Bees are flying insects, closely related to wasps and ants. Bees are a monophyletic lineage within the superfamily Apoidea. There are slightly fewer than 20,000 known species of bee, though many are undescribed and the actual number is probably higher. They are found on every continent except Antarctica.
Many species of bees are poorly known. The smallest bee is the dwarf bee (Trigona minima) and it is about 2.1 mm (5/64") long. The largest bee in the world is Megachile pluto, which can be as large as 39 mm (1.5"). The most common type of bee in the Northern Hemisphere are the many species of Halictidae, or sweat bees, though this may come as a surprise to people, as they are small and often mistaken for wasps or flies.
The most well-known bee species is the Western honey bee, which, as its name suggests, produces honey, as do a few other types of bee. Human management of this species is known as beekeeping or apiculture.
Bees are adapted for feeding on nectar and pollen, the former primarily as an energy source, and the latter primarily for protein and other nutrients. Most pollen is used as food for larvae.
Bees have a long proboscis (a complex "tongue") that enables them to obtain the nectar from flowers. Bees have antennae almost universally made up of thirteen segments in males and twelve in females, as is typical for the superfamily. They all have two pairs of wings, the hind pair being the smaller of the two; in a very few species, one sex or caste has relatively short wings that make flight difficult or impossible, but none are wingless.
Honey Bee collecting pollen from tree at end of winter. Location: Rušanj near Belgrade, Serbia. Bee collecting pollenBees play an important role in pollinating flowering plants, and are the major type of pollinators in ecosystems that contain flowering plants. Bees may focus on gathering nectar or on gathering pollen, depending on their greater need at the time, especially in social species. Bees gathering nectar may accomplish pollination, but bees that are deliberately gathering pollen are more efficient pollinators. It is estimated that one third of the human food supply depends on insect pollination, most of this accomplished by bees.
Bees are extremely important as pollinators in agriculture, especially the domesticated Western honey bee, with contract pollination having overtaken the role of honey production for beekeepers in many countries. Monoculture and pollinator decline (of many bee species) have increasingly caused honey bee keepers to become migratory so that bees can be concentrated in areas of pollination need at the appropriate season. Recently, many such migratory beekeepers have experienced substantial losses, prompting the announcement of investigation into the phenomenon, dubbed "Colony Collapse Disorder", amidst great concern over the nature and extent of the losses.
Many other species of bees such as mason bees are increasingly cultured and used to meet the agricultural pollination need. Bees also play a major, though not always understood, role in providing food for birds and wildlife. Many of these bees survive in refuge in wild areas away from agricultural spraying, only to be poisoned in massive spray programs for mosquitoes, gypsy moths, or other pest insects.
Most bees are fuzzy and carry an electrostatic charge, thus aiding in the adherence of pollen. Female bees periodically stop foraging and groom themselves to pack the pollen into the scopa, which is on the legs in most bees, and on the ventral abdomen on others, and modified into specialized pollen baskets on the legs of honey bees and their relatives. Many bees are opportunistic foragers, and will gather pollen from a variety of plants, but many others are oligolectic, gathering pollen from only one or a few types of plant. A small number of plants produce nutritious floral oils rather than pollen, which are gathered and used by oligolectic bees. One small subgroup of stingless bees (called "vulture bees") is specialized to feed on carrion, and these are the only bees that do not use plant products as food. Pollen and nectar are usually combined together to form a "provision mass", which is often soupy, but can be firm. It is formed into various shapes (typically spheroid), and stored in a small chamber (a "cell"), with the egg deposited on the mass. The cell is typically sealed after the egg is laid, and the adult and larva never interact directly (a system called "mass provisioning").
Visiting flowers is a dangerous occupation with high mortality rates. Many assassin bugs and crab spiders hide in flowers to capture unwary bees. Others are lost to birds in flight. Insecticides used on blooming plants can kill large numbers of bees, both by direct poisoning and by contamination of their food supply. A honey bee queen may lay 2000 eggs per day during spring buildup, but she also must lay 1000 to 1500 eggs per day during the foraging season, simply to replace daily casualties.
The population value of bees depends partly on the individual efficiency of the bees, but also on the population itself. Thus, while bumblebees have been found to be about ten times more efficient pollinators on cucurbits, the total efficiency of a colony of honey bees is much greater, due to greater numbers. Likewise, during early spring orchard blossoms, bumblebee populations are limited to only a few queens, thus they are not significant pollinators of early fruit.
Bees vary tremendously in size. Here a tiny halictid bee is gathering pollen, while a giant bumblebee behind her gathers nectar from a lily.Bees, like ants, are essentially a highly specialized form of wasp. The ancestors of bees were wasps in the family Crabronidae, and therefore predators of other insects. The switch from insect prey to pollen may have resulted from the consumption of prey insects that were flower visitors and were partially covered with pollen when they were fed to the wasp larvae. This same evolutionary scenario has also occurred within the vespoid wasps, where the group known as "pollen wasps" also evolved from predatory ancestors. The oldest definitive bee fossil is Cretotrigona prisca in New Jersey amber and of Cretaceous age. The recently reported bee fossil, of the genus Melittosphex, is in fact a wasp stem-group to Anthophila but cannot be considered an actual bee as it lacks definitive bee traits and no information is available on whether or not it fed its larvae pollen.
The earliest animal pollinated flowers were pollinated by insects such as beetles, so the syndrome of insect pollination was well established before bees first appeared. The novelty is that bees are specialized as pollination agents, with behavioral and physical modifications that specifically enhance pollination, and are much more efficient at the task than beetles, flies, butterflies, pollen wasps, or any other pollinating insect. The appearance of such floral specialists is believed to have driven the adaptive radiation of the angiosperms, and, in turn, the bees themselves.
Eusocial and semisocial bees
Eusocial honeybee swarmBees may be solitary or may live in various types of communities. The most advanced of these are eusocial colonies found among the honey bees, bumblebees, and stingless bees. Sociality, of several different types, is believed to have evolved separately many times within the bees.
In some species, groups of cohabiting females may be sisters, and if there is a division of labor within the group, then they are considered semisocial.
If, in addition to a division of labor, the group consists of a mother and her daughters, then the group is called eusocial. The mother is considered the "queen" and the daughters are "workers". These castes may be purely behavioral alternatives, in which case the system is considered "primitively eusocial" (similar to many paper wasps), and if the castes are morphologically discrete, then the system is "highly eusocial".
There are many more species of primitively eusocial bees than highly eusocial bees, but they have been rarely studied. The biology of most such species is almost completely unknown. The vast majority are in the family Halictidae, or "sweat bees". Colonies are typically small, with a dozen or fewer workers, on average. The only physical difference between queens and workers is average size, if they differ at all. Most species have a single season colony cycle, even in the tropics, and only mated females (future queens, or "gynes") hibernate (called diapause). A few species have long active seasons and attain colony sizes in the hundreds. The orchid bees include a number of primitively eusocial species with similar biology. Certain species of allodapine bees (relatives of carpenter bees) also have primitively eusocial colonies, with unusual levels of interaction between the adult bees and the developing brood. This is "progressive provisioning"; a larva's food is supplied gradually as it develops. This system is also seen in honey bees and some bumblebees.
Highly eusocial bees live in colonies. Each colony has a single queen, together with workers and, at certain stages in the colony cycle, drones. When humans provide a home for a colony, the structure is called a hive. A honey bee hive can contain up to 40,000 bees at their annual peak, which occurs in the spring, but usually have fewer.
Bumblebees (Bombus terrestris, B. pratorum, et al.) are eusocial in a manner quite similar to the eusocial Vespidae such as hornets. The queen initiates a nest on her own (unlike queens of honey bees and stingless bees which start nests via swarms in the company of a large worker force). Bumblebee colonies typically have from 50 to 200 bees at peak population, which occurs in mid to late summer. Nest architecture is simple, limited by the size of the nest cavity (pre-existing), and colonies are rarely perennial. Bumblebee queens sometimes seek winter safety in honey bee hives, where they are sometimes found dead in the spring by beekeepers, presumably stung to death by the honey bees. It is unknown whether any survive winter in such an environment.
Stingless bees are very diverse in behavior, but all are highly eusocial. They practice mass provisioning, complex nest architecture, and perennial colonies.
A Western honey bee extracts nectar from an Aster flower using its proboscis. Tiny hairs covering the bee's body maintain a slight electrostatic charge, causing pollen from the flower's anthers to stick to the bee, allowing for pollination when the bee moves on to another flower.Main article: Honey bee The true honey bees (genus Apis) have arguably the most complex social behavior among the bees. The Western (or European) honey bee, Apis mellifera, is the best known bee species and one of the best known of all insects.
Africanized honey bee
Africanized bees, also called killer bees, are a hybrid strain of Apis mellifera derived from experiments to cross European and African honey bees by Warwick Estevam Kerr. Several queen bees escaped his laboratory in South America and have spread throughout the Americas. Africanized honey bees are more defensive than European honey bees.
Solitary and communal bees
Most other bees, including familiar species of bee such as the Eastern carpenter bee (Xylocopa virginica), alfalfa leafcutter bee (Megachile rotundata), orchard mason bee (Osmia lignaria) and the hornfaced bee (Osmia cornifrons) are solitary in the sense that every female is fertile, and typically inhabits a nest she constructs herself. There are no worker bees for these species. Solitary bees typically produce neither honey nor beeswax. They are immune from acarine and Varroa mites (see diseases of the honey bee), but have their own unique parasites, pests and diseases.
Western honey bee collecting nectar from small flowers. Location: McKinney, Texas.Solitary bees are important pollinators, and pollen is gathered for provisioning the nest with food for their brood. Often it is mixed with nectar to form a paste-like consistency. Some solitary bees have very advanced types of pollen carrying structures on their bodies. A very few species of solitary bees are being increasingly cultured for commercial pollination.
Solitary bees are often oligoleges, in that they only gather pollen from one or a few species/genera of plants (unlike honey bees and bumblebees which are generalists). No known bees are nectar specialists; many oligolectic bees will visit multiple plants for nectar, but there are no bees which visit only one plant for nectar while also gathering pollen from many different sources. Specialist pollinators also include bee species that gather floral oils instead of pollen, and male orchid bees, which gather aromatic compounds from orchids (one of the only cases where male bees are effective pollinators). In a very few cases only one species of bee can effectively pollinate a plant species, and some plants are endangered at least in part because their pollinator is dying off. There is, however, a pronounced tendency for oligolectic bees to be associated with common, widespread plants which are visited by multiple pollinators (e.g., there are some 40 oligoleges associated with creosotebush in the US desert southwest, and a similar pattern is seen in sunflowers, asters, mesquite, etc.)
Solitary bees create nests in hollow reeds or twigs, holes in wood, or, most commonly, in tunnels in the ground. The female typically creates a compartment (a "cell") with an egg and some provisions for the resulting larva, then seals it off. A nest may consist of numerous cells. When the nest is in wood, usually the last (those closer to the entrance) contain eggs that will become males. The adult does not provide care for the brood once the egg is laid, and usually dies after making one or more nests. The males typically emerge first and are ready for mating when the females emerge. Providing nest boxes for solitary bees is increasingly popular for gardeners. Solitary bees are either stingless or very unlikely to sting (only in self defense, if ever).
While solitary females each make individual nests, some species are gregarious, preferring to make nests near others of the same species, giving the appearance to the casual observer that they are social. Large groups of solitary bee nests are called aggregations, to distinguish them from colonies.
In some species, multiple females share a common nest, but each makes and provisions her own cells independently. This type of group is called "communal" and is not uncommon. The primary advantage appears to be that a nest entrance is easier to defend from predators and parasites when there are multiple females using that same entrance on a regular basis.