collapse. Coarse, yellowish-orange sawdust-like droppings, called frass, often spill from an entry hole near the crown. The larvae eventually leave the vine and pupate in the soil, where they can survive until the next growing season. The number and timing of generations vary by region. Because the larvae are concealed inside stems, finding eggs or fresh entry holes early gives gardeners the best chance to prevent serious damage.

Symptoms​

  • Wilting
  • Yellowing leaves
  • Stunted growth
  • Stems chewed or cut through
  • Soft or rotting stems

How to Identify​

Look near the base of a wilting vine for a small hole surrounded by coarse, yellow-orange frass. If a stem is split open, the culprit is a pale, wrinkled caterpillar with a brown head. Adults are day-flying moths with orange-red bodies and largely clear hindwings; their flight can resemble a wasp’s. Squash vine borer damage often starts on one vine, which may wilt in the afternoon and recover overnight before declining permanently. Drought can also cause wilting, but it does not produce an entry hole or frass. Bacterial wilt likewise lacks frass and is more commonly associated with cucumber beetles.

Prevention​

Inspect stem bases frequently during the local moth flight period, and remove the small, flattened eggs before they hatch. Cover young plants with insect netting or floating row cover, securing the edges; remove it when flowers open so pollinators can reach them, or hand-pollinate while the cover remains in place. Do not cover soil where borers may already be overwintering. Rotate squash to a different area when possible, and promptly remove and dispose of infested vines after harvest to reduce surviving larvae. A loose wrap around the lower stem can make egg-laying there more difficult, but must not constrict growth. Consider butternut squash and other Cucurbita moschata varieties, which generally tolerate borer pressure better than zucchini and many pumpkins.

Treatment​

First, check the lower stems and remove any eggs or newly hatched larvae you find. If frass marks an entry hole, carefully slit the stem lengthwise near the hole, locate and remove the caterpillar, then cover the damaged section with moist soil. Where a trailing vine is still healthy beyond the injury, mound soil over a node to encourage new roots. Water the plant consistently while it recovers, but severely wilted vines may not survive. For plants not yet infested, a product containing Bacillus thuringiensis var. kurstaki or spinosad may help against newly hatched larvae if its label permits use on squash and the application is timed before they enter the stem. Follow the label exactly and keep spinosad off open flowers to protect pollinators. Sprays generally cannot reach caterpillars already feeding inside a vine, so repeat inspection matters more than spraying after extensive damage.

Plants Affected​

Acorn Squash, Butternut Squash, Delicata Squash, Pattypan Squash, Pumpkin, Spaghetti Squash, Zucchini

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