By Robin E. Owen
Everyone knows the large, brightly coloured, fuzzy bees that appear every spring and which can be found foraging on the early blooming flowers. These are queens of various species of bumble bees that have emerged from hibernation after surviving the long winter months. However, what most people do not know is how many species there are in any one particular area, or how they are recognized and distinguished. Even less thought is given to the entomologists who have described and classified these species. In this essay, I am going to tell you about one species of bumble bee which was first collected in Alberta one hundred years ago by an eminent entomologist, F.W.L. Sladen, and why this species is of interest to us now. This story will also illustrate, for one small example, how science actually progresses. First, I need to explain a bit about bumble bee biology. Bumble bees comprise a distinctive group of bees consisting of only about 300 species worldwide. The North American continent is home to over 50 species and more than 20 are found in Alberta alone. They are adapted to cool conditions and are confined mainly to the northern hemisphere. They are common at high elevations and at high latitudes; in fact, they extend all the way north to the arctic, and in Canada two species are found on Ellesmere Island. Scientifically, they are all classified in one genus, the genus Bombus. Their relatives, the honeybees, consisting of seven species, comprise the genus Apis. The domesticated honeybee, Apis mellifera, is what most people think of when they hear the word “bee.” Generally, most bumble bee species do not have common names, so I will be referring to them by their scientific names. Bumble bees have an annual life cycle. The large queen bees emerge in the spring after being produced by the colonies in the late summer and then hibernating through the winter months. All the other bees in each colony—that is the foundress queen, the workers and the males—die by the end of the summer, and it is only the young queens that will survive. The newly-produced queens mate with the males and store the sperm in a small sac called the spermatheca. Following hibernation, the queens are found searching for nest sites and feeding on pollen and nectar to develop their ovaries in preparation for egg laying. Many bumble bees nest underground in abandoned rodent burrows, but some species will nest above ground in balls of dry grass. They do require some type of nesting material to be present; thus, many are opportunistic and will nest in and around human habitation. For instance, the insulation under the eaves of a house is perfect nesting material. Once a suitable nest site has been found, the queen collects pollen which she fashions into a small lump on which she lays eggs, about six to ten initially. She continues to forage to feed the developing larvae, and she also builds a small honey pot out of wax which she fills with nectar in case of a rainy day. Once the first workers emerge after about three weeks, the queen remains “at home” and the workers (all females) take over the foraging and nest maintenance duties. The queen continues to lay eggs and the colony then grows rapidly. Workers are produced for the next month or so and then the colony switches to producing young queens and males. Because of the short life cycle, bumble bee colonies cannot grow very large; even the largest consist of only a few hundred workers, but a successful colony can produce a hundred or so queens and males. Bumble bees are essential pollinators of wild flowers and thus are key components of boreal ecosystems found in Canada. They pollinate many plants which are not pollinated, or are poorly pollinated, by honeybees. Honeybees are efficient and major pollinators of crops in North America and Europe; however, they are not good pollinators of most wild species of plants, many of which are dependent solely on native pollinators. Bumble bee species, with their varying tongue-lengths, ability to forage at lower temperatures, and capacity to buzz pollinate, are some of the most effective pollinators of wild plants and many crops. The species I am going to tell you about is called Bombus moderatus Cresson. As I have already mentioned, Bombus is the genus name (this is much like our surname) and moderatus is the species name (like our first name). This is the binomial system of nomenclature which was established by Linnaeus in 1758 and is universally used by biologists today. All scientific names of organisms are given in a Latinized form and should be written in italics. The name Cresson (referred to as the author) following the species name is the name of the biologist who first described and named the species. In this case, it is Ezra Townsend Cresson, an American entomologist (1838-1926), who described and named Bombus moderatus in 1863.1 B. moderatus occurs in the northern and western regions of the continent, being recorded from Alaska, the Northwest Territories, Nunavut, the Yukon, British Columbia and Alberta. In the science of taxonomy and classification (sometimes just called taxonomy or systematics), once a species has been described and named it is not the end of the story; indeed, it is often just the beginning! Taxonomic revision of species and whole groups of organisms is constantly being undertaken by biologists. Sometimes, affinities of species are unclear from the start, but a description and a name is a necessary first step; however, in some cases different taxonomists may simply disagree with opinion of the author. Often, more detailed study using new or different techniques yielding new information may result in a reassessment. In particular, the use of molecular methods over the last 40 to 50 years has revolutionized many areas of taxonomy and systematics. As we shall see, molecular information has been crucial for understanding the status of B. moderatus. Although Cresson described B. moderatus as a distinct species, some entomologists noted its similarity to certain European species of bumble bees. One of these was Frederick William Lambert Sladen (1876-1921). Sladen came to Canada in 1912 from England and was appointed Dominion Apiarist. Naturally, he was interested in the relationship between North American species of bumble bees and their European counterparts. Although he was based in Ottawa, he went on a number of collecting trips to western Canada and to the Arctic. In Alberta, Sladen collected B. moderatus in Jasper, at Maligne Lake, and in Banff. When I became interested in B. moderatus, I found some specimens2 collected by Sladen in Banff dating to around one hundred years ago. One of these specimens is shown in Figure 1.
Figure 1: The bee on the right is a specimen of B. moderatus collected by F.W.L Sladen in Banff, Alberta in 1911. The specimen on the left is B. lucorum collected by Prof. Adolf Scholl in Switzerland in 1984.
In 1908 Sladen collected one queen near Middle Spring on July 8, and another on July 15 on the grounds of the Banff Museum. In 1911 he collected a queen on June 12 at Tote Road, and in 1915 he collected another queen at Sulphur Mountain on August 24. These were the last recorded collections of B. moderatus until the 1980’s and Banff was the most southerly location recorded. Unfortunately, Sladen did not have many years of collecting left as he died prematurely in 1921.3 Sladen, in 1919, had written:
moderatusCr. a species of the terrestris group with colour pattern the same as that of lucorum, is found at Banff, Alta. and in Northern B.C., Yukon Territory and Alaska. It undoubtedly is a lucorum form. It differs from lucorum only in the somewhat paler tint of its yellow bands.4
Although Sladen considered moderatus to be a form of lucorum, he still regarded moderatus to be a distinct species, as his specimens are labelled “moderatus.” The latter was the accepted view until 1971, when Herbert Milliron5 published his revision of the bumble bees of the Western Hemisphere. In his treatise, Milliron did two things: one was to rearrange the subgeneric grouping within the genus Bombus; the second was to combine or synonymize separate species into one. It is this latter aspect which concerns us here. Milliron went one step further than Sladen and synonymized B. moderatus with B. lucorum (L.). He noted that “little significant structural variation is evident in this species.”6 Milliron’s interpretation was widely accepted and most books and papers on North American bumble bees appearing after 1971 use the name B. lucorum instead of B. moderatus. B. lucorum is extremely widespread, occurring throughout Europe and the Palaearctic into the Orient, and therefore, under this view, would also extend into North America. I moved to Calgary in 1984 and in 1985 began to collect bumble bees for my research. At that time, I was collecting queen bumble bees of all species as they emerged from hibernation in the spring. I would start collecting as soon as the first queens were seen (usually around the beginning of May) and continue until the end of June when the later-emerging species appeared. With the help of two field assistants, collecting was done virtually every day (weather permitting). This intensity of collecting ensured that all species in a particular location were found and gave a very good idea of the phenology of each species. My main collecting areas were in and around Calgary, and in the Kananaskis, where one of the best sites is at Barrier Lake on both sides of the dam. This spot warms up quite early in the spring, bringing out the bees and there is plentiful forage throughout the spring and summer months. In the early spring it is easy to find queen bumble bees because they will be foraging on any plant in bloom at that time, and usually there is not much! One of the earliest in bloom is the pussy willow7 and these are plentiful almost anywhere. About 21 species8 of bumble bees occur at Barrier Lake. Other collection sites in the Kananaskis included Fortress Mountain (on the ski slopes at the top), and Highwood Pass9. In 1985 and 1986, out of the several hundred queens collected, I found no B. moderatus at any of the locations. However, in 1987, one worker and three males were collected at Barrier Lake in July and August. Then, on June 1, 1988 one queen was collected. These specimens established Barrier Lake as the then southernmost limit of the species’ distribution that had been recorded.10 My interest in this species now was stimulated not only by its appearance at Barrier Lake, but also by the visit in 1988 of Professor Adolf Scholl from the University of Bern in Switzerland. He was visiting Canada and the U.S.A. to collect bumble bees for biochemical systematic studies. He was interested to know that I had found B. moderatus locally, and we decided to collaborate on a study to try to resolve the status of B. moderatus. We spent a few days collecting queens of this species and of others, and we found that although B. moderatus was rare at Barrier Lake, it was much more abundant further north, particularly at Ya-Ha Tinda Ranch11which lies just east of Banff National Park, where Sladen had collected B. moderatus from 1908 to 1915. At the time, both Prof. Scholl and I were using the technique of enzyme electrophoresis to analyse the genetic relationship between bumble bee species. Electrophoresis can be used to separate proteins on the basis of difference in net electrical charge12. In most organisms, including bumble bees, different species can have different forms of some enzymes present and so their “electrophoretic profiles” can be used to identify and distinguish species. For our study of B. moderatus Prof. Scholl and I examined 26 enzymes and we ran samples of B. moderatus and B. lucorum (plus other related bees) independently in our laboratories in Bern and Calgary respectively. We found exactly the same, exciting results: B. moderatus and B. lucorum were clearly distinct genetically! There were fixed differences at three out of the 26 enzymes (see Fig. 2), giving a genetic identity value of 0.88 (or difference of 0.12), which is comparable to that found between other pairs of bumble bee species. Therefore we concluded in our paper13 that there was no reason to lump them together and that B. moderatus deserved specific status. This is where the story was left in 1990, but things often move slowly in science and our view was, and still is, not accepted by everyone in the field. Some publications still continued and continue to
Figure 2: An electrophoretic gel showing the movement of differently charged forms of the enzyme phophoglucomutase (PGM) in various species of bumble bees, in particular Bombus lucorum (l) and Bombus moderatus (m). Each purple dot is the electrophoretic form of the PGM enzyme for an individual bee. Ten bees of various species were run on this gel and the unlabelled dots are species other than B. lucorum and B. moderatus.
use the name B. lucorum, although in others the name B. moderatus is adopted. However some recent developments with this and another species have emphasized the importance of this question. The first is that some recent molecular evidence confirms the results of our electrophoretic studies. Direct analysis of the genetic material, deoxyribonucleic acid (DNA), now has largely replaced enzyme electrophoresis for many evolutionary studies. As part of a broader study, Dr. Andreas Bertsch and colleagues of the Philipps-University Marburg in Germany sequenced the DNA of a portion of the cytochrome oxidase14 subunit I gene, revealing differences too great for B. moderatus and B. lucorum to be regarded as conspecific.15 The second development is that B. moderatus has, over the last 20 years, dramatically expanded its distribution. I collected bees extensively last summer (2010) and found that B. moderatus was still common at its well-known locations, for example on the road to Ya-Ha Tinda ranch (see Fig. 3), and it had become very abundant also at Barrier Lake where, 20 years ago, it had only started to appear. However, the most striking observation was that the species has spread east from Barrier Lake to Calgary, a distance of about 76 km in about 20 years, corresponding to a rate of about 4 km/yr. In Calgary, where it had never previously existed, it is now not only present, but also very common, both north and south of the Bow River.
Figure 3: A Bombus moderatus worker foraging on Red Clover found on the side of Forestry Trunk Road 940, north of Waiparous, Alberta, August 10, 2010.
The third development is that another species, B. occidentalis Kirby, once common in Alberta and across Western North America, has drastically declined in numbers over the last 10 years or so. This is one of three bumble bee species (the others being B. affinis andB. terricola) which have undergone drastic reductions in their ranges and abundances over this time period16. B. occidentalis, also known as the Western Bumble Bee (it is one species that has been given a common name), was, 20 years ago, abundant at Barrier Lake and in Calgary. It is now just barely present at these locations, where now B. moderatus is common. The decline in these species is very worrying as it has happened very rapidly and the exact causes are unknown. In Europe and in parts of North America, bumble bees have declined in some areas as a result of pesticide use, loss of habitat through urbanization and agriculture, spread of parasitic diseases from commercial bumble bee cultures,17 and possibly climate change. However, the decline of species such as B. occidentalis in essentially natural areas is difficult to understand. Because bumble bees are essential pollinators of native plants (as well as many agricultural crops), a significant decline in numbers would have a severe effect on the boreal forest ecosystem. What is the connection between the expansion of B. moderatus and the decline in B. occidentalis? There are three possibilities. One is that B. moderatus has outcompeted B. occidentalis, causing the latter’s decline. This is unlikely given that B. occidentalis is declining in areas where B. moderatus is not present. Another possibility is that B. moderatus has expanded its range to “fill the vacuum” where B. occidentalis was previously found. Finally, it may simply be coincidence. Assuming B. moderatus (or its progenitor species) dispersed via the Bering Strait land bridge 10,000 years ago, it has therefore spread a distance of about 3000 km to reach Calgary. Thus what we are observing now is just a continuation of the natural expansion of its range following migration from Eurasia after the end of the last ice age. Certainly, the current rate of spread of about 4 km/yr is much less than the long term average of 0.3 km/yr, but over such a long time period it is very unlikely that dispersal rates would be constant. We really do not know exactly what is happening and only further observation and study will tell. It is now about one hundred years since Sladen became interested in this bumble bee, and now we have some answers, but also more questions. This is typical of science. Also, given the changes that are occurring to wild species and their environment, it is essential that we can correctly identify species so that they can be studied and protected.
- Cresson, E.T., List of the North American species of Bombus and Apathus. Proceedings of the Entomological Society of Philadelphia 2 (1863): 86-116.
- These specimens are lodged in the insect collection of the Department of Biological Sciences at the University of Calgary.
- Sladen died on September 10th 1921 near Duck Island in Lake Ontario. The events of his death are recorded in his obituary published in The Canadian Entomologist (53: 240) in 1922. Arthur Gibson writes:
On August last he left Ottawa for the above island which is about 20 miles distant from Kingston, Ont. He had been in the habit of bathing in shallow water close to the shore; unfortunately he could not swim. On the above date he went swimming as usual and it is assumed that his heart, which had troubled him for many years, failed to function, thus causing his death. The body, which was recovered about seventy feet from the shore, was partially floating, unswollen, and no water was found in the lungs.
- Sladen, F.W.L., Notes on the Canadian representatives of British species of bees. The Canadian Entomologist 51 (1919): 124-130.
- Milliron, H.E., A monograph of the Western hemisphere bumblebees (Hymenoptera, Apidae, Bombinae). I The genera Bombus and MegaBombus subgenus Bombias. Memoirs of the Entomological Society of Canada. 82 (1971): 1-80.
- Milliron (1971), p. 47.
- Pussy willow refers to various species of willow in the genus Salix; in North America one of the most common, widespread species is Salix discolour.
- B. nevadensis, B. appositus, B.californicus, B. balteatus, B. rufocinctus, B. moderatus, B. occidentalis, B. terricola, B. bifarius, B. centralis, B. huntii, B. ternarius, B. flavifrons, B. melanopygus, B. sylvicola, B. frigidus, B. mixtus, B. perplexus, B. sucklei, B. insularis, B. fernaldae.
- Collecting insects in a Provincial or a National Park requires valid permits from the appropriate government authority. Also, if you are importing live insects into the country you require a permit from Agriculture Canada, Plant Quarantine. You do not need a permit of any kind to bring in dead insects (either pinned or preserved in alcohol, etc.); however, I found that whenever I had a box of pinned insects the customs officers at the border would often be curious and be unsure if any documentation was required. So, in order to make things easier for them, and for me, I requested and received a letter from Agriculture Canada stating that a permit was not required for the importation of dead insects. Subsequently, when I produced this somewhat Orwellian non-document, this did the trick and there were no questions asked!
- Scholl, A., Obrecht, E. and Owen, R.E., The genetic relationship between Bombus moderatus Cresson and the Bombus lucorum Auct. species complex (Hymenoptera: Apidae). Canadian Journal of Zoology, 68 (1990): 2264-2268.
- Ya-Ha Tinda in the Stony-Assiniboine language means “Little Prairie in the Mountains.” The ranch was originally within the boundaries of Banff National Park, but in June 1911 the park boundary was changed, leaving the ranch outside the park. Today, the 3,945 hectare ranch is the only federally-operated working horse ranch in Canada, where horses are wintered and trained for patrolling Canada’s Western National Parks. It is in a spectacular location on a plateau overlooking the Red Deer River and well worth a visit.
- Enzymes are an important class of proteins which catalyse steps in biochemical pathways. If a mutation occurs in a gene coding for enzyme involved in a pigment pathway (e.g. Drosophila eye pigment), then a defective form of the enzyme will be produced. This will mean that one step in the pigment synthesis will not occur and a different (mutant) eye colour will result. In this case the difference will be visible at the gross phenotypic level. However, suppose that we could detect enzymes and different genetic forms of enzymes directly without having to rely only on those that produce large phenotypic effects. This is what the technique of gel electrophoresis allows us to do. Electrophoresis (the word comes from the Greek, and means “transport by electricity”) is based on the principle that charged molecules in solution will move toward the appropriate pole when an electrical charge difference is applied across the solution. Gel refers to the substrate or supporting medium for the solution. Electrophoresis can be used to separate fragments of DNA itself (in this case on the basis of size), or to separate proteins (in this instance mainly on the basis of charge difference). Because each enzyme acts on a specific reaction, we can identify and localise a particular enzyme on the gel by providing the substance that the enzyme works on (e.g. alcohol) and coupling the reaction to a dye that changes colour when the reaction has proceeded, giving a purple spot on the gel (see Fig. 2). In the laboratory, the whole technique involves extracting some tissue from the organism (in this case, thoracic or wing muscle tissue was taken from each bee analyzed), grinding it up in a buffer solution, applying the sample to the gel and letting it run for a few hours under the influence of an electric current. The result is that the differently charged forms of the enzymes are separated, as shown in Figure 2.
- Scholl, A., Obrecht, E. and Owen, R.E., (1990).
- A critical enzyme required for energy production found in the mitochondria in every cell.
- Bertsch, A., Hrabe de Angelis, M. and Przemeck, G.K.H.. A phylogenetic framework for the North American species of the subgenus Bombus sensu stricto (Bombus affinis, B. franklini, B. moderatus, B. occidentalis & B. terricola) based on mitochondrial DNA markers. Beitrage zur Entomologie 60 (2010): 229-242.
- Evans, E.,Thorpe, R., Jepson, S. and Black, S.H. Status review of three formerly common species of bumble bee in the subgenus Bombus. The Xerces Society, 2008.
- A few species of bumble bees (including B. occidentalis) are reared commercially and are used to pollinate tomatoes in greenhouses. Many of these cultures are infected with the protozoan parasite Nosema bombi, and the spread of this parasite into wild populations via escaped bees has been documented (see Otterstatter, M.C., and Thomson J.D., Does Pathogen Spillover from Commercially Reared Bumble Bees Threaten Wild Pollinators? PLoS ONE 3(7) (2008): e2771. doi:10.1371/journal.pone.0002771).