Managing squash vine borer in the garden

Managing squash vine borer in the garden. Colorful adult vine borer moth laying egg on squash stem
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There are a variety of insects that will feed on squash. One of the more troublesome, and potentially devastating, is the squash vine borer. If you've grown squash and had a runner or two start wilting, there's a good chance you've had an encounter with squash vine borer.

Squash vine borers (Melittia cucurbitae) will feed on both summer and winter squash and pumpkins as caterpillars. The adult moths are 5/8- to 1-inch long and colorful. Their abdomens are usually orange with black dots. Their hind legs are orange and black and hairy. Their front wings are greenish-black, while the hind wings are colorless with dark veins.

The adults fly during the day, often darting around in a zig-zag pattern. Because they fly during the day, they are commonly misidentified as wasps. The adults will emerge from mid-June to early July and lay eggs, primarily near the base of stems, but they can be found on other parts of the plant (elsewhere on stems and petioles). 

After the eggs are laid, they will start to hatch in about 10-14 days. The larvae (caterpillars) will bore into the stems and begin to feed. When they enter the plant, they will leave a small hole, and as they feed, they will push sawdust-like frass (insect excrement/poop) out of the hole, which will accumulate over time. While the larvae are most commonly found near the base of the plant, they can be found throughout the plant and even in fruit, particularly later in the season.

As the caterpillars feed, they will tunnel through the stems. This will often cause wilting of vines, especially during the heat of the day. As feeding continues, the vines the caterpillars are feeding on may eventually die. The larvae will grow to be about 1" and have a whitish body and brown head. After feeding for 4-6 weeks, they will emerge and burrow into the ground, where they will eventually pupate. In southern Illinois, there can be two generations.

Often, by the time gardeners notice their damage, little if anything can be done. Fortunately, there are steps you can take to prevent them from attacking your plants and, if caught early enough, steps you can take to control them:

  • Covering plants with floating row covers can help keep squash vine borers off of plants. If you have had trouble in the past, make sure you are growing in a different area since they overwinter in the soil. Covers will need to be removed, or plants will need to be hand-pollinated once plants begin producing female flowers.  
  • Altering your planting date or planting a second crop in early July to avoid peak egg-laying can help reduce the likelihood that plants will be attacked. 
  • Some cucurbits are more susceptible to squash vine borer than others. Summer squash, zucchini, acorn squash, and Hubbard squash are some of the most susceptible. On the other hand, cultivars of Cucurbita moschata like butternut squash, calabaza, crookneck squash, and some pumpkins, along with other cucurbits like cucumbers and melons, are far less susceptible.
  • If you are growing vining cucurbits, bury a few nodes on each vine. This will cause the plants to root and can lessen the impact of any squash vine borers that may attack the plant.
  • Scout your plants for the presence of larvae. If you notice any frass, you can cut the vine lengthwise near the entry hole and remove the larva. Once the larva is removed, cover the stem with soil.   
  • If you decide to apply pesticides, apply them to the plants' crowns and runners when they begin to run. Apply late in the day to avoid pollinators. Once caterpillars are inside of the plants, foliar pesticide applications won't have any effect on them.

 

Good Growing Tip of the Week: Good sanitation can help reduce squash vine borer populations. When you are done with your squash and pumpkins in the fall, remove the plants and shred or otherwise destroy them. This will help get rid of any caterpillars that may still be present in the vines.

 

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MEET THE AUTHOR
Ken Johnson is a Horticulture Educator with University of Illinois Extension, serving Calhoun, Cass, Greene, Morgan, and Scott counties since 2013. Ken provides horticulture programming with an emphasis on fruit and vegetable production, pest management, and beneficial insects. Through his programming, he aims to increase backyard food production and foster a greater appreciation of insects.