Squash vine borer (SVB) is a common squash pest. SVB is a type of clearwing moth (Melittia species, family Sesiidae) that is black and orange colored and somewhat wasp-like in appearance. Eggs are laid on many plants in the squash family, including winter squash, summer squash, pumpkins, and gourds. Occasionally cucumbers and muskmelons are attacked. (Adult SVB image by Whitney Cranshaw, Colorado State University, Bugwood.org)
SVB eggs are small (about 1 mm or .04 in long), reddish brown, flat, and are laid on vines near the bases of plants and on leaf petioles starting in early summer. In Wisconsin, adults typically emerge around the time that chicory begins blooming. Eggs hatch after about two weeks and the caterpillars burrow into vines where they feed on internal tissue for about a month. Infected vines often wilt, and there will be sawdust-like “frass” (droppings) surrounding a hold in the stem near the base of the plant.
After they mature, the SVB caterpillars emerge from the stems, burrow into the soil, and pupate. In mild climate regions, adults may emerge and produce a second generation, but in cold climates, the pupae overwinter and do not emerge until the following season.
Gardeners should begin looking for signs of the borer by mid June in most areas. When scouting, check the bases of plants and petioles of the lower leaves for signs of eggs or small holes in the stem with frass (insect droppings) or ooze around the hole. Any eggs that are discovered can be scraped off with a fingernail to destroy them.
Floating row covers can be used to cover plants to exclude adults, but the covers need to be removed before plants bloom to ensure fruit production. Crop rotation should be done when row covers are used, to ensure that adult borers do not emerge from the soil under the covers.
Making a second planting of summer squash after the egg laying period has ended can also help to extend the harvest if the first planting is lost to SVB damage.
Fall cultivation after squash has been harvested will help to kill overwintering SVB pupae in the soil.
Thought it seems a bit daunting, it is possible to carefully remove SVB larvae from stems without harming plants. By the time vines have started to wilt, it is generally too late, so scout plants thoroughly to find the larvae while they are still small.
At an entrance hole where a larva burrowed into the stem, carefully slice the stem lengthwise a short distance and kill any larvae inside. A small piece of wire or the knife blade can be used to to stab and remove larvae. After surgery is complete, cover the cut stem with soil to encourage it to root. Plants will usually recover from their treatment quite well, if larvae are destroyed early enough. Remove and destroy any plants that are wilting from borer damage, to reduce the overwintering pest population.
You can also cover a node (where a leaf emerges) with a soil mound 3 to 5 nodes up from the base of the vine. This will allow the plant to root from this area so it can keep growing if the base of the main stem is attacked by SVB.
There is some variation in susceptibility among different types of squash to SVB damage. Solid stemmed squashes like butternut squash and other Cucurbita moschata varieties are less commonly damaged by SVB. Crookneck summer squash and cushaw (Cucurbita argyrosperma, aka Cucurbita mixta) are also generally resistant to damage by this pest. Zucchini, pumpkins, and hubbard squash (Cucurbita maxima) varieties tend to be quite susceptible to SVB damage.
As eggs are usually laid close to where the larvae burrow into the stems, there is a small window where the larvae are active and can be exposed to pesticides. Spraying the bases of plants and lowest leaf petioles puts the pesticide where SVB larvae are most likely to encounter it. Weekly applications during the egg-laying season may be needed to ensure good control. Spray late in the day to reduce the chances of harming beneficial pollinating insects if the plants are flowering.