Professor of Biology Heidi Dobson and her students study the relationship between bees and the flowers they visit and pollinate, revealing more about the world of insects. Dobson describes her work in her own words:
She flies into the flower patch in front of me, lands on a flower, immediately extends her proboscis into one of the five nectaries at the base of the petals, and then rotates around the flower, probing into one nectary after the other, all the while scraping the anthers with her legs and packing pollen into the special long and abundant pollen-carrying hairs on her hind legs. Then she flies off, her body shining in the sunlight, to another nearby flower.
There are many, mostly small, black bees flying among the blossoms, every individual doing their own thing, in their own way. Bees never cease to amaze me, totally capturing my wonder and fascination. I have been hooked on them since I was an undergraduate, when I conducted an independent study of bees pollinating high-elevation heath plants in Yosemite National Park. I can observe them for hours, just sitting in front of plants and watching the different species move about, approaching certain flowers, inspecting them, and then either flying off or landing and foraging in a species-characteristic manner. Some bees rarely land, cruising rapidly among the flowers—these are males, following often-regular flight paths, searching for females. A patch of flowers has so much to reveal.
Bees stand out among other insects in their total dependence on flowers for their sustenance: nectar, consisting of a sugar-water solution, provides mainly a source of energy, whereas pollen provides all of the life-essential proteins, fats, minerals and vitamins, and hence is a key link between bees and flowers. There are approximately 20,000 species of bees worldwide. Around 4,000 species are native to the U.S. and 600 species to Washington state. These small insects are essential to the pollination (and seed production) of the majority of our wild plants. Bees are also required for the production of many agricultural crops, such as consumable fruits—including many that we commonly categorize as vegetables, such as tomatoes, squash— and oilseeds, such as canola, as well as for the production of seeds that are planted to grow crops.
While all insects that visit flowers are often called pollinators, in fact only some are effective in pollinating: namely, in transporting pollen (which contains the plant’s sperm) from one flower to the stigma (female receptive surface) of another flower of the same plant species. Whether a particular bee actually pollinates a flower depends both on the bee species (its size and behavior on the flower, which determine whether it effectively picks up pollen and deposits it on the next flower’s stigma) and on the plant species (the flower’s shape, size, position of sexual parts). Uncovering these complex relationships requires patient, close-up observations and a keen eye.
When one mentions bees, most people think of honey bees, but in fact the honey bee that is used in agriculture and honey production in the United States is only a single species. Furthermore, it is not even native to the Americas (it was introduced from Eurasia). Our native plants depend for pollination on wild native bees, most of which are solitary. The honey bee is a highly social insect, living year-round in hives that consist of a queen and large numbers of workers. Among our native bees, only bumble bees are truly social; they differ from honey bees in that their colonies are annual (all bees die at the end of each species’ flight season except for newly mated queens, which overwinter under litter and start new colonies in the spring or early summer the following year).
In contrast to these social bees, most bee species worldwide, including 95 percent of Washington state’s native species, have a solitary lifestyle, where each female makes her own nest. Typically, each species has a particular six- to eight-week long flight season; at the start, adults emerge from their nests, with males emerging several days before the females. After mating, each female builds a nest, either in the ground or above-ground in pre-made cavities, where she makes individual nest “cells” in which she deposits sufficient food (pollen mixed with nectar) to feed one bee larva during its entire development, lays an egg on top of these mass provisions, closes the cell, and has no further contact with her offspring. The egg then hatches, the larva consumes all of the food, and the immature bee goes dormant until the following year, when it emerges from the nest as an adult. Solitary bees are immensely diverse; they come in various sizes (many are less than 1 centimeter in body length), colors (black, black with white/yellow markings, iridescent blue to green) and hairiness. In all bees, only the females can sting, and many solitary bees do not sting at all, making them easy to work with and handle.
“Getting a feeling” for the organisms one studies, namely how, where and when they live, is perhaps the most important step in carrying out biological research. I encouraged my current summer research student Lindsey Brodeck ’18, who came to me eager to study bees for her thesis, to start by simply observing the flower-visiting bees at the Whitman Campus Water-Wise Garden (located on the corner of Isaacs and Penrose, and designed several years ago by two students, Nicole Goehring ’09 and Sarah McVicar ’09), which is planted with native, low-water species and managed minimally so as to create natural habitat for insects. Returning after her first day of observation, Lindsey excitedly recounted her amazement at the diversity of bee species she saw and how each visited not only different plants but also moved from one plant to another as the day progressed. This powerful observational experience inspired her to conduct a baseline community study of the bee-flower associations at the garden by setting up a regular weekly schedule: spending some days observing bees and other days collecting them, for the month of June.
Lindsey’s study is the start of a multi-year project that will offer opportunities for future research students to conduct bee surveys covering different periods within the spring to fall seasons, right here on the Whitman campus. Insect and bee faunal compositions can vary widely on a year-to-year basis, making it necessary to conduct surveys over a course of years to determine which species are established local inhabitants, either nesting within close proximity of the garden or flying in from nest sites up to around 1000 meters away.
Insect surveys are never a simple affair. The diversity of insects and our limited knowledge about them is baffling, and this rarely becomes evident until one tries to make an insect collection. Establishing the identity of bees as they visit flowers can be very difficult, or often impossible, but identifying them once they are collected is also a challenging endeavor! One can key bees to family and genus with practice (and strong familiarity with bee morphology), but knowing the species depends both on the availability of species keys for a particular genus in our geographical region and on their ease of use. Most often, one must send collected voucher specimens to entomologists that specialize in particular bee groups. The three main factors that determine which bee species one might encounter in Walla Walla include a bee’s species-specific geographic distribution, the availability of appropriate nesting substrates (ground-nesting bees need undisturbed and often bare ground with a certain soil texture, whereas above ground cavity-nesting bees require particular materials with pre-formed holes, such as wood, or protected spaces), and the availability of food—flowers that meet the needs of each bee species across its six- to eight-week long flight season.
My passion for bee-flower interactions permeated my graduate studies, which encompassed the fields of both entomology and botany, and continues today to provide the foundation and guiding theme of my research, which I truly love and never tire of pursuing. Understanding how bees select which flowers to visit and identifying the possible roles that pollen plays at the interface of bees and flowers have been the long-term focuses of my research. It requires studying, on the one hand, the flowers: both what food they offer that bees seek, and their “form” (shape, color, scent); and on the other hand, the bees: both what floral cues and sensory stimuli they use to select flowers and what food (nectar and/or pollen) they obtain from each. I feel fortunate to study two such beautiful-to-behold groups of organisms: flowers and bees! While my interests are broad and I guide student research examining diverse aspects of plant-insect ecology and plant reproductive ecology (including pollination, pollen biology), my deepest fervor lies in questions that arise from the perspective of the bee: how it uses the floral resources in its surroundings and what adaptations it has evolved in its particular associations with flowers.
One food that has been of particular interest to me is pollen, the bees’ only source of protein (and other essential nutrients). Some solitary bee species are pollen specialists (oligolectic), where the females collect pollen to provision their nests from only a few closely related plant species. My fascination in the myriad of questions concerning the biology of oligolectic bees was instilled by my professor Robbin Thorp during my graduate studies at the University of California, Davis, and has been a strong thread in my research. After I conducted a community study of the spring bees visiting chaparral shrubs in California for my master’s thesis in entomology, I returned to the University of California, Berkeley, for my Ph.D. in botany to study the chemistry of pollen and investigate if and how scents from pollen might influence bees, especially oligolectic species, in their flower visitation. The pollen grains of animal-pollinated plants are covered with a sticky, oily material, and I showed that it is here that species-specific mixtures of volatile compounds reside, giving the pollen of each species a distinctive scent that is usually different from the scents emanating from other parts of the flower, and that bees can recognize the pollen odors of their host flowers.
At the time, the chemical study of floral scents was still very young, and methodological techniques were just being developed to analyze chemicals emitted by organisms into the air. Determined to demonstrate that pollen actually releases odors by capturing and analyzing air-borne volatile chemicals, I headed to Sweden for a post-doctoral fellowship to collaborate with chemical ecologists working at the cutting edge of floral scents—and to continue my studies of oligolectic bees. I landed in an idyllic research station located on the rural island of Öland, in the Baltic Sea, where natural habitats and bees are plentiful. Initially associated with Uppsala University, the station is now independent under the name Station Linné, and this has been my main research site ever since. It is a beautiful place to take Whitman students for thesis research, and to teach field-intensive pollination courses.
Since 1994, I have welcomed a total of 60 Whitman students to join me on Öland during 17 summers to conduct collaborative research for their senior theses in biology. The projects have focused mainly on using behavioral experiments to investigate questions revolving around how bees select which flowers to visit. We work with wild populations of bees, whose flight seasons last around six weeks, and this lends itself well to students gathering data for an undergraduate thesis. However, it also means that the big-picture research goals require multiple years to complete, with each student contributing one section. Many of the projects are aimed at determining how oligolectic bees, both when newly-emerged from their nests and after they have experienced foraging on their flowers, use vision (such as flower color) and olfaction (such as flower scents, including pollen odors) to locate and recognize their host-flowers. We have worked on five different bee species, some in more depth than others.
Another research theme encompassing a large number of theses has focused on non-ornamental roses, which offer only pollen as a food reward (the flowers are nectarless), to determine what parts of the flower (petals, anthers) influence pollen-seeking bees (especially bumble bees) to visit only certain individual flowers on a bush. My research with students helps us understand what selective forces shape the evolution of the biology of both plants and bees, and one common point that emerges is the wide diversity and variability of bee-flower interactions and the need to be careful not to jump to generalized conclusions. All of the projects involve both field and lab work, emphasizing the complementarity of what are sometimes viewed as disparate approaches to answer fundamental, natural-history based questions in biology.
By watching bees going about their business on a patch of different plants one can discover so much about them, the flowers and life around us.
During the past six years, I have turned my attention to a new and exciting research direction that investigates pollen feeding by adult solitary bees: while we know that larvae eat pollen collected by their mothers, there has been virtually no documentation of whether adult bees also consume pollen. Feeding on floral nectar by adults is well studied, but somehow pollen feeding has been neglected, leaving a surprising gap in our understanding of bees—their biology and ecological interactions with plants. One advantage of this research is that it can be done on the home front, in the Walla Walla Valley, where we have a wonderful outdoor bee laboratory in and around the many fields cultivated for alfalfa seed production in Touchet and Lowden, Washington. It is right here that two different solitary bee species are managed commercially to pollinate alfalfa (honey bees are not effective pollinators); these are the only solitary bees managed on a large scale for crop pollination. With the local abundance of the native ground-nesting alkali bee and of the Eurasian cavity-nesting alfalfa leafcutting bee, my students and I have been documenting pollen feeding by collecting bees at different times of the day and season, and dissecting them to determine when they feed on pollen and how much they consume.
It is exhilarating to watch students hone their fine motor skills (even those who initially claim to have none) and become expert dissectors of these small insects, delicately removing the entire, intact digestive tract to record pollen presence. We have likewise extended these studies to solitary bees in Sweden in an attempt to establish a broad foundation of knowledge for bee species from different genera and families. Overall, it is clear from the research conducted to date with 13 students that female bees consume pollen throughout their adult life (they also need it for egg production); in contrast, males consume much less, and it seems mainly at the start of the season, when mating activity is highest. Some of our preliminary research suggests that pollen may be most important in providing males with vigor needed for mating success, rather than playing a role in sperm production.
This summer, Lindsey will be examining pollen feeding in a population of a small-sized, ground-nesting, sweat bee species that has a large aggregation of nests on the Whitman College campus, along the railroad tracks next to the Science Building parking lot. She mapped the nesting area and was dumbfounded when she counted more than 500 nests in her five sample plots—suggesting that the entire aggregation includes over 1,000 nests. To determine which pollen this flower-generalist bee is eating, she will identify pollen in the bee guts by comparing it to reference pollen she collected from flowers. In several oligolectic bee species that we have examined in this manner in Sweden, the females eat the same pollen that they collect for their larvae, which is perhaps not surprising. The findings from our pollen feeding studies have led to numerous new questions, as research always does, and, together with my other projects, will keep me and my students busy in the years ahead, both in Walla Walla and in Sweden.
The encouragement and support I have received over the years from Whitman College in my desire and effort to invite students to conduct bee research with me in Sweden, providing both an educational and culturally enriching experience, have transformed my 24 years of teaching at the college. The opportunity for students to live for five to six weeks during the summer at the Station Linné, where they interact with other students and scientists from diverse countries, buy groceries at local stores, cook on their own, partake in cultural festivities and in many cases also explore other parts of Scandinavia or Europe before or after their research, represents another brick in the path of the Whitman experience aimed at providing a liberal arts education and laying the foundation of the global citizen.
In addition, I was fortunate to obtain pilot funding from Whitman College to teach an intensive, short-term field course in Pollination Biology at the Station Linné in 2011; a group of eight biology majors participated in this first-time course, located in a pollination-diverse setting, where students live surrounded by the “field” and where the lab requires only stepping outside of the station’s buildings. This offered a unique chance for me to teach a course integrating field botany and entomology, which are difficult to offer within the seasonal time constraints of the academic year. Following up on this, I am excited to teach the course again for five weeks in summer 2017 with funding from the new Crossroads Courses. Thanks to programs like these, students have access to varied learning experiences; and pollination, insects, and the plant life on our planet, receive the attention they need.