Scientific Name: Bombus sp.
Common Name: Bumblebee
(Information for this species page was gathered in part by Ms. Katie Kirstein as an assignment for Biology 220M in Spring 2007).
“Bumblebee” is a common name given to the 200 + species of bees of the genus Bombus. Bumblebees are wild bees that accomplish many of the same ecological functions as their close relatives the domesticated honey bees. The pollination of a wide range of flowering plants, the gathering of nectar and pollen, and the making of honey are shared roles between these two diverse groups of bees. Bumblebees, though, although quite vigorous in their pollination activities, carry out their honey making activities on a much smaller and economically less advantageous scale. Their colonies are much smaller, their hives are much less organized, and the individuals in the colonies have much shorter lives (hence the lack of an abundant, over-wintering food store of honey which the honey bee keeper “shares” with his customers!). Many economically important plants, though, including red clover, alfalfa, field beans, tomatoes (both field and greenhouse forms), cotton, apples, plums, raspberries, sunflowers, and more are primarily pollinated by bumblebees. In fact, colonies of bumblebees are available commercially for the pollination of greenhouse tomatoes!
Appearance and Nests
Bumblebees are large, orange (or red) and black (or, sometimes just black), “hairy” bees that are abundantly seen in both natural and agricultural habitats. They make their nests in a variety of natural and human created places. These nests are usually small (often housing 50 to 400 individuals depending on the abundance of local food supplies) and transient (lasting only a few months). Since only the new queens overwinter there is no need for the species to build more substantial hives or accumulate large surpluses of honey to feed hive populations. Bumblebee nests are often found in old rodent burrows, in rock or tree crevices, under porches or in the eaves of roofs, in clumps of grasses or thatch, and in almost any other dry, protected spot.
Bumblebee colonies go through distinct life cycles that are very adaptable to local climate conditions and food resources. Queens hatched at the end of the previous year’s colony cycle that mated in the early fall of that year are the only bumblebee individuals that survive the winter. They hibernate in the soil typically in a location that will not warm up too quickly or too early in the spring (to avoid a potentially lethal early emergence). Once these queens do emerge they begin to look for a suitable nest location and then begin to vigorously gather both nectar and pollen. Proximity of the nest site to a rich patch of flowers is critical for the energy economy of the bees.
Once a nest site has been chosen, the queen builds a small, wax, “honey pot” which she fills with gathered nectar and pollen. This food reserve is sufficient only for a day or two and represents a safety cache in the event that bad weather prevents foraging. The small size of this reserve reflects both the expected mild nature of the bumblebee’s habitat and also the robustness of the bees and their ability to forage and gather food under a very wide range of weather conditions. The queen then begins to lay eggs on waxy balls of pollen and carefully broods them (optimal temperature is 30 degrees C (86 degrees F) for 4 days. The queen must, as a “single parent,” alternate between brooding and food gathering and is only able to make short, hopefully highly productive foraging trips. The proximity of a rich flower source is critical for the success of this early stage of the colony.
After hatching, the larvae feed exclusively and extensively on the gathered pollen and go through 4 or 5 instars in a period of about a month. All of these early offspring develop from fertilized eggs (the queen stored sperm from her previous fall’s mating). Therefore, all of these early larvae develop into adult females. These females are also known as “workers.” The workers live for 2 to 4 weeks and rely primarily on sugar-rich nectars for their food. These workers, and the subsequent cohorts of more and more workers, take over the food gathering tasks from the queen who is now able to concentrate exclusively on egg production. A few workers may remain in the nest doing “household” tasks. These workers are typically smaller than the foraging workers and may even have reduced to non-functional wings.
Foraging and Pollination
Bumblebees forage in a wide range of flowers. They have excellent color vision, UV spectrum vision, and good color/pattern memories. Many flowers have adaptations (deep corollas etc) that select for pollination by bumblebees. Further, foragers, via trial and error learning and nectar/pollen gathering success may develop flower preferences. Entire colonies, in fact, to the energetic benefit of the bees and the pollinating benefit of their flower symbionts, may become quite specialized in the types of flowers that they visit. An individual forager may visit many thousands of flowers in order to gather sufficient nectar and pollen for its colony. Some foragers may even “overnight it” in the shelter of the flower petals. Bumblebees are strong fliers that can forage up to 2 km away from their nests and can fly in winds as strong as 15 km/hr (compared to honey bees that can forage only 700 m away from their hives and can only fly in winds of less than 8 km/hr). Further, bumblebees can generate sufficient metabolic heat to be able to fly very efficiently in cool mornings or even in the relatively chilled temperatures of the early spring
Egg Laying and Caste Determination
As the nest becomes larger and its workers become more efficient in accumulating pollen, the queen lays more and more eggs. Toward the end of summer, though, the queen begins to lay and brood these eggs differently. By withholding a pheromone from fertilized eggs and by increasing the amount of the food supply given to the developing larvae, female bumblebees begin to develop into new queens. And, by laying unfertilized eggs, which then begin to develop into larvae and adults, the queen begins the formation of male bees. These new queens and males are signs that the colony has begun its inevitable, seasonal decline. Once fully emerged as adults, the males consume the nest’s honey stores and disrupt the organizational integrity of the colony. Although they leave the nest and begin to cluster in the patches of the surrounding flowers, the colony never recovers from the chaos of their activities.
Mating and Communication
The males place pheromone tracts for the soon to emerge new queens and mating occurs usually on the ground or atop vegetation. The males die soon after mating. The mated queens, though, begin to consume abundant quantities of nectar in order to build up their body energy reserves. They then seek out a soil hibernation chamber and wait out the winter in order to begin a new colony cycle the next spring.
Recently it has been discovered that bumblebees, like their honey bee kin, are able to communicate directions and distances to food sources via “dances.” Though not as elegant or as sophisticated as the honey bee dance, these ritualized forms of visual communication represent significant evolutionary steps that greatly increase the foraging efficiency and eventual success of a colony.