Reading between the grains

What pollen can tell us about bees and the plants that love them

This piece explores how scientists have been studying pollen to see how bee behavior may be changing in a period of over 100 years.

By Ryder Diaz

With a quick slip of the wrist, I swooped up a shiny green bee from atop a flower into my insect net. It was a warm, clear day in the foothills of Northern California and the meadow was quite literally buzzing with activity. Bees zoomed from one flower to another, sounding like a chorus of tiny motorboats, barely taking note of my presence. I carefully transferred the bee to a small clear vial. On a closer look the bee was not only green, portions of her body were dusted with a fine golden yellow powder—pollen. This defines a day’s work for many a bee, collecting pollen and nectar from plants in order to feed her developing offspring.  I took a small sample of the bee’s glistening golden pollen and released her back into a sea of yellow, white, and red flowers.

Pollen grains from an array of plants, magnified 500 times their normal size. Source: http://www.wikipedia.org

A few days later, I found myself looking through the lenses of a high-powered microscope at a slide containing the pollen I collected from that fastidious bee.  As I brought the blurry image into focus, an array of pollen shapes began to emerge. Tiny blue spheres and yellow triangles floated before my eyes. This sight was not unusual, as pollen grains from different types of plants can be fairly distinctive in their sizes and shapes, ranging from subtle to bizarre. Some pollen grains appear similar to coffee beans, while others look like tiny pufferfish, covered in spikes. Still some grains are boomerangs frozen in flight. As I peered through the microscope, it dawned on me that in these unique features laid a secret code of pollen. We could learn the types of plants a bee has been fraternizing with, simply by matching up pollen with plant. If we can crack this code then we might gain more insight into the fascinating affairs that bees have with flowers.

 The birds and the bees

The obligatory human adolescence sex talk is often dubbed the talk about the birds and the bees, in which these animals get very friendly with flowers. Conversely, when scientists talk about “plant sex”, the analogies are often framed in the context of animals. Scientists fondly liken pollen to sperm which, as in humans, is one-half of the mixture that is necessary to create new life. Similar to sperm cells, which are highly abundant in humans, incredible amounts of microscopic pollen grains are produced by flowers in special organs called anthers. In order to spawn baby plants, a pollen grain must find its complementary partner, a cell in the ovary called an ovule.  This term ovary is also reminiscent of human anatomy. Although the early botanists may not have been terribly creative when describing plant reproduction, the process inherently makes sense to us. The pollen grain and the ovule are the necessary components for new life, just like a sperm cell and an egg cell are vital for a spanking new human being.

So, how does the pollen grain meet its fated partner, the ovule? Well, this is where the birds and the bees from your adolescence play a role. In many cases, it is necessary for an animal, such as an insect, to pick up the pollen grain and move it to a place where it can access the ovary. When the ovule is fertilized by pollen, a plant can then produce seed or fruit. This is the process of pollination and the insect transporting the pollen cargo, like a freight train on the floral railway, is the pollinator. Insect pollination is how we get many of the foods we eat. Peppers, tomatoes, almonds, squash, and strawberries are just a sliver of the bounty of foods that result from this magic process. Insect pollination is also a positive interaction for many plants, without which these species would not survive.

 Dusting off Charles Robertson

Charles Robertson was fascinated with documenting insects that visit flowers but was less concerned about pollen’s intricacies. Robertson worked on and off as a professor at Blackburn College in Carlinville, Illinois during the late 1800s. Between 1884 and 1899 he could most likely be found traveling, slowly, by horse and buggy over unimproved roads into the forested areas surrounding the small city. It was in these forests surrounding Carlinville that Robertson meticulously documented individual insects landing on individual flowers, for years. This may sound a bit inane but this type of work had never been done before. Robertson was creating a catalog of the interactions between specific plant species and insect species, a kind of Facebook of the 1800s, where the connections between the actors in the environmental network were drawn. In the end, Robertson published Flowers and Insects, a book listing a whopping 15,000 interactions between insects and plants. Many of the insects that Robertson collected were species that were completely new to science and more than a handful ended up being named after him.

Charles Robertson in the late 1800s.

Although Robertson contributed enormously to science, in the end his book merely suggested that a plant species and an insect species “interacted.” This is a pretty broad, amorphous label for scientists. For all they know, these interactions could be good, bad, or neutral, from the plant’s perspective. “Good” would be insect pollination, the “plant sex” mentioned before, where insects transport pollen.

However, for some observations in Robertson’s impressive list of interactions, it is possible that the “good” act of insect pollination was not occurring. An insect that Robertson noted on a flower may have merely been making a rest stop to bask in the warm summer sun or was even harming the plant. No pollen was dropped off or picked up and the plant would need to wait for a different animal to help with its reproduction.

In fact, plants have evolved some sneaky ways to entice insects into helping with this feat of pollen transport. Some flowers offer sugary nectar rewards to insects, attracting them to the flowers in hopes that some pollen will stick to them and make the journey to another flower. Other plants produce large amounts of pollen that insects use to feed their young and the plants bank on some pollen grains inadvertently falling onto other flowers as insects are busily stock-piling this high-protein resource.  Still, other flowers try to look like a female insect in order to trick a male insect to “mate” with it. “I tried to love you but all I got was this lousy pollen,” the male bemoaned. In this respect, orchid flowers are famously known for their sordid ways.

How then, I asked Dr. Laura Burkle, Assistant Professor of Ecology at Montana State University, do we know if Robertson’s insects were just hanging out or were actually helping to produce the next generation of plants? “That’s where the pollen comes in and can tell us a little bit more than what the interaction can’t tell us,” she replied. If I found a bee covered with lots of that pufferfish-looking pollen, then like a fingerprint, we could match up that pollen with the plant it came from. Burkle continued, “What we try to do is collect pure samples of pollen from known [plant] species so that we have a reference collection. We can compare what we see on the pollinator’s body to what we see in the reference collection and say this looks more like this or more like that.”

Burkle conceded that pollen identification is sometimes not as easy as it sounds: “Plants that are more related to one another tend to have pollen that looks more similar to each other.” This would be like, if your facial features were more similar to your parents than they were to your favorite Hollywood actor. We can distinguish roses from clovers but amongst species of clover the differences in pollen could be more understated and difficult to distinguish.

Over one hundred years since Robertson’s meticulous collections, Burkle and a group of researchers from Washington University in St. Louis attempted to retrace his steps. “We can’t go back in time and ask Robertson what the heck he was thinking or what he saw,” rued Burkle.  But with the help of some microscopic clues, they were trying to understand the relationships in Robertson’s Carlinville and figure out if anything changed.

I’m forever yours, maybe

Several treasured cabinets at the Illinois Natural History Survey are occupied by Robertson’s collection of preserved insects from his travels around Carlinville. Although mostly unobservable to the naked eye, a dusting of pollen covers these insects. “It’s an added layer of data that can tell us a little bit more and give us a sharper picture of what [was] actually happening,” explained Burkle.

Flowers and Insects documented 441 flowering plant species and 663 insect species, most now residing in the Illinois Natural History Survey collection. With so many plants and insects collected, the research team had to find somewhere to begin. They decided to look first at bees species in the genus Andrena, often called mining bees, because they had a bunch of them in Robertson’s collection. They were interested in how much these bees relied on the flowering plant Eastern spring beauty or Claytonia virginica, if you speak latin binomials. From Robertson’s records, the researchers believed that spring beauty carpeted the forest in the late 1880s. This plant remains the most common plant in these same forests today. “It’s a generalist flower,” Burkle remarked, which means “it’s a really important floral resource for a lot of different bees.”

To collect pollen from each bee, the 100 year old insects were basically given a shower. The pollen gathered was examined under the microscope. To Burkle’s amazement, the pollen held up to the test of time. The features of the pollen grains looked the same as fresh pollen collected today.

100 year old bees were rinsed over slides to collect historic pollen samples.

“We looked at the proportion of pollen grains that were [spring beauty] as an estimate of bee fidelity,” remarked Burkle. Bee fidelity, this is exactly how it sounds.  The team was interested in how promiscuous a bee was with its flower visits. Were bees ever-faithful to only spring beauty plants or did they have more of an open-relationship with their floral partners, and visit flowers on a bunch of different plant species?

Why on earth would you care about bee faithfulness?” I prodded Dr. Burkle.

She quickly clarified, “A pollinator with lots of different kinds of pollen on its body might be a poor pollinator because if you get pollen from the wrong species of plant on the female parts of a flower it can sort of clog the process up with all that incorrect pollen.” So the most monogamous, faithful bee will always transport the “right” pollen and the plant will be able to produce lots of happy seeds, which will turn into more happy plants. But a promiscuous bee can transport the “wrong” pollen, which could exclude the “right” pollen from fertilizing the flower. “Wrong” pollen will gunk up the process and would lead to fewer happy seeds and fewer happy plants. In short, the faithfulness of bees could have a big impact on the survival of plant species.

Many bee washings later, the team knew how faithful the bees were to the spring beauty flower in Robertson’s time but wanted to know if bees still behaved the same. “The area now is much less forested and much more cornfield than it was historically, so we basically went to all the forest patches that were left and sampled them,” explained Burkle. In true Roberston fashion they collected insects visiting flowers but this time they focused on the pollen carried by those mining bees.

What they found was that, over 100 years later, mining bees were dramatically less faithful. In the 1800s, if they visited spring beauty, they just visited spring beauty. Now those same bee species not only visited spring beauty, but also took a larger sampling from the floral menu.

The explanation may lie in the loss of bees. “About half of the bee species that Robertson observed in his 1880 study, we no longer see,” lamented Burkle. To give some perspective on the scale of this, Roberston documented approximately 300 bee species in that area. Burkle and her colleagues have not figured out why each of these bee species disappeared, but it may very well have been related to the loss of forested areas affecting the most sensitive of bee species, those whose lifestyles could not be supported by the changes in the environment. As these bee species vanished over time, they were no longer found visiting the plant species to which they were once faithful. However, few things in nature go unused, and the bees that remained stepped in to use those flowers. As more and more bee species disappeared, the remaining bees accumulated a larger number of floral options.

Although the bees that remain may now have a wider variety of plants to visit, this may not last. “If bees are changing their behavior to be ‘more promiscuous,’ they are going to be less effective as pollinators and then the plants would suffer lower reproduction or lower seed set,” argued Burkle. With bees now visiting a wider array of plant species, the odds are greater that the “wrong” pollen will be deposited on a flower. This translates into lower plant reproduction. And lower reproduction may mean fewer insect pollinated plants in a forest that has already suffered a dramatic size reduction. All these plant species may not be able to suffer the storm and as the researchers have already seen, within several places in the forest, some species have been locally lost.

This troubled Burkle, “the species are still there, they are in the same site, they used to interact with each other and now they don’t. That to me is a red flag [that] this is unhealthy.” In this work she and her colleagues have documented a kind of abandonment of a long-time partnership between the mining bees and spring beauty. Therefore, a question lingers as to how the degradation of this close relationship will impact the future of spring beauty, a plant that is dependent on insect pollination for its survival. Burkle and her colleagues focused on illuminating the changes in the mining bee and spring beauty partnership; however if we take a closer look, will we find other insects becoming less faithful to plants which they were once loyal?

At the same time, this group of scientists discovered that amidst the loss and reshuffling, there was flexibility in bee and plant interactions with novel relationships formed; interactions between bees and plants that Robertson never saw. Scrutinizing pollen helped scientists detect these changes but still there is much unknown about how all the plants and insects in Carlinville’s forest fragments will sort themselves out. Time will tell which plants will survive the restacking of the deck of pollinators.