Squash Vine Borer

by Ginny Burkhart, University of Illinois Master Gardener Intern, 2011

an image of squash vine borer courtesy Missouri Botanical Garden

Squash vine borers, photo courtesy Missouri Botanical Garden

Zucchini is my children’s favorite vegetable, and if it grows in abundance in my summer garden, they will eat it every day!  This season, I harvested only a few zucchini before my plants were destroyed by the squash vine borer (Melittia Cucurbitae).  I decided to research this bug and wanted to share my findings with you.

The squash vine borer is first sighted in the garden around mid-June as a moth.  Its looks are deceiving though, as it looks like a red and black wasp, a little more than a half-inch long.  Most moths fly at night, but this adult moth flies during the day. This moth also buzzes, another reason it is often confused with a wasp.  After mating, the female moth will deposit very small, flat, reddish brown eggs on the stem of your zucchini plant.

an image of the moth of Adult Squash Vine Borer

Adult Squash Vine Borer moth, courtesy Missouri Botanical Garden

The eggs will hatch in 7 to 15 days as larvae, and bore into the stem of the plant.  The larvae are worm-like creatures that are cream colored with a brown head.  Larvae will tunnel into the stem, feasting on the stem tissue, thus inhibiting the plant from receiving the water and nutrients it needs to survive. A tell-tale sign of the squash vine borer is the orange colored frass it leaves behind on the stem.  Additionally, a bacterial rot may soon follow its infestation, which causes the plant to become soft from decay.  If you notice a sudden wilting of your zucchini plant, the squash vine borer is probably the culprit.

The larvae feed on the plant anywhere between two to six weeks, until they mature to about 1 inch in length.  They will then return to the soil, creating a silken brown cocoon around themselves.  These pupae will stay in the soil and overwinter until the spring when pupation begins and the adult moth emerges once again.

There are a few natural controls you can use to thwart the squash vine borer.  Chemical treatments have been used so often that the borer has developed a resistance to many insecticides.

Be sure to rotate your crops and do not plant zucchini in the same area as the previous growing season.  After planting, cover the plants with a row cover.  Be sure that you have anchored or buried the edges of the cover below the soil line so the moth doesn’t get under the cover. This will prevent the moth from laying eggs on your plants.  However, be sure to remove the covers once flowers appear so that pollination can occur.  You can remove the eggs by scraping them off the stem when you find them, however, at about 1/25th of an inch in length and laid singly, they are very hard to spot.  If you don’t use a row cover, you can wrap the base of the stem of your zucchini plant with foil to prevent the borer’s entry.

Another interesting control is the use of a trap, recommended by the University of Minnesota Extension.  Fill a yellow pan or bowl with water and place near your plants in June.  The yellow color attracts the moth, and when they fly to the container, they can be trapped by falling into the water.  Once you see the moth in your trap, you know they are out there laying eggs.

Be sure to quickly remove any infected plants from your garden and throw them away.  It makes sense to do this as soon as you see an infestation, otherwise, if left to mature, you’ll have borer pupae in the soil of your garden ready to emerge again next spring.  Or, after infestation, you can use this control:  vertically slice the stem of the zucchini plant, remove the worm-like larvae, and then bury the stem in moist soil.  This will promote rooting and the plant may survive. (Ed. note–you can also cover the vine with soil every 3-4 leaves. The vine will root at those points, and even if borers wilt the vine leading up to that point, the vine beyond the buried point will often survive.)

You can plant another crop of zucchini in July to avoid the borer.  I’m trying this in my garden in late July.  The caution here is that there may be a second generation of the borer within the same season in southern Illinois.

Once the growing season is over, be sure to till the soil in your garden.  This may destroy the pupae and also cause exposure to the cold, thus killing it as well.  Do this again in the spring before you plant.

I hope you find these tips interesting and helpful.  Zucchini is a tasty, versatile vegetable.  A healthy plant will provide you with an abundance of zucchini for you and your family to enjoy!

Ginny Burkhart is a 2011 intern in the Illinois Extension Master Gardener program. After suffering a loss to her zucchini vine this summer, she researched the problem of squash vine borer, and wrote this article on her findings.