After a dry summer in Maine, it would be easy to dismiss any withered or dying squash vines as nothing more than victims of arid conditions or a lack of watering. But that might not be the whole story. In fact, those dying vines could reveal the presence of a squash loving pest.
The squash vine borer is native to North America. Its range covers the eastern United States from Florida up into southern Maine. But this summer, gardening experts have gotten reports about the pest from growers in central Maine. They say the warm, dry conditions may be playing a part in extending the borers’ range northward.
“This year I have gotten quite a few calls from gardeners about it,” said Jim Dill, pest management specialist with the University of Maine Cooperative Extension. “Far more calls than in the past and that may be due to the nice dry weather making it a good year for them.”
A good year for the vine borers can make for a bad year for squash fans. Dill said some home gardeners have seen their entire squash crops wiped out.
It’s the larvae of the vine borer that do the damage, according to Dill. The adult female moth lays her eggs at the base of the squash plant in early June. After about two weeks the eggs hatch and the larvae bore into the stem of the squash plant where they will spend the next four to five weeks.
Not only do the larvae feed on the material inside the stem, they block the flow of water through those vines, Dill said.
“Because they block the water, that section of the plant will begin to wilt,” Dill said. “If there are a lot of them, they can really do serious damage to a crop.”
There are two ways to determine if you have squash vine borers lurking in your garden. One is observing the presence of the adult moth, which Dill said can easily be mistaken for a wasp or bee.
Dill describes the adult moth as striking. The wings are covered with scales that give them a metallic green or black sheen. The head and abdomen is orange or reddish. It has black dots on its back.
“Unlike most moths, the vine borer is active during the day not at night,” Dill said. “So you will see them flitting around your garden during the day and their wings beat so fast, they actually make a buzz that sounds like a wasp.”
The moths are drawn to the color yellow, so Dill said placing a yellow pan of water mixed with a bit of dish soap will attract and trap them.
The second sign that you have an infestation is the presence of frass — insect droppings — around the base of the plant. It will look like wet, greenish-orange sawdust. Closer inspection of the stem may reveal tiny holes made by the larvae through which it pushes the frass out.
If you catch an infestation early enough, you can perform a sort of squash vine surgery to try to save the plants.
“On any stem or vine you see a hole and frass at the base you can carefully take a knife, split the stem and remove the larvae,” Dill said. “Then cover that portion of the stem over with soil and water it well and it may survive the procedure.”
Organic or chemical pesticides may also be used eliminate the adult moths, Dill said
Other than that, the best you can do is prepare for the following season.
“Once the larvae are fully grown they will leave the vine, drop onto the ground and burrow into the soil to pupate,” Dill said. “So I recommend that if you had them this year, be on guard for next year.”
Since the larvae over-winter beneath the soil, Dill said a concentrated tillage effort in the late fall can turn the dirt over enough to bring the caterpillars topside where they become prey for birds and grub-eating mammals.
For next year’s crop of squash, Dill recommends using a commercial vine cover over the rows to prevent the female moth from getting to the vines to lay her eggs. The females are active in June and July, so keep the covers on for those two months.
It’s also a good idea to remove all squash vines from your garden that exhibit any serious wilting or die-off due to the vine borer. This will prevent any larvae that are still in the stems from dropping into the soil in the garden.
While the vine borers main target are squash, they will lay their eggs in cucumber or melon vines if they are the only vines available, Dill said.
“I have no idea why, but they don’t care much for butternut squash,” Dill said. “So if you really want squash, you might want to plant a lot of that.”