If you like squash, you can thank this one particular pollinator

Living on Earth

The honey bee gets all the headlines, but other species of bees pollinate our plants and help sustain food production. Take the squash bee, for instance.

There are 20 species of bees that specialize in squash pollination — and one of them, Peponapis pruinosa, is making headlines of its own lately.

Researchers have found that this squash bee expanded beyond its native range as humans spread the cultivation of indigenous squash throughout North America. Over a period of thousands of years, as ancestral farmers spread the cultivation of squashes through the Americas, the squash bee followed.

It’s the first time scientists have shown that cultivating a specific crop led to the expansion of a pollinator species.

North Carolina State biologist Margarita Lopez-Uribe, who co-authored the paper, says she and her colleagues theorized the bee had expanded its range as squash moved from Mexico into North America.

“If you look at the distribution of the bee today, they are mostly co-distributed with plants that were domesticated by humans,” she explains. “We looked for genetic markers to see if there were signatures at the genetic level that could corroborate this hypothesis, and that's what we found.”

Peponapis pruinosa specializes in pollinating just one plant genus: Cucurbita — the genus that produces squash, pumpkin and zucchini. Unlike the honey bee, who collects pollen in a kind of basket on its hind legs, the squash bee has long hairs on its hind legs, where the pollen gets stuck. Squash pollen has large grains that easily attach to these hairs.

The rapid expansion of the squash bee may have been good for humans planting the squash, but it may not be so great for the squash bee itself. The bees’ genetic markers had signatures of severe “bottlenecks,” Lopez-Uribe says.

A genetic bottleneck is an event that drastically reduces the size of a population, which also reduces a species’ genetic variability. Low genetic variability can make a species vulnerable to changes in the environment.

Squash bees have been in North America for thousands of generations, yet they still show very low genetic variability, Lopez-Uribe says. “I'm really curious to keep investigating this. I believe these bees are so tightly associated with crop management and agricultural systems because we're doing something with the crops the bees rely on that is keeping the genetic variability of these populations extremely, extremely low.”

Plowing and soil tillage could be the culprit. Squash bees nest underground, close to the plants they pollinate. Once a female bee has made a nest, it starts collecting pollen and nectar. It lays eggs, closes its nest and then dies off later in the summer. The following year, the eggs develop, emerge as adults and the cycle starts again.

“I think what’s probably driving some of the low genetic diversity in the populations is that agricultural systems of squashes and pumpkins include what we call crop rotation and soil tillage,” she says. “I think a large number of these bees die every year as a result of these agricultural practices.”

This article is based on an interview that aired on PRI’s Living on Earth with Steve Curwood.  

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