Insert: Borer in stem.
Squash vines suddenly wilt. Holes in the stems are filled with a tan sawdustlike material. Slitting infested stems lengthwise with a knife reveals fat white worms up to I inch long.
BRIEF ANALYSIS: Squash vine borers (Melittia cucurbitae)
These insect pests primarily attack squash and gourds and only rarely attack cucumbers and melons. Hubbard squash is especially susceptible. The larvae damage and kill the plants by tunneling in the stems, preventing the rest of the vine from receiving the water and nutrients it needs. The metallic green adult female moths lay eggs on the vines. Egg laying occurs in April and May in the southern United States and in June and July in northern areas. When the eggs hatch, the white larvae bore into the stems and feed for four to five weeks. They then crawl out of the stems and into the soil to pupate.
Insecticides applied after the borer is inside the stem are not effective. Instead, slit the infested stems with a knife and destroy the borer. If the plant has not already died from the tunneling, cover the damaged portion of the stem with soil. Keep the soil moist to encourage new roots to grow. The vine may recover. Next year, during the egg-laying period, dust the plant with an insecticide containing methoxychlor.
DETAILED DISCUSSION: (text excerpt)
The Squash Vine Borer is common to some parts of Southern Canada and the United States east of the Rocky Mountains. Its prey all seem to be among the Cucumber Family in which squash, gourds, cucumbers, melons and pumpkins belong.
Its life cycle is fairly simple, and a knowledge of it is essential in controlling its numbers. From egg to larvae to pupae to moth, the Squash Borer lives a life in which it is ever changing in appearance. The female adult moth is about the size of a wasp with a greenish brown body, translucent wings, a red to orange striped abdomen and 5 red spots positioned in a row down the length of her back. She moves with darting and flickering movements which are reminiscent of a Dragon Fly. She lays eggs singly on leaf stems and vines. These eggs are oval, flat and brownish The eggs hatch out white larvae with brown heads which quickly bore into vines and begin to feed on the tissue inside. You can usually find evidence of them by looking for frass (excrement) and holes along the vine. This greenish-yellow frass is your first tip that you have an invader. Once you see this, you may not be able to control the borer with insecticides, and may have to perform surgery on the vine to extract the borer. Once inside the vine, the borer is protected from chemical sprays. Plants plagued by vine borers soon wilt and die, while the larvae spin cocoons in which to hibernate as pupae to await the next season.
These over-wintering pupae are normally found in the top 1-5" of soil and they will predictably emerge from the soil as adults between June 8th and the 15th in southern New England. They will emerge up to 3 weeks earlier in the South and somewhat later in the North. The important thing to understand is that their adult life is fairly short, perhaps 6-8 weeks. The emerging female will immediately begin laying eggs on stems and vines, and will continue for about 5 weeks. This means that effective control can be obtained if you are diligent during the egg-laying period. In southern New England, thoroughly spraying leaves, stems and vines from about June 10th until July 20th will give good control.
Most growers use either Sevin or Methoxychlor as the mainstays in their insecticide control programs. It is interesting to note that Sevin is not labeled as a control for Squash Vine Borer, but experience has taught most growers that it is very effective in reducing the egg-laying of adult moths on leaves and stems, and therefore successful in reducing vine borers. Since both of these insecticides have fairly short residual periods (the time in which they are active deterrents), they must be sprayed periodically and consistently throughout the growing season. A once-a-week spraying should give good control, with additional sprays required when excessive rain or overhead watering has washed chemical residues off of leaves and stems. Sprays should be directed at all of the plant parts, especially the leaf stems which seem to be the likely target of egg laying moths. Spraying should be done after the sun has begun to set in the evening. It is at this time that flower blossoms will begin to close and bees will be less active in the garden. Sevin is very toxic to bees, and any spraying during the day should be refrained from. Spraying after 7-8 PM prevents insecticide from penetrating the interior of blossoms, since they are closed. When these blossoms open the following morning, bees will be free to roam in and out of them without coming in contact with the insecticide.
In place of Sevin or other chemical insecticides, some organic growers will use Rotenone, diatomaceous earth and Bt. They are fairly effective in controlling the larvae stage, but do nothing in hindering the adult in her egg laying activities. Since she lays her eggs singly, the chances of finding all of these single egg locations are very slim. Once the larvae is inside the vine, only extraction can prevent it from totally killing that section of the vine. Extracting the borer can be as injurious to the plant as the borer itself, and much care must be taken to prevent the entrance of diseases into the plant's system after the extraction. It is better to prevent the borer from entering in the first place, and this can only be prevented by deterring the adult moths.
Other non-chemical procedures which will help in reducing borers are fairly simple practices. Since the pupae overwinter in the top 1-5" of soil, frequent cultivation before planting will reduce their numbers later. Begin this in the Fall, and continue right up until planting. Also, burning infected vines will kill any active borers that are contained in them. These practices should be followed by any grower, regardless of using chemical or organic insecticides.
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.
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.
Text and images adopted from theOrtho Problem Solver
Female moth and egg images courtesy of William M. Gardner.
Last Updated: Fri Sep 06 10:30:00PM CST 1996