Squash vine borers (Melittia cucurbitae) are a serious vegetable garden pest east of the Rocky Mountains. The small, wasplike, clear-wing moths come from pupae that have overwintered in the soil, emerging in April and May in the South and June and July in the North. The moths have metallic copper forewings and orange and black abdomens. Unlike most moths, they fly during the day. After mating, the females lay 150 to 250 oval, flat, brown eggs. These are deposited singly, typically on the main stem of the vine near the plant’s base, but sometimes on the leaf stalks or on the undersides of the leaves.
Summer squash, pumpkins, and gourds (Cucurbita pepo) are preferred hosts, as are Hubbard and related squash (C. maxima). Butternut squash (C. moschata), by contrast, is rarely attacked.
SYMPTOMS: When the eggs hatch, in about a week, the larvae bore into the stem and begin to feed. Signs of borers include greenish-yellow frass or excrement, emerging from holes in the stem, and a sudden wilting of the vine that can lead to death of the plant. Borers feed for about a month, reaching an inch in length, before leaving the plant to pupate in the soil. There is one generation in northern states and two generations in many southern states.
CONTROLS: Once inside the stem, borers are difficult to find. The best cure is prevention. Cover newly planted seeds or transplants with a layer of spun-bonded row cover to keep the adult moths at bay. You will have to remove this cover when female flowers are produced, to allow for insect pollination and at least a partial crop. Many squash cultivars produce secondary root systems at their nodes. Covering the nodes with a little soil will provide insurance against damage to the main stem. Crop rotation, staggered plantings, and handpicking of any observed eggs also provide some protection. Pesticides are of little use, since it is difficult to time their application precisely enough to catch the newly hatched larvae before they have bored into the stems.