Squash Vine Borer (Melittia cucurbitae) is a clearwing moth whose larvae cause the damage. Adult moths emerge in early summer and lay eggs near the base of squash stems. After hatching, the larvae bore into the stem and feed inside it, damaging the tissue that carries water through the vine. Plants may suddenly wilt even though the soil is moist. Look near the base of the stem for holes and piles of moist, sawdust-like frass. Mature larvae eventually leave the vine, burrow into the soil and overwinter.

How to Fix It
Watch for the moths: Check plants in early summer for the distinctive orange-and-black, wasp-like adults.
Check stems regularly: Look around the base for eggs, small holes or fresh frass so you can catch an infestation early.
Use row cover: Protect young plants while the moths are active, removing the cover when flowering begins for pollination. Don't use row cover over soil where squash grew the previous year because adults may emerge underneath it.
Remove the borer: If a vine has just begun to wilt, carefully slit the affected stem lengthwise, find and kill the larva, then close the stem and mound moist soil over the damaged section. The vine may recover and produce additional roots.
Remove infested vines: Pull and dispose of badly damaged vines so larvae cannot complete their development.
Rotate squash: Plant squash in a different part of the garden the following year because the insect overwinters in the soil.
Choose less-susceptible squash: Butternut squash is less susceptible than many other squash types.
Try a later planting: Where the growing season permits, a later planting of summer squash may avoid the main egg-laying period.