NATURE NOTES:
The Squash Bee: A West Texas Native with an Outsized Impact

By Andrew Stuart

The hum of a hive, the burning pain of a sting – honeybees are hard to ignore. But don’t let the buzz of these European imports drown out the broader bee story.

The native bees of the Americas are solitary, ground-nesting. And West Texas is the “Amazon” of native bees. From the Southwest through Mexico, arid North America has among the greatest bee diversity of any place on Earth.

One of those bees stands apart. Peponapis pruinosa – the eastern cucurbit bee, or squash bee – was critical to the birth of agriculture in North America. Over thousands of years, this desert native made a remarkable journey.

“I love squash bees. This is kind of my baby, my research baby, if you want to call it so.”

Dr. Margarita López-Uribe is an evolutionary biologist at Penn State. With other entomologists, she’s traced the squash bee’s singular story.

Peponapis pruinosa is 11 to 14 millimeters long. To the untrained eye, López-Uribe said, it might be mistaken for a honeybee.

“It has kind of a yellowish-orange thorax,” López-Uribe said. “And the size is similar, and it has stripes on the abdomen. But in many ways it’s very different than a honeybee.”

One difference is behavior. The squash bee is solitary. The female creates her nest underground. She gathers pollen, placing it, with her eggs, in subterranean cells. She dies in six weeks. She’ll never meet her young, but when they hatch, they feed on what she’s left behind.

What sets the bee apart is its complete dependence on one type of plant: Cucurbita, the New World family of gourds.

“Even though there are others bees who are specialists, in terms of their food requirements, peponapis is one of the few species that can only feed on one genera of plant,” López-Uribe said. “If that plant is not available in the habitat, they have no way of eating or feeding the following generation.”

Among wild plants, the squash bee relies on Cucurbita foetidissima – the “buffalo gourd.” They’re a familiar sight along roadways and arroyos across West Texas. Anyone who’s picked its yellow flower, or handled its vines or gourds, knows about the “fetid” – the odor is intense, and disagreeable. The plant repels almost all pollinators but the squash bee.

Peponapis can meet its needs with the buffalo gourd. But in ancient Mesoamerica, the continent’s earliest farmers changed the bee’s fate.

As early as 10,000 years ago, farmers in present-day Mexico domesticated wild gourds and began cultivating squash – pumpkins, zucchinis, marrows.

That cultivation depended on squash bees.

Squash became one of the “Three Sisters” of Native agriculture – along with maize and beans. Squash farming spread, and the bee spread with it. Five thousand years ago, wild gourds expanded north with a drying climate. Farmers in present-day Missouri domesticated new types of squash – acorns, scallops, crooknecks.

When Europeans arrived, Native peoples were growing squash as far north as New England. And the bee was there – 2,000 miles from its arid homeland. No other specialist pollinator is known to have expanded in such a way.

U.S. agriculture today relies on honeybee pollination. But there have been staggering honeybee die-offs in recent years. The cause of this “colony collapse disorder” is unclear. But scientists have turned their attention to other pollinators – including native bees.

López-Uribe specializes in DNA analysis. She found squash bees likely reached the Northeast from the Midwest – perhaps as squash farming moved up river valleys.

She also found dramatic genetic “bottlenecks.” Squash-bee populations outside the native range lack genetic diversity – which makes them less resilient. López-Uribe traces that to farming practices – specifically, the annual tilling of soil.

“These guys are ground-nesting bees that are nesting right underneath the plants,” she said. “The fact that we are disturbing the soil every year probably kills a good portion of the population, so it doesn’t let the genetic diversity increase.”

The extent to which current squash farming relies on the bees is uncertain – but it could be significant. Some growers are now changing their practices, to support the squash bee.

Scientists don’t know what enabled the squash bee to adapt to locales as different as Quebec and Mexico. But one thing is certain – this West Texas native has had an outsized impact.