Squash vine borers are a common problem in the squash, zucchini, and melon plants in my South Tennessee garden, and can be a nuisance in gardens across the country. The best method of treatment is prevention of squash vine borers, although there are chemical pesticides that can be used as well.
The squash vine borer, Melitta cucurbitae, looks similar to a wasp, but is considered a moth. They also attack gourds, melons, and cucumber plants.
Squash vine borer moths lay eggs that they glue to the stalks of squash, and the other aforementioned plant, vines. The eggs usually hatch within a week, and a fat white grub, the squash vine borer larvae, burrows into the stem of the plant and basically eats the stem from the inside out. Once they have grown enough to pupate, they eat their way out of the stem and bury themselves into the surrounding soil. The pupae hatch into adult moth, where the cycle begins again.
One of the easiest ways to tell if you have squash vine borers is to check for sawdust-like material at the point where the stem meets the soil. The stem, having been damaged by the squash vine borer, can no longer transport nutrients further up the plant, and the stem and leaves dry out and die.
As with most garden pests, prevention is more successful than treatment. Use row covers to prevent adult moths from laying eggs in the soil surround your plants. Place the row covers over your plants as they begin vining, or when you first notice squash vine borer adults. Keep the row covers on for at least two weeks, to prevent the adults from laying eggs in the surrounding soil. You will either need to remove the row covers to allow for pollination by insects, or keep the covers on and hand-pollinate your plants. You can also only plant varieties that are resistant to squash vine borers, although this will not guarantee that you will not have a borer problem.
Following the growing season, do not return spent vines into the soil where you plan to plant similar crops the next Spring, till or turn the soil several inches down to destroy pupae, and rotate your crops.
Chemical pesticides can also be used as a preventative measure. Check with your county extension office for recommendations on pesticides and appropriate application dates for your area. For the most part, pesticides will only be effective if they are applied before the squash vine borers have entered the plant’s stem.
If squash vine borers have already attacked your plants, you can gently slice open the stem vertically with a very sharp knife to seek out the grubs and kill them, then bury the sliced part of the stem in soil and mulch over. Keep the area well watered, and most of the time, the plant will fuse itself back together at the stem. Even if it does not, the plant was doomed to failure, due to the existence of the squash vine borer larvae, so you don’t have anything to lose by attempting to root out the larvae.