Common Name: Bumble Bees
Scientific Name: Bombus spp., Psithyrus spp.
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.
Adult worker body length about 1/4-1″ (6-25 mm), queens about 3/4-1″ (17-25 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 base of mandible. Hind tibia with apical spurs. Front wing with 2nd submarginal cell more or less rectangular, about as long as 1st 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.
(1) 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). (2) Some robber flies (Diptera: Asilidae) with only 1 pair of wings. (3) Some hawk moths (Lepidoptera: Sphingidae) with siphoning mouthparts.
Bumble bees are social insects which live in nests or colonies. The adults are represented by workers (Psithyrus spp. lack 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 Bombus queen fashions a honey pot 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 degrees F (30 degrees 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.
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 (11-20 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 thermoregulation procedure to warm up flight muscles before the sun rises and to also forage when temperatures are below 50 degrees F (10 degrees C; lowest observed flight at 26 degrees F/-3.6 degrees C) whereas, most bees stop foraging at 61 degrees F (16 degrees 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.
Bumble bees are considered beneficial insects because they pollinate the flowers of many plant species. However, if their nest is located in or close to an occupied structure or recreational area, then control is warranted. During the day find the location of each nest by observing where the bees disappear into the ground, grass clump, or structure. At night using background light and while wearing a bee veil, apply an appropriately labeled pyrethroid pesticide. Dusts work best when applied to an area 6″ (15 cm) around the nest entrance. For structural nests, treat with dust or aerosol but do not seal the entrance. Structural nests should be either retreated with a long-lasting repellent material and sealed or the void opened up and cleaned out within 1-2 days to prevent future problems with dermestid beetles, spider beetles, and/or psocids.