By Linda Zielinski
posted August 3, 2023
I wanted to find out what forces of pollination created the pile of zucchini on my kitchen counter, but may now have more questions than answers. As a person newly investigating our local native bees, I hoped to give you a concise look at the squash bee.
Well. There may be evidence that squash bees are local, since many of us have SO MUCH ZUCCHINI.
But let's look at this bee's history. 5,000 to 10,000 years ago, the folks in Oaxaca in southwestern Mexico farmed Cucurbita pepo as a domesticated vegetable, with Peponapis pruinosa being the pollinator (so far, the research bears this up. Although 5,000 years? 10,000 years? is a bit slapdash).
C. pepo (zucchini) cultivation and its pollinator P. pruinosa (the squash bee) slowly crawled up to northern Mexico, then up the intermountain west. Let's fast forward to Oregon, 2016, when a citizen scientist with the Oregon Bee Atlas took the first definitive photos of P. pruinosa! Now, we all know that many decades have passed since Washingtonians started eating zucchini in bread and muffins, or stuffed (ala The Moosewood Cookbook, with cheese and onions). So here's one of my questions: why can't I find a reference to squash bees inhabiting Washington? Or found one? I've been up at dawn looking for squash bee males sleeping in closed zucchini blossoms, like they do. (That's also where they have sex. Kind of romantic.) But no can find.
That made me ponder what other bees might mess around with my zucchini plants.... My very first research finding explained that few insects are attracted to, or can digest, squash pollen, as it contains very bitter compounds called cucurbitacins. Squash bees, on the other hand, can use ONLY squash pollen to create the pollen balls upon which they deposit each egg in their nest. No other pollen will do. And, I learned, there were no honey bees for the first mucho-thousands of years of cucurbits. Only in the last 400 years have we had honey bees, imported by colonists. Yep, I also learned that honey bees pollinate zucchini, as do some other generalist native bees, but only do so incidentally while they're foraging for squash blossom nectar, carrying some pollen from one squash blossom to the next.
Squash bees on stigma of female squash blossom. (Female blossoms have nectar; male blossoms contain pollen on a single anther, aka stamen).
So back to the squash bees.... If my garden has them, the female will create tunnel nests, about 15 inches deep, right under the leaves of my zucchini, or very nearby. She makes a small side tunnel in which to form a pollen ball (yep, pure squash pollen! The only kind her kiddos will eat! Fussy!!) and lays an egg on it, then seals that cell up. She might, in her short adult life, create up to 30 cells in a few tunnels. Each pollen ball can represent a full day's foraging. I've read that anywhere from 5 to 14 trips to one female squash blossom is needed to form a nice squash, fully pollinated.
Squash blossoms start to open before sunrise. The squash bees are early risers, unlike honey bees who rise late and have less time to spend poking around squash flowers, which close up after six hours or so. But for squash bees, pre-dawn until about noon is their foraging time.Their larger-than-average bee eyes help them see flowers in darker light.
I'll keep looking (yawn) before I've had my morning tea...who knows, I might get lucky and spot a squash bee! I've also started to look for pencil-width holes in the ground directly under the leaves of my C. pepo, or very nearby, indicating a P. pruinosa tunnel nest.
You were waiting for the "sex" part? First, here's a short video of the difference between a male and female zucchini flower:
And here's a very "graphic" explanation of the natural history of the squash bee:
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