by Bill and Mary Weaver
Interested in getting pollination assistance from wild ground-dwelling squash bees? Greg Donaldson regularly grows 80 acres of pumpkins. Donaldson Farms is a 500-acre vegetable and fruit farm in northern New Jersey. He has discovered that by following specific farming practices he has been able to establish large populations of wild squash bees. Scattered pumpkin growers from coast to coast have been able to achieve similar results.
“Over the past 20 years,” explained Donaldson, “we’ve been using 90 percent no-till. Our pumpkin fields are strip tilled, so only a small portion of the soil is disturbed. We grow pumpkins and zucchini every year. I spray, but my timing is common sense.”
As Donaldson has discovered, by following these simple steps, he has set up the perfect conditions for getting significant help for his pumpkin pollination from wild squash bees. A beekeeper himself, he still keeps about 10 hives on the farm but considers them pumpkin pollination “insurance.”
“When I go to the pumpkin fields very early in the morning, the plants will be literally buzzing with the sound of wild squash bees at work. There can be several in one flower.”
Squash bees are solitary Native American bees, Peponapis pruinosa. Each female burrows down a foot or more into the ground to build her nest cells. Each tiny cell, the size of a thimble, receives enough pollen and nectar to feed one grub-like offspring. There a single larva feeds, overwinters and later transforms to adulthood to work the next year’s midsummer squash bloom.
“Do avoid deep tilling,” stated Dr. James Cane with the USDA Agricultural Research Service, who has spent 40 years researching pollination by solitary bees like the squash bee and is heading a population study which recruits volunteers called Squash Pollinators of America Survey (SPAS). “But also take care not to till pumpkin fields in the morning while female squash bees are out foraging, as returning females can’t find a nest entrance buried by cultivation. Instead, cultivate in the afternoon or control weeds chemically. If females are at home they can dig their way out the next morning, reorient themselves to their surroundings and continue raising their young.”
As long as your farm has undisturbed nesting ground, any insecticide sprays are done at dusk with shorter-lived chemistries (avoid systemics), and your pumpkins are grown within ¼ mile of the previous year’s cucurbit fields, your squash bee populations should multiply. Nesting is more likely if you irrigate, and if your soils don’t contain heavy clay.
“I have met pumpkin growers,” continued Cane, “who alternated weekly sprays of Malathion and Sevin applied at dusk to control their squash bugs and vine borers. Bee activity was long over for the day and the next morning, the interiors of the day’s new flowers are uncontaminated.
“Because squash bees have been associated with pumpkin and squash growing by Native Americans for thousands of years,” he explained, “squash bees are especially well adapted for pollinating Cucurbita species quickly and efficiently.”
Although melons and cucumbers are in the same plant family — Cucurbitaceae — as pumpkins, squash and gourds, they are members of a different genus — Cucumis — which originated in Asia. Squash bees do not visit Cucumis species plants, like melons and cucumbers. They are only found on and around Cucurbita species such as pumpkins, squash and gourds.
“Squash bees will be pollinating about an hour earlier than honey bees, often at or before sunrise,” Cane continued. They also fly at cooler temperatures than honey bees and even fly in light rain. Also, nesting right in the pumpkin field cuts their commute time.
Squash bees are fast fliers and their hairy bodies and back legs catch the large, sticky pollen grains, even when only collecting nectar. They effortlessly distribute these hitch-hiking pollen grains from flower to flower.
“Squash bees are native to the New World tropics and subtropics. In our southwest deserts, they primarily use flowers of a perennial native gourd. From there they spread northward into most of the United States and parts of southeastern Canada, as well as initially following the spread of squash cultivation by Native Americans into both the Great Plains and what is now the Eastern U.S.
European settlers first brought squash cultivation to the Intermountain West. There squash bees hopscotched between pioneer gardens from the 4 Corners region all the way north to Boise and Caldwell, Idaho. Where homestead gardens ended, so did Peponapis’ spread. Unable to spread further, the bees’ colonization stopped, leaving the Pacific Northwest without squash bees,” said Cane.
Dr. Cane, as head SPAS researcher, personally did several population surveys in eastern Washington and western Oregon searching for squash bees. None were found. Fortunately, he pointed out, honey bee hives in sufficient numbers can pollinate cucurbits there.
Some areas of the U.S. have smaller populations of squash bees because of farming practices. “In parts of the South,” explained Cane, “pumpkin growers, to escape from burgeoning populations of squash vine borers, squash bugs and cucumber beetles, will rent and plant a field for one year only. The next year they move to a new area to escape the plants’ maladies. There simply isn’t time for squash bees to colonize and multiply.” The same thing may be happening in some parts of New York State. Also, in the Deep South, pumpkins seeded late for the Halloween market can bloom too late for the squash bee life cycle and cannot be used to build squash bee populations.
One problem with some population studies could be that assistants may arrive at a pumpkin field at 9:30 in the morning on a hot day, see no squash bees and conclude there are none. But to see squash bees, you need to be in the field near dawn. They do their foraging quickly and efficiently. Thereafter, females retire to their underground nests and males chose a closed flower in which to snooze the day away.
How do you determine what pollination benefits you might gain from wild squash bees? “People who grow zucchini year-in-year-out could have as many as one squash bee per every three flowers,” explained Cane, “but if you stop growing squashes, emerging bees will leave in hopes of finding their food plants. Populations can take years to recover after recolonization.”
Cane advises a cautious approach, since honey bee hives are equally competent pollinators of cucurbits. “Continue to order your usual number of hives each year for pumpkins and keep a close watch on insects in your pumpkin flowers near dawn through the period of pumpkin bloom. Over time, you’ll get a sense of your local abundance of squash bees, and like Greg Donaldson, can make informed choices about how many hives to have around.”
In zucchini and crookneck summer squashes Cane calculates that, based on systematic bee surveys and controlled pollination experiments over 13 years, one male squash bee per every 10 flowers is enough to regularly deliver the seven visits a female flower needs to set a fully seeded summer squash. Comparable results have been published for pumpkins and butternut squash, but without recommended Peponapis bee densities.
Want to provide needed help via a 10-minute Squash Pollinators of America Survey? Contact Dr. James Cane at the ARS Pollinator Research Lab, Utah State University, Logan, Utah at