Happy moths, very sad squash

This is a guest post by Annemarie Nagle

I planted yellow crookneck squash this year as an afterthought, after coming across a half-full packet of seeds and pushing last year’s disappointing crop out of mind in a hopeful bout of springtime enthusiasm.

By June those babies had grown big, lush and beautiful, and friends stopping by remarked on how great the garden was looking. A couple of weeks later and now those giant squash plants are looking more like an embodiment of how I feel after working in the NC heat for a few hours: pretty wilty.

The nemesis at work in my garden is an insect that will be familiar to aspiring squash (and zucchini and pumpkin) growers from Canada to Argentina. Melittia cucurbitae, the squash vine borer, and its host plants in the genus Cucurbita are native to North and South America. In fact, along with maize and beans, squash were cultivated by the Native Americans as one of the ‘Three Sisters’ that provided the agricultural staples for many tribes.

Wet, brownish frass volcanoes coming out of your squash vines are a sure sign there are borers inside. Photo: A.M. Nagle

M. cucurbitae adults are beautiful orange and black clearwing moths. They fly during the day (most moths are active at night) and are often mistaken for wasps when encountered in the garden. They lay their eggs along the base, stems, and leaves of squash plants, and the larvae burrow into the stem immediately upon hatching. They set up shop and feed for several weeks through the center of the stem, eventually cutting off water flow to the rest of the plant: thus, the wilting.

The fact that I grew yellow squash in my garden last year (and lost them to borers) didn’t do this year’s plants any favors either. The mature caterpillars, which at this point in the year look like chunky, inch-long grubs with amber heads, chew their way out of the stem and drop into the soil, where they will bury themselves a few inches deep and pupate. There are two generations per year in NC, so the first generation will re-emerge as adults in a few weeks to polish off whatever plants they didn’t get the first time (or any new ones you’ve been hopeful enough to plant). The second generation will overwinter in the soil as larvae or pupae encased in cocoons and will emerge come spring.

Part of the vine removed to reveal the nearly full-grown caterpillar within. Photo: A.M. Nagle

If you are a more dedicated gardener than I am (I’m a plant pathologist, and better at killing plants than keeping them alive…), there are several measures you can take to protect your squash plants from borers. Lightly tilling the soil in late winter can expose the overwintering pupae to the elements. Also, be sure to destroy squash plants when they are done producing to prevent any borers inside from developing, and rotate locations or years for growing squash. For vining varieties such as pumpkins, which can grow new roots at each node, burying portions of the stem encourages the growth of extra roots that can help portions of the plant survive, even when other portions are eventually attacked.

Keep an eye on your squash for the first signs of borer attack: the presence of eggs, then pin holes and brownish, wet frass accumulating at the base. You can ‘deworm’ your vines by making lengthwise slits in the vine near these holes and killing the larvae inside with a knife or pin. Pile an inch or so of soil or compost around the cut to prevent the vine from drying out and encourage rooting.

M. cucurbitae adult hanging around my cucumber vines. Photo: A.M. Nagle

Insecticide treatments can prevent hatching caterpillars from burrowing into the stem, but are ineffective if you miss this window, so careful monitoring of adult moth activity is important. Adults are attracted to yellow (think squash flowers) and you can monitor when they become active in your garden by placing out a yellow bowl filled with soapy water in late May. Also bear in mind that squash depend on beneficial insects—bees—for pollination, so insecticides should not be applied to flowering plants.

There are some Cucurbita varieties that seem to be less preferred by the borers, particularly those derived from C. moschata, including butternut squash and some types of pumpkin. I’m thinking perhaps a packet of butternut seeds should be waiting for me next year when planting fever hits.

About the Author:

Annemarie Nagle
I’m responsible for making sure the Frank Laboratories run smoothly. I also help with research, web content management, and creation of outreach and extension materials.