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Lawn & Garden

Ask Dayle

8/21/2014 - West Side Leader
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By Dayle Davis

This larva was found eating away at a zucchini plant.
Q: I pulled up a zucchini plant in my garden recently and saw the end was mushy and what looked like some type of larva eating away at it (see photo). Can you tell me what it is and how I can avoid a repeat appearance next season? I would rather not use pesticides, if possible.

A: I suspect you have some squash vine borers at work in your zucchini patch. That caterpillar-like worm in the photo appears to be the larval stage of this pest.

Here’s a shortened life cycle description of how that worm gets in the plant.

The squash vine borer overwinters as a fully grown larva in a cocoon in the soil 1 to 6 inches deep. It pupates in the spring, and the adult emerges in June.

The adult squash vine borer is a moth that looks a bit like a wasp — a black body with some orangish-red markings and feathery hind legs with black and orange hairs. The front wings are metallic green, and the hind wings are transparent. Male and female moths are similar, although the male is more colorful, smaller, has a narrower abdomen and more feathery antennae.

These moths mate in June. After mating, the squash vine borer moths fly slowly in zig-zags around plants and lay eggs singly on stems at the base of squash, zucchini, pumpkins and gourds. Eggs also can be found on leafstalks or on the undersides of leaves. (The borer prefers Hubbard squash over other host plants. Butternut squash is less susceptible than other squashes.)

The moths are active for about two months, with a peak in about early July.

Eggs hatch in nine to 14 days. Larvae enter the stem at the plant base within a few hours after the eggs hatch. Larvae feed inside the stem for four to six weeks and can make their way into the fruit to feed during that time. Fully grown larvae leave the stems and crawl into the soil to pupate. There is usually one generation of squash vine borers per year in Ohio.

To help with identity, know that eggs are oval, flattened, dull red in color and 1/25-inch in diameter. The larva is a fat grub-like caterpillar with a white, wrinkled body and a brown head. A fully grown larva is 1-inch long. The pupa is brown and 5/8-inch long and contained inside a cocoon that is made of earth-covered black silk and is 3/4-inch long. Attack by squash vine borer is characterized by a sudden wilt of the plant. Larvae bore within stems, usually in the lower one foot of the stem. When gourds are grown on a trellis, borers can be found at any stem node, even several feet above ground. Stems can be girdled by borers, which prevents water and nutrients from circulating in the plant. The point where a borer enters a stem is marked by a hole with yellow granular or sawdust-like material nearby. Injured vines often decay and become wet and shiny. Infested plants can be weakened or they can die; the ultimate effect on the plant depends on the number of borers and their location.

Methods of control:

  • Destroy vines soon after harvest to destroy any larvae still inside stems.
  • Till or disk the soil in fall or spring to destroy overwintering cocoons.
  • Cover vines at leaf joints with moist soil to promote formation of secondary roots that will support the plant if the main root and stem are injured.
  • A trap crop of very early planted Hubbard squash can be used to alleviate pest pressure on other susceptible plants.

For small plantings:

√ Run fingers down the stems and crush the eggs by hand before they hatch.

√ Stems can be covered with a barrier, such as strips of nylon stockings or toilet paper rolls, to prevent egg laying.

√ Borers can be removed from vines if detected before much damage is done. Examine stems in early summer; once holes are detected, slit the stem longitudinally with a fine, sharp knife, remove the borer, then cover the wounded stem with moist soil above the point of injury to promote additional root formation.

√ Catch and destroy the moths, especially at twilight or in early morning when they are resting on the upper side of leaf bases.

Organic controls are neem oil, insecticidal soap or BtK (a caterpillar-targeting bacterium). Or, cover your young plants with floating row covers to prevent the moths from reaching your young plants. Remove the lightweight covers just after Independence Day so pollinating insects can pollinate the female flowers that give you your zucchini crop.

For more horticultural information, call The Ohio State University Extension’s Summit County Hotline Tuesdays between 9 a.m. and noon at 330-928-4769, ext. 3. Request Fact Sheet HYG-2153-09, “Squash Vine Borer.”

Please note fact sheets are sent out free, but there is a fee for bulletins. Many bulletins are available online at ohioline.osu.edu and can be printed from home or accessed at a public library.

Dayle Davis is a freelance writer and avid perennial gardener, with a B.A. in communications and coursework in botany, geology and wildflowers. Davis is certified as a Master Gardener under The Ohio State University’s Horticultural Extension. Readers can send in questions regarding lawn and garden issues, which could be featured in a future edition. Questions can be emailed to kcollins@akron.com, faxed to 330-665-9595 or sent to Leader Publications, 3075 Smith Road, Suite 204, Akron, OH 44333. Please do not send any leaves, seeds or other organic material.

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