Gardening Q&A: Preventive steps best way to fight squash vine borers

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Q. Squash vine borers ruined my zucchinis this year. Can you recommend steps I can take to avoid this problem next year?

A. Squash vine borers are pests of summer and winter squash and pumpkins and, to a lesser extent, cucumbers and melons. They overwinter as pupae in the soil under host plants, and adults become active mid- to late June or early July. There is a single generation yearly in Pennsylvania.

Adult squash vine borers are clearwing moths that resemble wasps more than anything. Unlike most moths, they are active during the day. They have a 1- to 11/2-inch wingspan, with metallic green forewings. The rear wings are transparent, with black or brown margins and veins. The body is orange and black.

Adult female moths lay their eggs on the main stems and sometimes the leaf stalks (petioles) in July and August.

The oval eggs are reddish-brown and flat and usually laid singly or in small groups.

The small white larvae hatch in a week to 10 days and bore into the stem where they feed for about a month. The mature larva is a thick, white wrinkled worm with a brown head and is about 1 inch in length. They exit the stems and burrow into the soil to pupate.

The leaves on infested stems wilt when the borers' feeding destroys the plant's vascular system.

Once that occurs, the plant is unable to take up sufficient water to support the leaves. Upon close examination, you can see frass -- sawdust-like excrement -- coming from holes in the infested stems.

Prevention is the best course of action because squash vine borers cannot be controlled with insecticides once they get inside the stems.

Crop rotation is important because they overwinter in the soil under their host plants. Avoid growing susceptible crops in the same area from year to year.

Also, be sure to remove spent cucurbit crops at the end of the season to remove any larvae that may still be present.

Till up the garden to expose overwintering pupae to predators and the winter freeze and thaw cycles.

Be sure to mulch or plant a cover crop to avoid leaving the soil bare over the winter, which could allow erosion.

Cover transplants or seedlings with a floating row cover (Reemay, Garden Blanket) to exclude adults from laying their eggs on the plants. Allow enough excess material to allow for the growth of the plants and seal the edges with soil. The covers must be removed when the plants bloom because the flowers have to be pollinated for fruit to set.

Preventative insecticide applications are another option.

Applications must be carefully timed to catch the larvae before they bore into the stems. Trapping adults is the best way to determine when to begin spraying and is relatively easy. The adults are attracted to the color yellow.

Simply place a yellow container (pan, bowl or bucket) filled with water in your garden in mid- to late June. Check the trap daily and begin making applications when you find adult squash vine borers in the trap.

Azadirachtin (BioNeem, others), carbaryl (Sevin, others) and spinosad (Captain Jack's Deadbug Brew) are labeled to help control this pest in the home vegetable garden.

Repeat applications at labeled intervals will be necessary for control over the egg-laying period.

Sprays should be directed toward the base of the plants and stems rather than leaves.

Even if some borers have gotten into the stems, all is not lost.

Find the entry hole, slit the stem lengthwise with a sharp knife, then remove and destroy the borer. Squash often produce secondary root systems at the nodes on their stem. Push the slit ends together and cover the injured stem with soil.

The plant may recover and still provide plenty of zucchini for you to enjoy.

Send questions to Sandy Feather by email at slf9@psu.edu or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.


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