How to Prevent Squash Vine Borers From Killing Your Squash

How to Prevent Squash Vine Borers

To prevent squash vine borers, keep the moth from laying eggs on the stems. Cover plants with floating row cover from the day they emerge or go in the ground until they flower, then wrap the base of each stem in aluminum foil or nylon hose. Plant butternut where borers are a yearly problem, and sow a second crop of summer squash in early July, after most egg laying is over. Sprays do not reach a larva once it is inside the vine, so everything that works happens before the eggs hatch.

That window is short. University of Maryland Extension notes that moths can lay eggs three days after they emerge. Hatching takes 7 to 10 days according to Wisconsin and Connecticut extension services and 10 to 14 days according to Illinois, and the larva bores into the stem soon after.

Know the Pest and Its Timing

The adult is a day-flying moth that looks like a wasp. Maryland describes it as a fat, black, clear-winged moth with orange legs and an orange and black abdomen. Wisconsin gives a wingspan of 1 to 1.5 inches. The eggs are tiny, flattened, dull red to brown, and attached one at a time, mostly at the base of the stem.

The larva is a wrinkled white caterpillar with a dark head, up to 1 inch long. It feeds inside the stem for 4 to 6 weeks by the Illinois and Connecticut accounts, or 14 to 30 days by Wisconsin’s, then leaves the vine and pupates in the soil. The signs are greenish-white, sawdust-like frass pushed out of a hole near the base and a runner or whole vine that wilts quickly. If you are unsure whether the stem or the leaves are under attack, squash bugs versus squash vine borers sorts out the two pests.

Flight dates differ by state, so use the one nearest you.

State (source)	When adults fly	Generations
Maryland (University of Maryland Extension)	Mid-May to mid-June	1 to 2
Connecticut (University of Connecticut)	Emerge mid to late June	1 in the Northeast
Illinois (University of Illinois Extension)	Mid-June to early July	2 possible in southern Illinois
Ohio (Ohio State University Extension)	Typically June, after 1,000 degree days (base 50°F)	1
Wisconsin (University of Wisconsin Extension)	Late June and July, at 900 growing degree days (base 50)	1

Where there are two generations, the threat returns later in summer, and late plantings are not automatically safe.

Cover Plants Until They Flower

A floating row cover is the most dependable barrier because the moth never touches the plant. Connecticut’s fact sheet says to apply covers as soon as plants emerge or are transplanted and to remove them when flowers appear. The same sheet says to secure the cover well along all edges, so leave no gap at ground level. Our guide to row covers for pest protection compares the fabric weights.

One caution applies. Squash needs bees, so a cover left on through bloom means no fruit unless you pollinate by hand, which Illinois Extension offers as an alternative to uncovering. The method is shown in our zucchini hand-pollination guide. Ohio State makes the same point: covers exclude the moths and the pollinators alike, and they must come off at flowering.

Watch for the First Moth

A cheap trap tells you when the flight begins in your own yard. Connecticut describes filling a yellow container with water: the adults are attracted to the color and drown. Set one near the squash before the dates in the table. Ohio State, which uses pheromone traps for the same purpose, deploys them before the expected flight in late May or early June and checks them at least weekly. A yellow bowl is worth a look every day or two, since a moth can lay eggs three days after emerging.

The first moth in the bowl is the signal to confirm that covers are sealed or stem wraps are in place.

Wrap and Bury the Stems

When the covers come off, protect the part of the plant the moth aims for. Maryland recommends a collar of aluminum foil around the lower stems, and Connecticut lists aluminum foil or nylon hose wrapped around the base of the vine to prevent egg laying. Start the wrap at the soil line so no bare stem shows beneath it.

Soil works as a barrier too. An NC State Extension county article suggests burying the stem under mulch or soil so the moth cannot find it. On vining types, burying does a second job. Illinois Extension advises covering vine nodes with soil to encourage rooting, and Ohio State notes that vining plants root along their stems and compensate better for larval feeding. A vine with roots at several points can keep growing after the original base is hollowed out.

Check the stems during the flight. Ohio State advises inspecting the base of vines for eggs, frass and early wilting, and Connecticut notes that eggs can be removed before they hatch.

Plant Around the Flight

Timing the crop is free. Illinois and Connecticut both suggest a second planting of summer squash in early July to avoid the peak of egg laying. Maryland’s advice for its earlier flight is to use transplants or to plant squash seeds in mid-June.

This works best in the one-generation states. In Maryland and southern Illinois, where a second generation is possible, pair late planting with a stem wrap.

Choose Squash the Borer Avoids

Variety choice is the least work of any method.

Susceptibility	Squash	Source
Resistant	Butternut, Tromboncino, cushaw	University of Maryland Extension
Resistant	Butternut (Cucurbita moschata)	Ohio State University Extension
Less susceptible	Calabaza, crookneck, some pumpkins, cucumbers, melons	University of Illinois Extension
Most susceptible	Zucchini and other summer squash, acorn, Hubbard	University of Illinois Extension
Commonly infested	Pumpkin, zucchini, crookneck, straight neck, acorn, patty pan, banana, buttercup, Hubbard	University of Wisconsin Extension

The sources disagree on yellow crookneck. Maryland rates it less susceptible than zucchini and Illinois agrees, while Wisconsin places crookneck on its list of commonly infested squash. Treat it as a moderate risk and protect it. Butternut is the one squash that no source here lists as susceptible.

Sprays: Timing and Limits

Insecticide only works on the stem surface before the larva bores in. Maryland’s home garden guidance is to spray the lower stems and base of the plant with pyrethrins when adults are flying, mid to late May there, and repeat 14 days later. Wisconsin describes two to three treatments spaced 5 to 7 days apart during the three-week egg-laying period. Connecticut lists kaolin clay applied in mid to late June and again 7 to 10 days later.

Aim at the lower stems and base of the plant, as Maryland specifies, and apply late in the day, as Illinois advises, to avoid pollinators. Follow the label. Where plantings mix flowers and vegetables, as in a companion planting layout, keeping spray on the squash stems alone protects the bees on everything else.

Clean Up After Harvest

Next year’s moths come from this year’s larvae. Connecticut says to remove infested vines immediately after harvest and to till the soil to expose overwintering cocoons. Maryland adds a detail: seal infested vines in a plastic bag before the larvae leave to pupate. Maryland puts the overwintering stage 2 inches under the soil, shallow enough for a fork or tiller to reach.

Rotation helps less than gardeners hope. Ohio State puts an effective rotation distance at more than 5 miles, which no backyard can manage. Cleanup and tilling do the real work in a home garden.

If a Borer Gets In Anyway

A wilting vine with frass at the base can sometimes be saved. Maryland’s method is to make a vertical slit upward from where the frass appears, remove and kill the borer, and mound soil over the wound. Connecticut describes a lengthwise slit halfway through the vine near the base. Keep the mounded soil moist so the stem roots there. If the plant collapses instead, pull it and bag it before the larva leaves the stem, so it does not become one of next summer’s moths.