What to do now
Mildest step first.
- Watch for orange-and-black, wasp-like moths flying low around the vines.
- Check stem bases every few days for tiny flat brown eggs and scrape them off.
- Keep young plants under row cover until flowers open, or wrap the lower stems in foil.
- If you see sawdust-like frass, slit the stem, remove the larva, and bury the cut in moist soil.
When the moths fly
Squash vine borers spend the winter as pupae in the soil and come out as day-flying moths with an orange abdomen marked with black dots; they look more like wasps than moths. When they appear depends on accumulated heat. University of Wisconsin–Madison Extension says moths emerge in late June and July in Wisconsin once about 900 (base 50°F) have built up, which coincides with full bloom of roadside chicory, and that egg laying runs about three weeks around 1,000 degree days. Iowa State puts the flight in early to mid-June, and University of Maryland Extension says to watch for moths from mid-May to mid-June. Farther south there are two overlapping generations a year, and University of Florida notes adults can be out from mid-May into late October. Your county extension office can tell you when they fly where you live.
Eggs and the first signs
Females fly low and slow around the plants and lay tiny, flat, brown eggs one at a time, mostly on stems near the base, Maryland notes; Iowa State adds that they're hard to see and are often laid on leaf stalks near the soil. Wisconsin says the eggs hatch in 7 to 10 days, and the larvae bore straight into the stem and feed there for two weeks to a month. The first signs are entry holes near the base with light brown or greenish, sawdust-like pushed out, and a stem that feels mushy. Then a runner or the whole plant wilts, often first in the heat of the day. For other reasons squash plants wilt, see the wilting page.
Keeping eggs off
Insecticides can't reach larvae once they're inside a stem, so the goal is to keep eggs off. Maryland and Wisconsin both suggest covering young plants with while the moths fly and removing it once the plants flower so bees can reach the blossoms; if you leave the cover on, you'll have to pollinate by hand. Because the pupae overwinter in the soil, put squash in a bed where it didn't grow last year, or moths can come up under the cover. Maryland also suggests wrapping a collar of aluminum foil around the lower stems. During the flight, check stem bases every few days and scrape off any eggs.
Planting time and less-favored squash
Planting time can help, but sources differ on which way. Wisconsin finds early plantings tend to have fewer problems than later ones. Maryland suggests using transplants, or sowing seed in mid-June so young plants miss the moths' peak. Which works better depends on when the moths fly where you live. Choosing the right squash is more reliable. Maryland lists butternut, 'Tromboncino' and cushaw as resistant and yellow crookneck as less susceptible than zucchini, and University of Florida also names butternut among the least preferred. Wisconsin lists pumpkins, zucchini, acorn, Hubbard and most other summer and winter squash as commonly attacked, and notes Hubbard is attractive enough to use as a trap crop, destroyed while the larvae are still inside.
Removing borers
If you find frass, act quickly. Maryland suggests slitting the stem lengthwise, upward from the frass, with a sharp knife, cutting about halfway through, then removing and killing the white, dark-headed larva and mounding soil over the wound so the stem can root. Wisconsin adds keeping that soil moist and notes many plants recover if caught early. Iowa State says insecticides don't help plants that already have borers in the stem. Maryland suggests sealing badly infested vines in a plastic bag before the larvae leave to pupate, and Iowa State suggests removing vines as soon as harvest is done and tilling the soil in fall and spring to disturb the pupae.
Read the whole label first: it must list your crop, and its directions (amount, timing, gear, bees, days before harvest) are the law. If this page and the label differ, follow the label.
- ZucchiniThe summer squash that turns a garden into a produce stand — if you can keep the vine borers away.
- Korean Zucchini (Aehobak)A vining summer squash picked small and tender — kin to butternut, not to Italian zucchini.
- PumpkinA sprawling vine that needs room to breathe and soil warm enough to germinate without rotting.
- Acorn SquashA smaller winter squash with a thinner skin — and a shorter shelf life than its butternut cousin.
- Delicata SquashThe winter squash with an edible skin — no peeling, no fighting the cutting board.
- Squash Vine BorerSudden vine wilt with sawdust-like frass at stem base — larvae feeding inside the hollow stem.
- Squash BugShield-shaped gray-brown bugs and egg clusters on squash; leaves develop bronze patches and wilt.
- Bacterial WiltCucurbit vines wilt rapidly despite moisture; cut stem shows sticky ooze that threads when pulled apart.
- Why are my squash plants wilting?Check the stem base first: holes with sawdust-like frass mean squash vine borer. Gray bugs and copper egg clusters mean squash bugs. Ooze from a cut stem suggests bacterial wilt. Recovery by morning points to heat.
- Why does my zucchini flower but not fruit?Usually it's pollination. The first zucchini and squash flowers are often all male and fall off a day after opening, which is normal. If female flowers (tiny fruit at the base) open and then shrivel, they weren't pollinated. Hand-pollinate them, and watch for heat.