Squash vine borer
Click for larger image Squash vine borer caterpillars (Lepidoptera) in summer squash (Cucurbita); the caterpillars are encircled in yellow

The squash vine borer, Melittia cucurbitae, is native to Missouri. It is a serious pest of both summer and winter squash. The insect will also attack cucumbers, pumpkins, muskmelons, and watermelons.

Symptoms and Diagnosis

The first symptom is generally wilting of part or all of the plant. At the base of the plant, moist, sawdustlike debris (frass) can be seen piled outside small holes in the stem. If the stem is split lengthwise, frass and one or more fat, white caterpillars with brown heads can be found tunneling in the stem.

Life Cycle

Squash vine borers overwinter as pupae in a silken, dirt-covered cocoon one or two inches below the soil surface. Adult squash vine borers are day-flying clear wing moths a little over 1/2 inch long with reddish-white bodies and black bands on their abdomen. They have a distinctive wasp-like appearance. Their forewings are greenish-brown but their hind wings are transparent. The insect has a wing span of about 1-1/2 inches. The hind legs are orange and black, long, and hairy. Adults emerge in early to mid-June through July, first appearing about the time cucurbits begin to bloom, and lay 1/10 inch long, brown or reddish-brown eggs in rows or clusters on all parts of the plant but predominately on the stem. Upon hatching, the larvae immediately bore into the stems and remain there until full grown; then full-grown larvae move into the soil to overwinter. There is one generation a year.

Integrated Pest Management Strategies

1. Remove and destroy the borers. Once detected the borers need to be killed or removed from the stem. The easiest way is to slit the stem above where the hole and frass is noticed and remove the insect or insects from the stem. Cover the stem area with soil to encourage new roots to form above the damaged area. This may save the plant, depending upon the severity of the damage. Dip your knife in a 10% solution of bleach before making the next cut in order to prevent transmitting wilt or rot organisms, which might be present.

2. Biological control. Borers can also be killed by injecting a solution of Bacillus thuringiensis (Bt) into the infested stem, using a disposable syringe or wood working glue injector. Follow the disinfecting and mounding procedures discussed above. This method can also be used as a preventive if injected into the stem 1-1/2 inches above ground level when the plants first begin to flower.

3. Promote rooting along stems. About every five leaves or so, place a small mound of soil over the stem to encourage rooting. Then if borers kill the lower part of the plant, the upper parts will have roots to recover.

4. Clean up plant debris. Plants killed by borer activity should be pulled up. Place these plants in a large, clear, plastic bag and leave it in the sun for a week or so. This will kill any borers still in the debris. In the fall, clean up and dispose of old plant debris. Leave the soil uncovered for a day or so to give birds a chance to clean up any insects on the ground.

5. Protect plants with a row cover. In subsequent years, early in the season, plants can be protected with floating row covers secured on all edges to prevent flying adults from laying eggs on the plants. The cover needs to be removed when the plants begin flowering.

6. Use chemical controls with caution. Properly timed chemical pesticides can be effective. Scout plants early and often, watching for the first signs of borer’s frass at entrance holes in the stems indicating egg laying has occurred. Two insecticide applications spaced 5 to 7 days apart will control the majority of newly hatched larvae before they enter the vines. Chemical controls are not effective once the borer is inside the stem. Sevin can be applied to crowns and runners when the plants begin to vine. Apply late in the day because Sevin is very toxic to bees that frequent vine crops and facilitate pollination. Bees carry grains of Sevin dust back to the hive where it can kill many bees.

Organic Strategies

Strategies 1, 3, 4, and 5 are strictly organic approaches. For an organic approach to Strategy 2, consult the Organic Materials Review Institute (OMRI™) for appropriate Bacillus thuringiensis (Bt ) products.

More images:

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Damage to stem of squash (Cucurbita) caused by squash vine borer (Lepidoptera)
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Close-up of adult squash vine borer, a moth (Lepidoptera), on squash (Cucurbita)
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Adult squash vine borer (Lepidoptera) on squash (Cucurbita)
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Collapsed summer squash (Cucurbita) caused by squash vine borer (Lepidoptera)
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The rotted crown of this summer squash plant (Cucurbita) was caused by squash vine borers (Lepidoptera)
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Caterpillars or larvae of squash vine borers (Lepidoptera) can be very difficult to see because they the same color as the pith of this summer squash (Cucurbita); there are two in this picture.
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Squash vine borer larva (Lepitdoptera) on squash (Cucurbita)
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Squash vine borer adult (Lepitdoptera) on squash (Cucurbita)