A squash vine borer (Melittia cucurbitae) is a colorful daytime moth that zigzags through your garden. This swift flier mimics a wasp, and so you would be excused if you dodged this insect when you spot it. This clearwing moth is about half an inch long, orange, with black spots. It has two pairs of wings. The front wings are covered with scales that make the wings look metallic greenish-black. The second pair of wings (without scales) are clear.
Squash vine borers do exactly what the name implies: They bore into your squash plants. They especially like hollow-stemmed varieties, such as summer squash, zucchini, pattypans, acorn squash, pumpkins and winter squash (especially Hubbard). They are less attracted to solid-stemmed varieties, such as butternut squash, cucumbers, gourds or melons. Because squash vine borers affect home gardens more than large farms, there has not been a great deal of research conducted on these insects.
Squash vine borers overwinter as cocoons about an inch or two down in the soil. In the spring, adults emerge from the cocoons. This emergence coincides with the time that squash plants begin to produce vines.
The emerging insects mate, and soon thereafter the female glues a reddish-brown egg at the base of a squash plant. Each egg needs a squash plant to complete its development. The female will glue eggs in up to 200 locations! The eggs hatch in a week or two, and the larvae (caterpillars), about one-16th of an inch in length, tunnel immediately into the center of plant stems to feed. This process can block the flow of water and nutrients to the rest of the plant, causing it to wilt. Eventually the damaged plant rots and dies.
Each whitish larva has a brown head and feeds on a squash plant for four to six weeks. It grows and grows (shedding its outer shell as needed) until it is about an inch or more in length and looks like a maggot. Then it drops off the plant and burrows into the soil. In the soil, it spins a brownish cocoon, pupates and emerges as an adult the following growing season. If a person plants squash in the same soil as the year before, the borers will emerge to find a ready squash supply for their eggs.
Although the caterpillars bore into squash stems, the adult moths feed on nectar. You may spot squash vine borers nectaring, for example, on your milkweed plants during daylight hours.
If you want to know if your squash plants house squash vine borers, check for symptoms such as wilting leaves during midday heat and holes near the base of the plant. As a caterpillar feeds, it pushes its sawdust-like brownish or orangish poop (frass) out of holes in the stem. As the infestation worsens, the base of the plant might rot and eventually die.
If you spot signs of vine borer caterpillars in your plants, there are some things you can do. Take a razor blade or X-Acto knife and slice above the hole in the stem until you find the borer. Remove the borer, then mound soil up to cover the cut area. This will encourage new root growth and help the plant survive. You can also use a needle to try to kill a caterpillar without slitting the stem, but it can be difficult to tell if you have actually stabbed the larva.
It is easiest to deter vine borers before they lay eggs rather than after. You can turn the soil over in the fall and again in the spring looking for vine borer cocoons. The cocoons are dark brown and about three-fourths of an inch long. Removing the cocoons from your soil will prevent the caterpillars from feasting on your plants later in the season.
Starting in late June, look for the large, colorful adult vine borers in your garden. They are attracted to the color yellow, so set out yellow buckets or pans filled with water and a drop of dish soap (to break the surface tension). The squash vine borers will fly to the water and drown.
You can sprinkle diatomaceous earth on lower stems to prevent egg laying. Or you can wipe the base of the stems with a damp cloth every few days to remove eggs before they hatch. You might also try wrapping the base of the stem – from just beneath the soil to just below the lowest leaf – in a few inches of aluminum foil to prevent the moth from laying eggs in the stems.
You can also outsmart the insect. Squash vine borers have a predictable cycle that parallels the squash cycle. Therefore, you can plant your squashes later in the season than usual – in early July instead of late spring. These late squash plants will mature after the vine borer has laid its eggs.
Or you can cover your plants to prevent adult vine borers from flying to the plants to lay eggs. Remove the covers once the plants flower so that bees can pollinate the squash plants. If you planted squash in the same area the year before, though, this method will not work. In this case, the vine borrower cocoons are already in your soil and will emerge whether or not there is a cover above.
After you harvest your squashes, remove the squash plants completely from your garden. This will prevent any remaining caterpillars from completing their development in your yard.
Squash vine borers are a native species found from Canada to South America. But squashes are native, too, so the squash vine borer has evolved to complete its life cycle in your zucchini. People in Central America raised squashes more than 10,000 years ago, meaning that the battle between vine borers and gardeners probably has been ongoing for a very long time.
Reader photo
Stuart Roelke spotted this water snake on the Somerville Community Path near School Street on July 10.
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Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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Thank you so much for this in-depth information. And terrific photos! I watch gardening videos but you’ve used fewer words with precision and used up so much less of my time to help me understand this creature and my options. Last year I used the foil-at-the-base method but first wrapped the base of the stem with a layer or two of a bit of panty hose, putting the foil on top. I had enough squash to eat that summer but I did realize it’s important to readjust those protective layers every couple of weeks as the stem expands. Heavily pruning excess leaves from squash plants on a regular basis allows easier access to those base stems to inspect and monitor them as the plant grows. (And I’ve read that heavily pruning the lower leaves helps the plants focus on fruit making and allows more air circulation to prevent powdery mildew.) This summer I happen to have no summer squash at all in my garden — needed the space to try out other types of plants — so I’m hoping that means I won’t have to worry about any cocoons in the soil. I’m curious to try planting summer squash in early July and will try it next year, but I assume that means transplanting young squash plants rather than starting from seed in early July (because our season here is so short). Lastly, what is it that keeps squash borers away from large-scale squash production? I’ve read that conventional summer squash bought at the store is generally low in harmful pesticides compared with other vegetables and fruits. Thanks again, I’m glad this article is here on Cambridge Day for me to refer to when I need it. Editor: perhaps create a “food gardening” tag so any “wild things” articles that intersect with vegetable and fruit gardening issues can be searched easily.