INTRODUCTION.
The common name of bumble bee possibly comes from their rather large, clumsy appearance
and/or the buzzing sound they make as they fly. In the urban setting, bumble bees do not
usually nest in structures but are of concern because of their abundance around the many
flowering plants typical of yards, and because they can sting. There are about 51 species
(45 in Bombus, 6 in Psithyrus) in the United States and Canada, and as a group they are
found throughout the United States.
RECOGNITION.
Adult worker body length about 1/4-1 " (6-25 mm), queens about 3/4-1 (1 7-25 mm)
long; robust in form. Color black with yellow (rarely orange) markings; with overall fuzzy
appearance, including top surface of abdomen. Head with distinct space between base of
compound eye and bass of mandible. Hind tibia with apical spurs. Front wing with 2nd
submarginal cell more or less rectangular, about as long as lst submarginal cell. Hind
wing lacks a jugal lobe (lobe on rear margin near body). Stinger relatively smooth, with
small barbs. In addition, Bombus with hind tibia modified into pollen basket (surface bare
and polished, marginal hair fringe) whereas, Psithyrus lacks pollen basket, hind tibia
slender.
SIMILAR GROUPS.
Carpenter bees (Xylocopa spp.) with top surface of abdomen largely
bare and shining, front wing with 2nd submarginal cell triangular, and hind wing with
jugal lobe (lobe on rear margin near body).
Some hawk moths (Lepidoptera: Sphingidae) with siphoning mouthparts.
BIOLOGY. Bumble bees are social insects which live in
nests Dr colonies, The adults are represented by workers (Psithyrus spp. lark workers)
which are sterile females, queens, and males (drones) which come from unfertilized eggs
and usually appear in late summer.Typically, only inseminated queens overwinter and do so
underground. In the spring, the queens of Psithyrus species wait until the Bombus nests
are moderate in size and then parasitize them. The Bombus queens select a suitable
subterranean cavity or surface grass clump as a nesting site. Then the Sombus queen
fashions a honey pat of wax scales near the nest entrance into which she regurgitates
nectar. Next she makes a pollen clump on the nest floor and lays 8-10 eggs on it. The
queen will periodically add pollen and nectar to the peripheral edges of the clump, and
eventually more eggs. Developmental time (egg to adult) is 16-25 days, with 4 larval
molts. Workers live about 2 weeks. Most first brood workers are small due to nutrition.
The queen will increase the number of eggs laid as the number of workers to care for them
increases. During the summer, parasitism may eliminate up to 50% of the colony's workers
each week. However, a mature bumble bee nest ultimately contains about 50-400 bees at any
given time; the largest known nest contained 756 bees and 385 brood (larvae and pupae).
The nest temperature is regulated to about 86' F (30' C). This thermoregulation is
accomplished by the bee relaxing the 3rd axillary muscle to its wings which unhinges the
wings from the main power-producing thoracic muscles. Then contractions of these large
muscles produces body heat without wing movement. In the late summer only males (drones)
and new queens are reared in the nest. Once these new queens emerge, they mate and find a
suitable place to overwinter. The males, workers, old queen, and any virgin new queens die
with the onset of cold weather.
HABITS. Depending on the Bombus species, the
overwintering queen will select an appropriate nesting site the following spring. The
queen of some Bombus species locate a dark cavity at least 3/4" (2 cm) high by 1
1/8" (3 cm) wide containing fine plant fiber; such a nest is usually underground and
often an abandoned mouse nest. Queens of other Bombus species select a dense clump of
grass on the surface for a nest, adding grass on top. The queens of Psithyrus species are
all parasitic on Bombus nests, so they bide their time until the Bombus nests are moderate
in size and can therefore support them. They then enter the nest, kill the Bombus queen,
and take over the nest using the Bombus workers to care for her young. Bombus queens of
later emerging Bombus species sometimes also parasitize the nests of earlier emerging
Bombus species.
Bumble bees foraging for nectar fly at 7-12 mph (1 1-20 km/hr) and
spend only 2-4 minutes inside the nest between trips. Probably they will travel at least 3
mi (5 km) if necessary for nectar. They orientate by the sky's polarized light via their 3
ocelli, so they can forage before and after light when objects and landmarks are not
visible. They use their theromoregulation procedure to warm up flight muscles before the
sun rises and to also forage when temperatures are below 50' F (10' C; lowest observed
flight at 26' F/-3.6' C) whereas, most bees stop foraging at 61' F (16' C). Each worker
forages independently, and bumble bees never exchange food. Old cocoons are used to store
both pollen and nectar. Only enough food (honey and pollen) for a few days is stored at
any given time which helps discourage nest predation by skunks, foxes, etc.
Defense is usually done by using their
relatively smooth stingers which can be used over and over. Some species will also spray
feces, and some cover the intruder with regurgitated honey. People sensitive to insect
venom should exercise care around bumble bee nests.
