Every summer morning, somewhere in North America, a little squash bee crawls into a squash flower and falls asleep. Around mid-day, the blossom closes, making a safe little golden chamber for the little bee. If you ever need reassurance that life can be beautiful and good, remember the squash bee, Peponapsis pruinosa.
The squash bee is a solitary bee that is a specialist pollinator: for their food and their offspring’s food they only use pollen and nectar from squash plants: pumpkins, summer squash, winter squash, butternut squash, zucchini, really most plants in the genus Cucurbita (with the exception of melons and cucumbers -for some reason, that’s where they draw the line).
Their specialization has resulted in a few fitting adaptations. Since squash flowers release their pollen at 48F, the pollen often becomes available before dawn. Because of this, squash bees evolved larger eyes to see in the dark and the ability to fly at cooler temperatures. Squash bees actually look a lot like honey bees, and one of the main ways to identify them is simply that they are flying so early in the morning. So if you’re out at 6am, and you see what looks to be a chubby honey bee flying around your squash plants, there’s a good chance you have found a squash bee.
Female squash bees also have long hairs on their rear legs that make it easier to grab onto large, spiky squash pollen grains for transport. Males didn’t evolve these hairs because they don’t collect any pollen. Now why are the females the only ones to collect the pollen?! Because being a female solitary bee is no joke.
All by herself, she collects pollen and nectar, digs her tunnels and nesting chambers, lays the eggs, and provisions them with a nectar and pollen loaf for the larva to eat. The males spend most of their time hanging out in flowers, waiting for females, sometimes mating. I’m not kidding! Now is this unequal? Clearly, yes. Is it unfair? Hmm… I don’t know. Seems like a complex philosophical question to me! I’ll spare you my thoughts here, but maybe add them below the Actual Information someday.
Squash bees are ground nesting solitary bees, meaning that the females mate with males, and then dig tunnels and chambers underground usually 6 to 18 inches below the surface. They start creating brood cells, and in each cell they lay a fertilized egg. For each egg they have gathered squash pollen and nectar, and made a little pollen loaf, for the larvae to eat when they hatch. This is all the food the young will need as they mature over the next year into adult bees.
Important Conservation Note for Squash Gardeners:
Squash bees typically dig their nest right under the squash plants that they love, and so the little growing squash bees are in your soil, under your squash plants. Say What! So it is important to not till a current year’s squash patch until next year’s squash bees have emerged. In the Midwest or New England the bees should definitely have emerged by the following August. So in terms of garden planning, it would be good to follow a squash bed one year with a no till vegetable the next year, like kale, rather than carrots or potatoes, if you see what I’m saying.
There are about 20 species of squash and gourd bees, almost entirely centered in Mexico. The only squash bee to come further north was Peponapsis pruinosa. This bee originally pollinated a wild squash Cucurbita foetidissima, also known as the buffalo gourd, stinking gourd, or my favorite, the Wild Pumpkin. But the squash bee was also happy to follow another squash, Curcubita pepo, as it was cultivated and traveled with native American peoples, in two distinct domestication events. 10,000 years ago, pumpkins and zucchini were cultivated in Mexico and began to move northwest, and 5,000 years ago in Missouri (I’m from Missouri, too!), another squash domestication event developed acorn, crookneck, and scallop squashes that began to move east. Ultimately the cultivation and movement of squash has led to P. pruinosa’s range extending far beyond the geographical range of its original host plant, from Mexico to Canada, and from coast to coast.
If you are a vegetable gardener, you know squash takes a little space, but other than that, it is pretty simple to grow. And if you’ve already got squash in the garden, I would recommend setting your alarm for early morning to try to catch these beautiful squash bees in action. Wouldn’t that be something. But beyond the vegetable garden, what an amazing native plant! This year I will be planting some squash in my flower beds to see how it might perform as a ground cover. I’m thinking almost all the flowers in my border will rise above the squashy fronds.
But here is the real, native plant deal: from what I’ve seen, the stinking gourd, or wild pumpkin, is beautiful, drought tolerant, and perennial to zone 4! What?! So I will be trying this interesting plant in among my flowers, and see how it does. I am looking forward to seeing how much it stinks.
Squash and Gourd Bees (Peponapis, Xenoglossa) and the Origin of the Cultivated Cucurbita. Hurd, P., Linsley, E., & Whitaker, T. (1971). Evolution,25(1), 218-234.
Squash Bees, U.S. Forest Service
Squash Bees in the Home Garden, NC State Extension
Crop Domestication Facilitated Rapid Geographical Expansion of a Specialist Pollinator, the Squash Bee Peponapis pruinosa. Lopez-Uribe MM, Cane JH, Minckley RL, Danforth BN. 2016 Proc. R. Soc. B283:20160443.