Peponapis pruinosa bees gather pollen from a squash flower (Photo: USDA Agricultural Research Service, Public Domain)
Thousands of species pollinate our plants and guarantee our food, but one particular bee specializes in the squash family. As ancestral farmers spread the cultivation of squashes through the Americas, the squash bee followed. North Carolina State biologist Margarita Lopez-Uribe explains the history to host Steve Curwood.
CURWOOD: It’s Living on Earth, and I’m Steve Curwood. The US Fish and Wildlife service has announced it will issue an endangerment ruling on North American Monarch butterflies within the next three years. The iconic butterfly is just one of many pollinators in trouble, thanks to human activities; the honeybee is another. But science has also found a pollinator that follows human activity. The squash bee moved beyond its native range in the Americas as people spread the cultivation of indigenous squashes. Margarita Lopez-Uribe studies evolutionary biology at North Carolina State University and co-authored the paper laying out this connection. She joins us from her lab. Margarita, welcome to Living on Earth.
LOPEZ-URIBE: Hi. Thank you.
CURWOOD: What are we really talking about when we say the squash bee? How different are these bees from the familiar honeybee?
LOPEZ-URIBE: Well, they are very different. There are about 20,000 species of bees in the world and the honeybee is only one of them. So when you're talking about the squash bees, actually there are about 20 species that specialize in squash pollination, and the one that I focus on is only one of those 20, and it's called Peponapis pruinosa.
CURWOOD: And what does it look like?
LOPEZ-URIBE: The bee is about the size of a honeybee, but it looks a little bit different to someone that has a trained eye. And one of the big differences morphologically is that the honeybee collects the pollen in a structure in the hind legs. It's called a cubicula It's basically a basket. So the honeybees visit the flowers they collect the pollen, they put a little bit of nectar in the pollen and then they make these wet balls of pollen that they store in those baskets in the hind legs. The squash bee does not have that basket. It has an incredibly long and very conspicuous hairs in the hind legs, and so the pollen gets actually stuck in those hairs, and one of the features of the squash pollen is it has very large grains of pollen, so it easily gets attached to those hairs in the hind legs.
CURWOOD: Now, tell me the crops that they pollinate for humans. When you say squash, what are we talking about here?
LOPEZ-URIBE: Well, they specialize in pollenization of one plant genus, the genus Cucurbita and that plant genus happens to actually be the genus of a lot of different crops. So we are talking about squash, pumpkin and zucchini. All of those crops are actually part of the genus, the plant genus Cucurbita and they're all pollinated by these squash bees.
CURWOOD: What's neat about your paper is that you figured out the bees spread their range thanks to the cultivation of squash. What prompted you to look at this?
LOPEZ-URIBE: Well, so if you look at the distribution of the bee today, for a big chunk of their distribution they are only co-distributed with plants that are domesticated by humans, and so we already predicted that the bee had expanded its range outside of the ancestral range of the plants that were not domesticated by humans. What I did was I look for genetic markers to actually see if there were signatures of the genetic level that could corroborate this hypothesis that we had, and that's what we found, that indeed this bee expanded dramatically thanks to the cultivation of these crops outside of the native range of the plants that are not domesticated.
CURWOOD: What surprised you most about your findings?
LOPEZ-URIBE: Well, there were a couple of things that were very interesting. One thing was the route of the movements of the bees. So, these bees are very very abundant in northeastern North America. One possible way they got there was actually kind of you know like along the east coast of North America, but actually what I found is that the bees moved through the midwest and then colonized the northeast of North America. So they kind of like took the longer route to get there. The other interesting finding was even though this was a rapid expansion, we did find some signatures of severe bottlenecks. What happens is that even though these bees have been in eastern North America for quite a while – we’re talking about thousands of generations - they still show very low genetic variability. This is interesting and it's something that I'm really curious to keep investigating because what I hypothesize is that the fact that these bees are so tightly associated with crop management and agricultural systems that means it probably there is something that we're doing with the crops of these bees are relying on that is keeping the genetic variability of these populations extremely extremely low. And that would make them really vulnerable to changes in the environment.
CURWOOD: You term these bees as being solitary, but, of course, how do they reproduce then?
LOPEZ-URIBE: Well, the lifecycle is very different. So what happens is that I told you these bees nest underground and they have a yearly lifecycle. Usually by midsummer, the females and males emerge from the ground, they mate, all females are fertile, and then the females who have mated, they start looking for areas where they can make their own nest. Once they find a good spot, they make the nest, they start collecting pollen and nectar, they lay eggs, they close those nests and they never see the babies. They die that summer, and then the next year those eggs of course go through the whole development and adults emerge and the cycle starts again.
CURWOOD: I imagine that if they build their nests in the ground, it's close to the plants. What happens when the plows come through?
LOPEZ-URIBE: Yeah, so that's one of the things that I'm worried about, and that I think it's probably driving some of these low genetic diversity populations is the fact that agricultural systems of squashes and pumpkins actually include what we call crop rotation and soil tillage, and so I think a large number of these individuals just dies every year as a result of these agricultural practices.
CURWOOD: So, if I understand this, honeybees can also pollinate squash, so what's the difference here?
LOPEZ-URIBE: Well, there are major differences. One of them is the time of the day that these bees forage. So Peponapis pruinosa is a very early morning bee. When I was doing fieldwork for the study, I would have to get up really really early because most of the foraging happens the first one or two hours of the day. Honeybees and other pollinators of these crops like bumblebees, they usually pollinate later in the day and for much longer. The other difference, and this is something we don't really know much about, is it seems to be the pollen of these crops has some chemical properties that make the pollen highly unattractive to most bees. Peponapis pruinosa pollinates squashes and pumpkins because the female bees are collecting the pollen. So in the movement between the flowers they are transferring pollen grains between flowers. The honeybee goes to the flowers only for nectar, and so it's a much less specialized behavior in terms of the foraging. We know a lot about the honeybee but very little to almost nothing about the other thousands of the species of bees in this planet.
CURWOOD: Margarita Lopez-Uribe is a postdoc researcher at North Carolina State University. Thanks for taking the time today.
LOPEZ-URIBE: Thank you so much. It was a pleasure.
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