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Squash Vine Borer

by Celeste Welty    

Damage

Attack by squash vine borer is characterized by sudden wilt of the plant. Larvae bore within stems, usually in the lower one meter (three feet) of the stem. 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 frass exuding from it. Injured vines often decay and become wet and shiny. Infested plants may be weakened or they can die; the ultimate effect on the plant depends on the number of borers and their location. Over 100 larvae have been found in a single plant.

If a plant wilts but there is no evidence of borers, other possible causes are root feeding by larval cucumber beetles, or a bacterial wilt infection.

Hosts

Squash, zucchini, pumpkins, and gourds are attcaked. The borer prefers hubbard squashes over other hosts. Butternut squash is less susceptible than other squashes. Cucumbers and melons are usually not attacked.

Classification

Melittia cucurbitae (Harris); Order Lepidoptera, Family Sesiidae.

Appearance

Eggs are oval, flattened, dull-red in color, and 1 mm (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 25 mm (1 inch) long. The pupa is brown and 16 mm (5/8 inch) long, and contained inside a cocoon that is made of earth-covered black silk and is 19 mm (3/4 inch) long.

The adult is a moth that looks like a wasp; the body is black marked with orangish-red, and the hind legs are feathery with black and orange hairs. The front wings are metallic green, and the hind wings are transparent; the wingspan is 25 to 37 mm (1 to 1.5 inch). Male and female moths are similar, although the male is more colorful, smaller, has a narrower abdomen, and more feathery antennae.

Life Cycle and Behavior

The squash vine borer overwinters as a fully grown larva in cocoons in the soil, 2 to 15 cm (1 to 6 inches) deep. It pupates in the spring and the adult (a moth) emerges in June. Moths are active during the daytime and in the evening they rest on leaves. This is different than the behavior of most moths, which are active at night. The moths fly slowly in zig-zags around plants, and lay eggs singly on stems; eggs are usually found on the main stem near the base, but are also found on leafstalks or on the undersides of leaves. Moths are active for about one month.

Eggs hatch in 9 to 14 days. Larvae enter the stem at the plant base within a few hours after hatching from the eggs. Larvae feed inside the stem for 4 to 6 weeks. Fully grown larvae leave the stems and crawl into the soil to pupate. There is usually one generation per year in Ohio, but a partial or complete second generation is possible.

Life stages of squash vine borer; adult male (a), adult female (b), egg (c), larva (d), earthen cell (f), pupa (g).

A trap baited with the squash vine borer's sex pheromone would be a useful tool in determining when the moths are active. This pheromone has been identified but is not yet commercially available.

Natural Enemies

The stage most susceptible to natural enemies is the egg stage, which is attacked by parasitic wasps. Larval and adult ground beetles (Family Carabidae) can attack larvae of squash vine borer, but do not appear to cause significant mortality.

Cultural Control

• Destroy vines soon after harvest to destroy any larvae still inside stems.
• Disk or plow 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 from other cucurbits.

Physical Control

The following are suitable in small plantings:

• 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.
• Stems can be covered with a barrier, such as strips of nylon stockings, to prevent egg laying.
• Catch and destroy the moths, especially at twilight or in early morning when they are resting on the upper side of leaf bases.
• Hand-pick the eggs before they hatch.

Chemical Control

Squash vine borer can be killed by chemicals but the trick is in the timing of the application. An insecticide is effective when applied at the time that eggs are hatching. A preventive treatment regime is to apply an insecticide when vines begin to run, and re-apply every 7 to 10 days for 3 to 5 weeks. The application should be directed to the base of plants, at crowns and runners.

Chemicals used for borer control in gardens are methoxychlor, rotenone, pyrethrum, malathion, or carbaryl (Sevin), applied as sprays or dusts. Restricted-use insecticides used for borer control by commercial growers include endosulfan (Thiodan) and pyrethroids (Ambush, Asana, Pounce). The biological insecticide B.t., in the forms currently available, is not effective because it cannot be applied to the plant parts that are eaten by the borer.

NOTE: Disclaimer - This publication may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.


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