If you have squash or related plants in your vegetable garden that suddenly wilt and die in midsummer, you may have squash vine borer. This insect will bore into the stems of squash, zucchini, pumpkins and gourds. Cucumbers and melons are usually not a target although both cucumbers and melons can be affected by a disease that causes similar symptoms, known as bacterial wilt.
The adult of this insect is a clear-winged moth that resembles a wasp. The forewings are a dark metallic green but the rear wings are clear. The abdomen is orange with black spots. The larva is cream-colored and rather wrinkled. Adults emerge in the spring and lay eggs on or near susceptible plants. Eggs are deposited singly on the underside of the vines and are often concentrated at the base of the plants. Larva bore into the plant and feed for about a month as they move toward the base. Mature larva will exit the plant, burrow into the soil and pupate where they remain until the next year. Each plant can have numerous borers.
If you suspect squash vine borer, split the stem of a collapsed plant near where it enters the ground. Infested plants will be hollowed out and mushy and may contain borers.
Unfortunately, there isn’t much you can do at this late stage. Control measures should center on
prevention. Suggested preventative controls would include crushing the dull red eggs before they hatch, excavating larvae from stems (carefully cut the stem and remove/kill the borer) before they cause much damage or using insecticide applications.
Applications should begin when the vines begin to run and reapplied every seven to 10 days through the end of June. Direct the spray at the crown of the plant and the base of runners.
Chemicals used for borer control in gardens are permethrin (Eight Vegetable, Fruit & Flower Concentrate; Lawn, Garden, Pet and Livestock Insect Control; Lawn & Garden Insect Killer) or bifenthrin (Hi-Yield Bug Blaster II, Bug-B-Gon Insect Killer for Lawns & Gardens) applied as sprays or dusts.
If plants wilt, look for the presence of holes and ooze. However, remember that in extreme heat, squash plants will temporarily wilt in the afternoon even if undamaged by this insect.