Field of Science

Wilting squash plants

About this time every year, people's squash vines will often wilt, by day at first and then severely, sometimes just collapsing completely and suddenly.  The problem is quite simple: the interior of the vines are being eaten by the larvae of a day-flying moth (Melitta curcurbitae).  As the specific epithet suggests this insect is specific for cucurbits (Cucurbita): squashes & pumpkins (so not cucumbers or melons).  TPP usually applies an insecticide (cabaryl) once a week to just the stems and petiole bases from mid-June into early July, but this year my routine was not followed.  This keeps the insecticide away from flowers, fruit, and pollinators.  When the wilting just begins, sometimes you can split the stems of affected plants with a sharp, thin-bladed knife and kill the larva(e), and the plant may survive, but mostly it's too late.  If you notice small entry holes near the base of the stem a small stiff probe (paper clip, toothpick, etc.) can kill the very young larvae.  Having access to sciencey stuff, TPP once injected the center of the stem with a bit of insecticide above the entry hole, and this worked fine.  The thicker the vine, the more susceptible, so the bush varieties of squashes are much more vulnerable than the viny varieties. Some people just replant summer squashes in mid-June for a later crop.  The plants are just too small when the moths are actively reproducing.  The saddest case was some neighbor kids who had a nice pumpkin vine with some softball-sized fruits growing, but then their vine wilted and you just knew what you were going to find and that nothing could be done for their pumpkin. 


James said...

My condolences! I witnessed synchronized wilting in my zucchini this year. This after harvesting 5 of the most beautiful little angels! I wonder if early sprinkling of diatomaceous earth would keep the cretins away.

The Phytophactor said...

No, diatomaceous earth won't help. If you want a non-pesticide solution, you might try bright yellow cards smeared with tangle trap. The moths are attracted to yellow, so you would put the cards right next to the base of your squash stems to trap them before they can lay eggs on the stem of your plants just above ground level. You can't miss the red & black moths.