Expert relates experiences with squash vine borers

AFP Correspondent

HERSHEY, Pa. (April 14, 2015) — Known as the “silent vine killer,” squash vine borer caterpillars can do considerable damage to plants before a grower knows they’re there. But measures can be taken to limit their destructiveness. 
University of New Hampshire Extension Field Specialist George Hamilton related his experiences with monitoring for the squash vine borer to a group of growers at the Mid-Atlantic Fruit and Vegetable Convention earlier this year.
Southern states typically experience two generations of that insect per year. Depending on the season, Delaware, Maryland and Virginia may be among the regions troubled by more than one generation.
Although New England growers have been expecting only one generation per year, during Hamilton’s monitoring, he found in late August a late peak in the years 2010 and 2013.
Hamilton said the orange and black moths fly by day. Soon after mating, the females locate squash and pumpkin plants by scent, and lay single eggs on the vines, stems and the undersides of leaves.
Each female can lay 150 to 200 eggs, which hatch in 10-15 days.
Then the caterpillars begin boring inside the cucurbit plants. This larva stage is the most destructive.
The larva takes four to six weeks to mature, and reaches about one inch long. They have cream-colored bodies and dark brown heads.
The caterpillars are often unnoticed until their frass is observed. Unfortunately, when the yellowish-orange frass pushes out of the vines, considerable damage has already occurred.
Usually the insect bores through the vines, Hamilton said, but occasionally they bore into the hard squashes and pumpkins. The fruit feeding is usually seen when a large flush of moths occurs. Severe damage reduces yields.
Hamilton pointed out that shallow pans painted yellow and filled with water can attract and capture moths, but he recommended using commercially available pheromone traps and lures.
At the University of New Hampshire, Hamilton said, they use Heliothis traps, which were originally designed for corn earworm. These white dacron net traps are baited with a lure that releases a sex pheromone that attracts male squash vine borer moths.
The number of moths captured determines whether to use an insecticide. For summer squash, the threshold is five moths per week.
Pumpkins can typically sustain more injury.
He recommends twelve moths captured per week for vining pumpkins. But for the bush types, five moths per week is the recommended threshold.
In New Hampshire, the traps had been typically placed in the field after July 4. However, Hamilton noted that some farmers now place the traps the first week of June.
His researchers have observed the first male squash vine borer the second week of June.
During the first week of July in 2013, 26 males were caught in Hamilton’s research plot.
Traps should be placed about two feet from the ground in the squash field with squash leaves below the opening.
He places duct tape over the tie points of the supporting pipe to keep the trap from sliding down. Also, the bracing line to the stake should be slightly slack to provide support in windy conditions.
The lure should be in the middle of the opening. Hamilton’s researchers use clothes pins, safety pins, or small binder clips to secure the lures. They store the individually wrapped lures in a freezer. The unwrapped lures release the odor for about four weeks once in the trap.
Hamilton said summer squash and zucchini are very susceptible to attack by squash vine borers. Giant pumpkins and Kubocha squash are also susceptible. Butternut squash is resistant.
Since vine varieties of squash and pumpkins can root at the nodes, the effects of the larvae bores may be lessened. The bush varieties, Hamilton said he has noticed, appear to suffer more.
Cultural controls can limit damage. Crop rotation, especially when placing the new field some distance away, can reduce problems.
Spun-bonded row covers exclude the moths, but the crop must be uncovered for pollination.
Tilling two inches deep after harvest or before spring planting may kill some of the larvae/pupae that overwinter in the soil.
Removing and destroying dying vines will prevent the larvae inside from maturing and emerging as moths the following year.
The beneficial nematode, Steinernema carpocapsae, can be injected into vines to attack the borers, he said.
If insecticide treatment is warranted, growers should contact their local Extension office for recommendations. Hamilton urged that growers not apply pesticides unless really needed.
He said growers should try to avoid pesticides injurious to bees, and spray very late in the day to reduce risk to honey bees.
If a particular variety such as a susceptible summer squash is desired, Hamilton suggested planting a trap crop. For example, plant a wide border of a highly preferred variety to the borers such as blue hubbard squash. Completely surround the field of summer squash.
Then when the borers are flying as determined by your monitoring traps, spray the blue hubbard squash plants heavily. As the borers focus on the blue hubbards, fewer will attack the summer squash.
Details on the trap crop system appear in a fact sheet by T. Jude Boucher, University of Connecticut Agricultural Educator, available at, under ‘crop specific articles,’ and scrolling to “Perimeter Trap Cropping for Yellow and Green Summer Squash.”