The Rise and Fall of Squash Bees

The Rise and Fall of the Squash Bees
August 8, 2016

Pumpkin pie, ratatouille, butternut squash boats filled with butter and bacon—all three of these fall favorites are largely reliant on a single group of pollinators: the squash bees.

Two genera of bees, the Peponapis and Xenoglossa, pollinate pumpkins, gourds and squash throughout North America. One species, the Peponapis pruinosa ranges from Mexico to Quebec and California to Georgia, foraging squash flowers everywhere in between.

SquashPruinosa’s wide range is not an accident either. New research from NC State University shows that the expansion of these bees is closely tied to humans and agriculture.

NC State postdoctoral researcher Margarita Lopez-Uribe collected bees from all over pruinosa’s range and tested their genomes to see how pruinosa’s genetics changed over the course of its expansion. 

A group of any species living in one area is called a population. As a species expands to new areas, new populations form, and depending on what the species needs to survive in the new habitat, the population’s genetics will change slightly as they adapt to the new environment.

By studying these minor differences in the squash bees’ genetics, Lopez-Uribe could track not only how the species changed over the course of time, but also where and when those genetic changes occurred. 

Healthy genetic diversity is important to a species’ long-term survival and learning how a species’ spread affects that diversity can give researchers insights into how to protect pollinators.

Lopez-Uribe found that the squash bees’ journey began more than 5,000 years ago in Mexico. Around that time, pruinosa began to migrate north into what is now the Midwest, following the spread of Native American agriculture. 

Squash BeesIn Iroquois tradition, the squash plant was one of the "Three Sisters”—along with beans and corn—of agriculture. The Iroquois were native to what is now New York and when cultivation of squash spread there, the squash bees followed. In fact wherever Native Americans grew squash, pruinosa bees followed to the point where their habitat matches today’s range.

Lopez-Uribe also found that as the squash bees spread, their genetic diversity gradually decreased. A possible reason for this is that when a species expands, a few individuals can start a whole population. Those individuals, however, do not have the entire genetic diversity of the original population. The new population, therefore, will also not have the diversity of the original.

One problem that can happen with low genetic diversity in a population is that there is a higher rate of sterility in the male bees, but the researchers did not see this happening in the squash bees. Their populations, however, are still declining and Lopez-Uribe says there are other genetic “bottlenecks"—spots where the genetic diversity of a species is dangerously winnowed—popping up in their populations; bottlenecks that come from the same thing that helped the squash bees spread in the first place: human agriculture.

Pruinosa and the other squash bees are different from other bees. They don’t follow a queen or live in a large hive. Each female squash bee digs her own nest in the ground and each female can reproduce. The squash bees set up their nests near their food source, squash and pumpkins, but every time a farmer tills a squash field, countless bees’ nests are destroyed. 

Some bees survive, but every time a large number of them dies, the diversity of the whole population suffers. Lopez-Uribe thinks these types of farming practices could be driving some of the more recent genetic bottlenecks and population declines popping up in the squash bee population, and hopes to study that in the near future.

Pollinator populations are declining around the world, and while scientists are not exactly sure what is causing the decline in each and every population, many hypotheses feature human involvement. Studies like this one could help us learn the status of these pollinators and see if there are in fact practices that we could change to protect them.

This study was published in the journal, Proceedings of the Royal Society B.

—Daniel Lane

Daniel Lane covers science, medicine, engineering and the environment and North Carolina.