C.J. Fellows, Reagan, Thomas E., Huval, Forest, Carlton, Christopher E.
A common eastern bumblebee resting on a flower. Note the yellow patch of hairs on the forward portion of the thorax.
(David Cappaert, Bugwood.org).
The common eastern bumblebee, Bombus impatiens, is an important native pollinator found in the eastern United States and southern Canada. Adult bees are covered with short, even hairs across their bodies. They are mostly black with a yellow thorax (middle section) and an additional yellow stripe at the base of the abdomen. As with other social insects, the different forms, or castes, of B. impatiens vary in size. Worker bumblebees measure from 1/3 to 2/3 of an inch in length (8.5-16 mm), while drones measure ½ to ¾ of an inch (12 to 18 mm). Queen bumblebees are the largest members of the three castes, measuring between 1/3 of an inch to 1 inch (17 and 23 mm) in length. Larvae are rarely seen and are enclosed with the nest brood cells for the duration of their development. They are pale, legless grubs around 1 inch (25 mm) in length.
At least six additional species of bumblebees have been documented in Louisiana, with another two species possibly present but not confirmed. Identifications are mainly based on the patterns of coloration of the body hairs, but these patterns can be subtle. Good, dry specimens are necessary for correct interpretation of the characters.
The life cycle of the common eastern bumblebee begins when a mated queen emerges from hibernation. This usually occurs during February or March in Louisiana. After the queen emerges, she forages for nectar and pollen from nearby flowers. Her next task is to find a suitable site to build a nest. While different species of bumblebees build nests in many different locations, B. impatiens queens always build their nests in abandoned underground burrows left behind by other animals, such as rodents or armadillos. The queen then gathers fibers, such as grass, moss or hair, and forms a hollow ball to contain her brood. As the abandoned burrows of rodents often contain soft beds made of these fibers, they are particularly suitable sites for bumblebee nest construction. Within the hollow fiber ball, the queen deposits a lump of nectar-moistened pollen. The queens also construct honey pots near the nest entrance using the secretions of her wax glands. She then fills the pot with nectar that she has gathered from flowers. After provisioning the nest with stored food, the queen lays fertilized eggs into the lump of pollen. Around four days later, larvae hatch from the eggs and grow as they consume the pollen lump over the next 10 to 14 days.
Bombus impations nest. Note hatched and empty brood cells in the bottom center of the image.
The larvae then spin silk cocoons around themselves and pupate. Approximately two weeks later, adult bumblebees emerge from the cocoons. The entire process of growth from egg to larva, pupa and adult takes around four to five weeks, depending on weather conditions. A few days after hatching, the first generation of workers takes over food gathering responsibilities. As the season progresses, additional generations of workers are produced, and the size of the colony grows. Later in the season, once the colony has reached sufficient size, they will begin to rear the reproductive castes of bumblebees (males and virgin queens). Notably, many nests never reach this stage, as the failure rate of bumblebee colonies is quite high. Once the adult reproductive castes emerge from their cocoon, their roles quickly diverge from those of their nonreproductive nestmates. Males leave the nest and never return, contributing nothing to the welfare of their siblings. They live the rest of their lives outdoors, searching for a mate and feeding on flowers as necessary. Reproductive females forage for nectar and pollen for their own use for a short period after emerging but make no contribution to the resources of the nest. By consuming a large amount of nectar and pollen, the virgin queen increases the stores of fat in her body. After this period, the virgin queen leaves the nest to mate, usually with a single male (drone). Unlike many bee species, B. impatiens mate while sitting on the ground or while resting on vegetation. After mating, the queen begins to search for a place to hibernate for the winter. She excavates a small oval-shaped chamber underground, where she remains until the following spring to begin the cycle anew.
The common eastern bumblebee is an extremely important pollinator of many agricultural crops in the United States as well as Mexico and Canada. While the honeybee, Apis mellifera, is the most widely managed crop pollinator species in these countries, bumblebees are capable of providing superior pollination services for many crop species. Some crops, such as tomatoes, possess a special type of flower with porous anthers. These anthers, or male flower parts, function like salt shakers in that they must be vigorously shaken in order to release their pollen. The pollinating insect must vibrate its body, or buzz, in a very specific manner to cause the pollen to fall through the pores in the anther of the flower. While honeybees are not capable of buzz pollination, several species of native bees, including B. impatiens, are able to effectively pollinate these crops. Consequently, colonies of B. impatiens are reared and sold commercially for pollination services in crops where honeybees prove inadequate. Bombus impatiens colonies are also used in combination with honeybees to improve pollination efficiency in other crop systems.
The common eastern bumblebee nest may be parasitized by a number of other bee species, most notably the cuckoo bee of the genus Psithyrus. Female cuckoo bees, which are believed to have evolved from within the genus Bombus, invade bumblebee nests and kill the queen. The female cuckoo bee then lays eggs inside the bumblebee nest, and begins to functions as a conqueror-queen. Bumblebee workers care for developing cuckoo bee larvae throughout their development. In addition to parasitic bees, B impatiens nests may be host to a number of parasitoid insects, prokaryotes, mites, fungi and other microbes.
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