By Rick Borchelt
July and August bring farmers market tables laden with squash — zucchini, patty pan, crookneck. Bushels and bushels. Dinner tables groan with them too: Think succotash. Squash fritattas. Gazpacho. Well-meaning neighbors leave bags of them on your front porch. By the end of summer, you wish the squash would all disappear.
Well, it would, were it not for one industrious little bee that paid attention as the indigenous peoples of Mexico domesticated squash thousands of years ago. This bee followed the fortunes of squash, gourd and pumpkin culture from the Southwest U.S. through most of the rest of North America wherever the days are long enough to support growing squash.
This bee is not the honey bee, our domesticated European bee that most people think of as our one and only crop pollinator. No, this bee lives for one thing and one thing only: to pollinate squash and squash relatives in the genus Cucurbita. Naturally, it’s called the squash bee.
There are actually about 20 different kinds of squash bees, but the one that has followed the fortunes of squash culture through the American landscape is the pruinose squash bee, (Peponapis pruinosa). The pepo in the genus name references Cucurbita pepo, a wide-ranging plant species that includes cultivated crops such as pumpkins, winter squash and zucchini. Together with plants like watermelon and ornamental gourds, botanists call this group of plants cucurbits.
Squash bees want only one thing from squash — and it isn’t zucchini bread. They want the pollen inside the handsome golden flowers, which they collect and pack into their underground nests to feed their young. The big, yellow pollen grains stick to long hairs on the squash bee’s hind legs. In the process of collecting pollen, the bees also fertilize the squash plants that then produce the bumper crops we see in farm stands.
Honey bees are terrible squash pollinators. All they want is nectar, and they bypass the pollen on their way to get to it. Plus, honey bees are slackers compared to squash bees. Because squash blossoms open at the crack of dawn and wither after the sun starts to bear down, squash bees have evolved special ocelli — little eyes — in addition to their large compound eyes. These ocelli help them fly in the dark, before dawn, to find squash. Squash bees do all their work in the first hour or two of daylight before honey bees even get going.
If you’re up at that ungodly early hour, you will almost certainly see squash bees among your squash flowers. If you keep honey bee hours, on the other hand, and don’t get out to the garden until the squash blossoms are toast, you can still often find male squash bees napping in the spent flowers. (They often hang out there so they don’t have too far to fly the next morning to find females collecting squash pollen.) They look superficially like honey bees — with the same general size and behavior — but they have a striped abdomen that looks more like prison garb than the furry golden butts of honey bees.
Also unlike honey bees, squash bees are solitary. Each female is fertile, not just the queen, and each digs her own tunnel in the ground to provision with pollen. She lays her eggs in the stored pollen, covers the tunnel up, and never sees her progeny. (The young bees won’t mature and emerge until squash season the following year.) You may even see female squash bees digging holes in the garden around your squash plants; they need loose, open soil for their nests, and garden dirt is just the ticket.
Margarita Lopez-Uribe and her lab at Penn State have been studying how squash bees evolved and moved across the North American landscape as Indigenous peoples spread squash culture. Lopez-Uribe is also interested in how these bees may be adversely impacted by modern agricultural methods. She and her team found that if the farmer — or the agricultural conglomerate — plows up a field after squash or pumpkin harvest, nests get plowed under. Similarly, if the farmer covers the ground around plants with plastic sheeting or landscape cloth, or even heavy mulch to control weeds, the bees likely can’t nest there.
Squash agriculture has literally shaped the evolution of this bee, notes Lopez-Uribe. Squash bees probably originated 50,000 to 100,000 years ago, before humans domesticated squash. Peponapis pruinosa likely harvested pollen from wild cucurbits like wild buffalo gourd before squash culture evolved. But once squashes and their relatives were widely cultivated, squash bees didn’t look back. Squash was cultivated in the eastern U.S. as long as 7,000 years ago, and genetic data from bees analyzed by Lopez-Uribe and her team suggest squash bee populations boomed as Native Americans ramped up agriculture as a primary way of life about 1,000 years ago.
This mutualistic relationship between cultivated squashes and squash bees has been a good one for both the bees and the squash — so far. Our increasing use of agricultural pesticides, which has had a serious downside for more generalist pollinators like bumblebees, has spared squash bees (for the most part) because of their very strict squash diet. These pesticides may, however, become a problem for squash bees if we more aggressively use them to control pests like squash bug and squash borer.
If you want to keep those bushels of zucchini and wagon loads of pumpkins coming, you need to take some care for squash bees. Don’t till your garden. Leave open patches of soil in your yard for squash bee (and other native bee) nests. And don’t use pesticides on your cucurbits — or in your garden at all.
You can hear an extended interview with Lopez-Uribe about squash bees and their relationship to squash culture on the radio program “Living On Earth” here: tinyurl.com/yc5hjr9h.
Have questions for Rick about the world of nature in and around the city, or suggestions for future ”College Park Wild” columns? Drop him a note at firstname.lastname@example.org.