Squash vine borer
Melittia cucurbitae
A caterpillar that bores into squash stems, making vines wilt suddenly from the base. Wrap stem bases in foil, slit stems to remove borers, and bury nodes to re-root.
How to know it
The adult is a striking day-flying clearwing moth that mimics a wasp — orange and black, buzzing low over the plants in early summer. The larva is a fat white grub with a brown head, found only by slitting open a hollowed stem.
What you'll see
- A vine that wilts in the heat of the day and recovers overnight, then wilts for good.
- Sawdust-like frass pushed out of a hole at the base of the stem.
- A stem that is soft, split and hollow when squeezed near soil level.
- A fat white grub inside when the stem is slit lengthways.
- Wasp-like orange moths flying over the plants by day in June.
Where it comes from
Moths fly in early summer and lay single flat brown eggs at the base of the stems. The grubs bore straight into the stem and feed inside for a month, severing the plant’s plumbing as they go, then drop into the soil to overwinter in a cocoon. One generation in the north, two further south.
Warm summers across eastern and central North America. Zucchini, pumpkins and hubbard squash are hit hardest; butternut and other moschata squashes have tougher, more solid stems and largely escape.
Dealing with it
- Slit the stem lengthways with a sharp knife at the frass hole, lift the grub out, then bury the wounded stem in moist soil so it re-roots.
- Wrap the bottom 15 cm of each stem in foil or old tights at planting so the moth cannot lay against it.
- Mound soil over the leaf nodes along the vine, so it roots in several places and can survive losing the base.
- Inject or drench the stem base with Bt or nematodes at the first sign of frass.
- Pull and burn a plant that has collapsed — the grub is still inside heading for the soil.
Keeping it out
- Grow butternut and other Cucurbita moschata varieties, which the borer largely leaves alone.
- Grow an edible bottle gourd such as cucuzza in place of a courgette — it is a different genus and the moth mostly ignores it.
- Sow a second, later batch in early summer to fruit after the moth flight has finished.
- Cover young plants with row cover until flowering, then remove for pollinators.
- Rotate cucurbits and cultivate the bed in autumn to expose the overwintering cocoons.
- Watch for the moths in June and start the foil wraps before the eggs are laid, not after.
What it goes for
Courgettes, marrows, pumpkins and hubbard squash above all; butternut and cucumbers much less so.
Who hunts it for you
- Beneficial nematodesSteinernema & Heterorhabditis spp. Microscopic worms watered onto damp soil that hunt down soil-dwelling grubs and release a bacterium that kills them within days. Timing is the whole thing: warm, damp soil and a small target grub, which means late summer for most of what they are used against.
- Tachinid flyTachinidae Bristly, house-fly-lookalike parasitoids that lay eggs on caterpillars, beetle grubs, and earwigs — the maggot consumes the host from within. Adults sip nectar from shallow umbels and daisies. Grow to recruit it: Cilantro,Dill,Feverfew,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com