The squash vine borer is a sporadic pest of pumpkin and squash (e.g. cucurbits). Activity of adult moths and larvae often occurs as the crop is expanding rapidly from late June until the first of August. Resulting damage can be difficult to diagnose prior to significant harm to the plant occurs. In years of heavy infestation squash vine borer can become a significant economic pest in some cucurbit crops.

Susceptibility to squash vine borer is variable among species of cucurbits. Varieties known to be suitable hosts are pumpkins and squashes. Commonly infested cultivars are pumpkin (standard and giant), zucchini, as well as crookneck, straight neck, acorn, patty pan, summer, banana, buttercup, and hubbard squashes.

Appearance

Adult squash vine borer is a day-flying clearwing moth (Lepidoptera: Sesiidae) that is commonly confused with a large wasp. Forewings of the adults are greenish-brown while the hindwings are transparent with a fringe of reddish-brown hairs. Wingspan is approximately 3 cm. The abdomen is typically rusty orange with black spots along the dorsal margin. Hind legs are typically covered with tufts of orange and black hairs. Larvae of squash vine borer are creamy white with brown head capsules. Mature larvae are about 2-2.5 cm length.

Life cycle

Beginning in late June to early July adult vine borers emerge from the ground. In the Midwest the pest typically emerges after 1,000 growing degree-days (base 50°C) have been reached. (An explanation of growing degree-day calculation, see page 14 in Growing Pumpkins and Other Vine Crops in Wisconsin (A3688) at http://learningstore.uwex.edu). Often, this degree day threshold will closely coincide with full bloom of the common roadside weed chicory (Chicorium intybus L.). Newly emerged female moths quickly seek suitable hosts and begin laying small, brown eggs singly at the base of susceptible plants. Each female is capable of laying between 150-200 eggs. Depending upon temperature, eggs will hatch within 10-15 days of being laid. Newly hatched larvae quickly bore into the vine stems to feed for four to six weeks. As the larvae feed, they leave behind characteristic light brown frass (insect feces) that resembles sawdust. Larvae typically feed at the center of host plant stems this internal feeding greatly restricts the plant’s ability to move water and nutrients. Fully-grown borers exit the stems and burrow into the soil to pupate. Squash vine borers produce one generation per year in Wisconsin.

Damage

The damage caused by squash vine borer larvae is often difficult to detect until the plant wilts and dies in late July and August. Initial signs of infestation are very difficult to detect. Scouting early often involves searching for entrance holes and frass at the base of vine crop stems.

Advanced symptoms of squash vine borer infestation are quickly wilting plants in the heat of the day. Since wilting may be confused with other pests of vine crops (e.g. bacterial or Fusarium wilts) scouting remains critical. Plants that infested plants may be diagnosed by splitting the base stems of the plant to confirm the presence of the larvae. Fields that have been damaged in past seasons have a good chance for recurring squash vine borer infestations annually.

Scouting

Shallow pans painted yellow and filled with water are effective to attract and catch Squash vine borer adults. Place bowls throughout the garden in late June. Monitor bowl catches daily to determine when adults are present in the garden. Adults may also be easily seen flying in the garden. A careful eye can quickly distinguish the diagnostic coloration and behavior of the day flying adult moths. Currently there are no treatment thresholds for the Squash vine borer.

Management

Cultural control

In Wisconsin, infestation risk can be greatly reduced by planting crops early in the season. Floating row cover placed on the crop when adults are actively laying eggs is an effective method to reduce problems. Understanding when vine borers are present is a critical component to successful management with floating row cover. Synchronizing row cover installation with peak adult activity will reduce damage to preferred host plants. Keep in mind that plants in bloom require bees to pollinate the flowers. Remove row covers daily to allow adequate access for pollination. Planting a trap crop such as summer squash can be an effective means of reducing damage in the primary crop. Trap crops should be planted early to provide a more attractive egg deposition area than less preferred cucurbit species. Trap crop residue should be destroyed before larvae exit vines to pupate, limiting next season’s infestation.

Chemical control

Squash vine borer is very difficult to manage with chemical insecticides since older larvae are protected within the plant stem. The target life-stage for conventional chemical management is newly hatched larvae that have not yet entered the stem. Effective control requires insecticide residue to be in place before and during the egg laying period (1,000 DD₅₀). Two to three successive applications of insecticide 5-7 days apart will adequately control most of the larval borers before entering the vines.

Refer to the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for a list of registered insecticides and management recommendations.