Melittia cucurbitae

Order: Lepidoptera
Family: Sesiidae

“This squash plant was fine yesterday morning; I picked two squash from it!  When I went to the garden this afternoon it had wilted and died.  This happens every year, usually not long after the plants start producing.  It looked like something was boring in the base of the plant, so I cut into it and found this big white worm.  What is this and what can I do to keep this from happening?”
 
Squash vine borer is one of two insect pests that cause squash and other cucurbits to suddenly wilt and die.  The other is squash bug.  Both of these pests love squash and are a curse to, and are often cursed by, squash-loving gardeners.  When one finds a fat, white caterpillar tunneling at the base of their squash plant, its logical to think it must have bored into the plant from the soil, but this is not the case.  A brief overview of the biology of these unusual caterpillar pests will help better understand how to control them.
 
Adults are colorful, black and orange moths that fly during the day and look a lot like red wasps; they even sound like wasps when flying.  They lay eggs on the leaves, stalks and vines.  Newly hatched caterpillars bore into the plant and feed down the vine toward the base of the plant.  Small caterpillars don’t eat that much and cause relatively little damage, but by the time they reach the base, they are large caterpillars and can eat a lot of plant tissue in a very short time.  This is when plants wilt suddenly as a result of the heavy tissue destruction these caterpillars cause.  Mature caterpillars exit the vine and tunnel into the soil to pupate. Note the caterpillars move from the plant into the soil, and not the other way around.
 
Control: The key to controlling vine borers is to kill eggs and hatching larvae before they bore into the plant.  Do this by spraying regularly with products that contain either permethrin or zeta-cypermethrin.  Weekly sprays are frequent enough during early to mid-season, but you may need to spray more often if you are trying to grow squash in late summer.  Spray the whole plant: leaves, stems, vines and base, thoroughly to control both vine borers and squash bugs. 
 
Be sure to pick all ripe fruit just before spraying.  Pick close!  Squash grow fast and you want to be able to observe the pre-harvest interval (PHI) without having to throw away a bunch of oversized squash.  Permethrin and zeta-cypermethrin both have 1-day pre-harvest intervals on squash, but it doesn’t hurt to allow an extra day.  There are several other effective products, but many have longer PHIs, and a 3-day PHI just does not work well for zucchini or yellow squash.  Spray in late afternoon to protect pollinators.  Spinosad is an organic option for vine borers, but spinosad has a 3-day PHI on squash and does not control squash bugs.  Early planting and use of floating row covers, especially on parthenocarpic varieties, which can set fruit without bee pollination, are other methods of control.
 
See Bug's Eye View No. 9 of 2015 for information on squash bugs.
 
See Extension Publication 2347, Insect Pests of the Home Vegetable Garden, for information on other garden insect pests, including brand name products that contain the active ingredients mentioned above.

Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service.
The information given here is for educational purposes only.  Always read and follow current label directions.  Specific commercial products are mentioned as examples only and reference to specific products or trade names is made with the understanding that no discrimination is intended to other products that may also be suitable and appropriately labeled.
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