Squash Vine Borers

Brian Jervis: Ask a Master Gardener

Sunday, June 21, 2020

Q: My squash plant was big and beautiful, and then one day, the leaves started turning yellow and in a few more days it was dead. What happened? RW

A: The culprit was likely the squash vine borer. These insects can be a problem for all cucurbit plants and are the reason many of us no longer grow squash but grow cucumbers instead. Yes, I know cucumbers are cucurbits too, but they don’t seem to bother cucumbers like they do the squash.

Squash vine borers overwinter in the soil as either larvae or pupae. Adult moths begin to emerge in June (possibly sooner if we have a mild winter like we have had) and remain active through August.

During this time, they find a mate. The female deposits her eggs, usually about a dozen or so at a time, on the stem of the squash plant near ground level. The eggs hatch and the larvae enter the stem to begin feasting on your beautiful squash plant. They essentially eat out the inside of the stem, destroying the plant’s ability to draw up nutrients into the rest of the plant.

Signs of this not only include the yellowing and wilting of the leaves on your squash plant, but you will also see something that looks like wet sawdust on the stem of the plant. This is the excrement (frass) of the borer inside of the stem.

These larvae continue to feed for four to six weeks and can migrate to other plants if their host plant dies or ceases to become suitable for their needs.

These borer larvae are white or cream colored and can be easily seen by slicing open the vine in the affected area. The fully grown moth is primarily black and orange and is easily mistaken for a wasp.

Squash vine borers are worthy adversaries and controlling them can prove difficult. The first line of defense will require diligence on your part. This entails visually inspecting your plant stems near the soil line for eggs. If eggs are found, they can be destroyed.

If you missed the eggs and are seeing evidence of the squash vine borer (frass), you can gently slit the stem and remove the larvae. Once removed, you should cover the damaged part of the stem with soil. With any luck, you will continue to get production from your plant.

Weekly preventative applications of an organic insecticide during the active months can also help. An organic pesticide like spinosad should be applied to the stalk of the plant near the soil level during their period of activity. But be careful to only spray the base of the stalk and spray late in the evening, as you don’t want to harm any of the pollinators working on your behalf to get you some squash.

As an alternative strategy to weekly pesticide spraying, pheromone traps can be placed in your garden near your squash plants. When the squash vine borer moth shows up on your trap, you know it is time to begin spraying the stalks near the soil with spinosad.

And last but not least, because squash vine borers overwinter in the soil, tilling the soil at the end of the season can help disrupt their life cycle.

Good luck!

Garden tips

·        Remain alert for insect damage. Add spider mites to the list. Foliage of most plants becomes pale and speckled; juniper foliage turns a pale yellowish color. Shake a branch over white paper and watch for tiny specks that crawl. Watch for first-generation fall webworm.

·        Some pests can be hand-picked without using a pesticide. Do not spray if predators such as lady beetles are present. Spraying insecticides early in the morning or late in the day will avoid spraying honeybees and other essential pollinators.