Bombus Biology


Natural history

The genus Bombus contains the bumblebees. The bumblebees are very familiar and ecologically important insects in colder regions. The following essay is an overview of some aspects of Bombus biology, natural history, and taxonomy. Much of my information about Bombus biology is from Heinrich (1979) and Thorp et al. (1983). I suggest these sources for a more complete overview of Bombus biology.

Sociality

All of the Bombus species are eusocial. Eusociality is characterized by a reproductive division of labor, consisting of queens, workers and males; overlap of generations; and cooperative care of offspring. Bombus workers are usually smaller than the queen, but otherwise they are similar morphologically. This is considered a relatively primitive level of sociality compared to other eusocial Hymenoptera (e.g. ants and honeybees) that have morphologically differentiated workers and queens (Wilson, 1971).

Colony cycle

The colony cycle of a bumblebees starts in the spring. Queens, which have over-wintered in protected locations, will initiate a new colony. Queens emerge as early as mid February in Western Washington (Kweskin, pers obs.) The queens will often locate a preexisting cavity in the soil, such as an abandoned rodent nest. Locating a suitable location often takes considerable time. Bombus queens are visible in the spring flying close to the surface of the ground searching for a cavity. It is common for Bombus queens, even of different species, to fight for a nesting location (Heinrich,1979).

Workers, which are smaller than queens and remain unmated, are produced during most of the summer. The queen will forage for pollen for her first group of workers. Following that workers will do most of the foraging. In the nest, the pollen is placed in a ball, and several eggs are laid in it. The queen, or workers, can incubate the pollen mass to speed the development of the young. As the summer proceeds, the colony will start producing males and new queens. The new queens mate in the fall, and then overwinter in protected locations.

Mimicry

General Mimicry

Mimicry is the similar appearance of one or more, not necessarily closely related, species. Mimicry systems are often described in terms of models and mimics, although assigning these terms to species in nature is often problematic. When an individual of a species looks more like a member of a dangerous, model species, these individuals may be more successful (i.e. they are less likely to become a prey item). These individuals may be more likely to have progeny than individuals that do not look like a model. Over time, the mimic may look extremely similar to the model.

In mimicry systems, at least one species, the model, is somehow noxious to predators, either through a painful sting (the case in Bombus) or bad taste. The predator is called the observer in mimicry systems. The observer learns, through experience, that the model species is not a defenseless food item, and avoids things that look like the model. The bad tasting or dangerous model species often has bright warning, or aposematic, coloring which may help the individual predator associate its unfortunate feeding experience with the model's appearance. The mimic species, by sharing the coloration of the model species, takes advantage of this avoidance by the observer.

The affect of the mimic species on the model species varies. At one extreme the mimic is a cheater, it is actually defenseless,but it is posing as a dangerous food item. By posing as the model the mimic species is hurting the model. Predators may become confused if similar looking prey are sometimes dangerous and other times harmless; it takes longer for the observer to learn the aposematic coloring. This type of mimicry system is called Batesian mimicry. These systems will only work when the mimic species is present at low levels. If the mimic species becomes too populous, the predators will be less likely to encounter a bad tasting model, and they will not learn the aposematic coloring.

The other extreme in mimicry is called Mullerian. In these systems, two or more unfavorable prey items will look similar. The individuals in these systems benefit by increasing the number of bad tasting or dangerous individuals in an area, that have similar aposematic coloring. An observer is more likely to encounter a bad tasting individual, and the avoidance behavior is reinforced.

Mimicry in Bombus

Because bumblebees are common and have a painful sting, there are many mimicry systems involving Bombus. On the Pacific coast of North America, there are several Mullerian mimicry systems, in which several species within the genus Bombus have converged in coloration, although they are not closely related. The coloration within a Bombus species often shows great geographic variation, correlating with local mimicry systems.

Thorp et al. (1983) identified four mimicry groups within the Bombus of California. Some of these can be illustrated with the Bombus known from The Evergreen State College, in Olympia, Washington. One clear example is that of B. californicus and B. vosnesenskii. These species have a mostly black abdomen with a prominent yellow band near the apex.

B. vosnesenskii B. californicus

Another mimicry system in the Bombus of Evergreen is B. sitkensis and B. flavifrons. These two species share extremely similar color patterns (see couplet 1.1.2).
B. sitkensis B. flavifrons

There are also many non-bee Batesian mimics of Bombus. There are several families of robust hairy flies that are considered Bombus mimics. These include flies of the families Syrphidae, Bombyliidae, and Tachinidae (Gabritschevsky, 1926).

Fly mimics of Bombus
Syrphidae Bombyliidae Tachinidae

Taxonomy

Bees are acuelate Hymenoptera. Acuelata also includes ants and wasps. They are united by their ovipositor being modified into a stinger in females.

Apoidea

Bees evolved from sphecid wasps. Sphecids, like many wasps, will feed their young insect prey as a protein source. Bees have given up animal prey as a food source. Instead, they feed their young pollen and nectar, which the adults collect, pollinating plants in the process.

The bees and sphecid wasps make up the superfamily Apoidea. In this superfamily, there are 11 apiform (bee) families (Goulet and Huber, 1993). Bees can be recognized by having some setae that is branched rather than the straight, unbranched setae found in most insects (Michener et al., 1994). All bees also have the basal tarsal segment wider than the following segments. In sphecoid wasps, the basal tarsal segment is about the same width as the other segments (Michener et al., 1994).

[Comment from Sydney Cameron, October 1999: Apidae has now been sunk into the larger family Apidae, which includes the old Anthophoridae. So now Apidae has 3 subfamilies: Xylocopinae, Nomadinae (the cleptoparasites), and Apinae (= 19 tribes which include the old anthophorids and the old apids). There is no new name for the monophyletic clade that comprises Apini, Euglossini, Meliponini and Bombini. See Roig-Alsina and Michener 1993. Studies of the phylogeny and classification of long-tongued bees (Hymenoptera: Apoidea). Univ. Kansas Sci. Bull. 55: 124-162.]

Apidae

Bombus is in the family Apidae, in the subfamily Bombinae. The family Apidae contains the bumblebees (Bombinae), honey bees (Apinae), stingless bees (Meliponinae), and orchid bees (Euglossinae). Most females of the family Apidae have a corbicula. This is a bare region on the outer surface of the hind tibiae that is surrounded by long hairs. It is used to carry pollen.

Bombinae

The subfamily Bombinae contains two genera, Bombus, the bumblebees, and Psithyrus, the cuckoo bumblebees. Psithyrus bees look very similar to bees of the genus Bombus. They are large bees (~3cm long), with a thick, black and yellow pile.

Psithyrus females are social parasites. They do not collect pollen; instead they lay their eggs in the pollen masses collected by Bombus. Because Psithyrus females do not collect pollen, they lack the pollen carrying modifications found in female Bombus. Psithyrus females lack a corbicula, the pollen carrying structure found in Bombus females. The corbicula is found on the hind tibiae. It is a bare region that is surrounded by long hairs. When the female Bombus is foraging, the pollen is pooled in the corbicula. When she returns to the nest, the pollen balls are pushed off of the corbicula. Because males do not collect pollen, both Bombus and Psithyrus males lack a corbicula. This makes determining the genera of the males of Bombinae more difficult than the females.
The hind tibiae of Bombus and Psithyrus
Bombus Psithyrus
Female
Corbicula present Corbicula absent
Male No image available
Corbicula absent Corbicula absent

Variation

Bombus are notorious for having considerable variation in coloring within a species. One reason for this is a single species may be involved in different mimicry systems in different parts of the species' range (see mimicry). This variation makes taxonomy difficult. It is often hard to define a species, and even harder to define subspecies. A common method is to look for discrete forms in sympatry (occurring in the same area), this reveals two distinct species. The variation in coloring within a species is more extreme with male Bombus than females. To identify males, it is usually necessary to dissect the genitalia.

The morphological variation within a species make it difficult to distinguish species when considered over their whole range. It is therefore helpful to have regional guides to the forms of Bombus common in a limited area. This web site tries to do this for the bumblebees known from The Evergreen State College, in Olympia, Washington.



Page author: Matthew P. Kweskin
Last modified: 23 October 1999
For more information about this site, please contact John T. Longino longinoj@evergreen.edu