What are they and what can I do about Squash-Vine Borers?
SVB attack primarily squash and gourds but cucumbers and melons are also possible hosts. The adult moth is a 1 1/2-inch wing-span, wasp-like moth that is quite colorful (metallic green-black colors with the hind wings fringed with black and orange hairs and similar colors on the abdomen). They are also called "clear-winged" as the lower 1/2 of the black spread wings are transparent. The moths are day fliers which hatch shortly after normal planting time (early to mid-June) from pupae (dark brown, 5/8th inch long, and found in an earthen cell) overwintered in the soil.
They lay single eggs which are a dull red to rose in color, 1/25th inch in diameter and glue them to the stalks and underside of the stems of squash vines around the base of the plants. Within 5-7 days the eggs hatch and the larve, a 1" white, heavy-bodied grub with a dark head then burrows into the stem of the plant where they feed till grown enough to burrow out and into the soil to pupate. The stem and then the rest of the plant quickly withers and dies. A classic sign of burrowing is a "sawdust-like" yellow frass pile near the base of the stem. In most parts of the country there is but one annual cycle, but midwestern states in zones 6 and 7 may experience a partial second hatch in late July.
Non-chemical controls include timed-planting according to the hatch dates in your area (info available from your County AG agent or note your dates for future reference). Early and late crops (after mid-July) are seldom affected by SVB. In addition, the use of polyester row covers to prevent moth's access to the plants. The covers can be removed briefly at bloom time to allow for pollination but retaining the covers and hand-pollination is recommended. Planting resistant varieties also helps. Certain varieties offer more resistance to and tolerance of borer attack. According to University of Illinois the most tolerant varieties are: White Bush Scallop, Acorn, Summer Crookneck, Dickenson pumpkin, Green Striped Cushaw, and Butternut. The least tolerant varieties are Hubbards, Golden Delicious, Sugar and Small Varietal Pumpkin, and most all zucchni.
After the borer enters the vines, home gardeners may use a sharpened wire to seek out and kill each tunneling larva; or may be able to slit the stem lengthwise with a pocket knife, remove and destroy the larvae, press the stem back together and cover with soil. In many cases the plant will heal and survive the injury if the damage is not extensive. Syringe injection of BT (bacillus thur.) into the stem will also kill the grub. To reduce the number of borers for the next year, destroy crop residue after harvest, rotate planting site, and a 2-3" deep fall and spring till is quite effective at destroying overwintering pupae.
Chemical controls require a weekly preventive treatment applied to the basal three feet of the plant as soon as the vines begin to run. Some recommend either the insecticide methoxychlor or malathion Always follow the manufacturer's directions carefully including "days till harvest". And, since recommended application dates vary from zone to zone and success of treatment depends on early and repeated treatment, contact your county Extension office for current pesticide controls and application dates recommended in your area. Once the larvae have entered the stem, insecticides have little effect.