Pollinators

A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains.

Insects are the major pollinators of most plants, and insect pollinators include all families of bees and most families of aculeate wasps; ants; many families of flies; many lepidopterans (both butterflies and moths); and many families of beetles. Vertebrates, mainly bats and birds, but also some non-bat mammals (monkeys, lemurs, possums, rodents) and some lizards pollinate certain plants. Among the pollinating birds are hummingbirds, honeyeaters and sunbirds with long beaks; they pollinate a number of deep-throated flowers. Humans may also carry out artificial pollination.

A pollinator is different from a pollenizer, a plant that is a source of pollen for the pollination process.

Background

Plants fall into pollination syndromes that reflect the type of pollinator being attracted. These are characteristics such as: overall flower size, the depth and width of the corolla, the color (including patterns called nectar guides that are visible only in ultraviolet light), the scent, amount of nectar, composition of nectar, etc. For example, birds visit red flowers with long, narrow tubes and much nectar, but are not as strongly attracted to wide flowers with little nectar and copious pollen, which are more attractive to beetles. When these characteristics are experimentally modified (altering colour, size, orientation), pollinator visitation may decline.

It has recently been discovered that cycads, which are not flowering plants, are also pollinated by insects. In 2016, researchers showed evidence of pollination occurring underwater, which was previously thought not to happen.

Types of pollinators

Bees

The most recognized pollinators are the various species of bees, which are plainly adapted to pollination. Bees typically are fuzzy and carry an electrostatic charge. Both features help pollen grains adhere to their bodies, but they also have specialized pollen-carrying structures; in most bees, this takes the form of a structure known as the scopa, which is on the hind legs of most bees, and/or the lower abdomen (e.g., of megachilid bees), made up of thick, plumose setae. Honey bees, bumblebees, and their relatives do not have a scopa, but the hind leg is modified into a structure called the corbicula (also known as the "pollen basket"). Most bees gather nectar, a concentrated energy source, and pollen, which is high protein food, to nurture their young, and transfer some among the flowers as they are working. Euglossine bees pollinate orchids, but these are male bees collecting floral scents rather than females gathering nectar or pollen. Female orchid bees act as pollinators, but of flowers other than orchids. Eusocial bees such as honey bees need an abundant and steady pollen source to multiply.

Honey bees

Honey bees travel from flower to flower, collecting nectar (later converted to honey), and pollen grains. The bee collects the pollen by rubbing against the anthers. The pollen collects on the hind legs, in a structure referred to as a "pollen basket". As the bee flies from flower to flower, some of the pollen grains are transferred onto the stigma of other flowers. Nectar provides the energy for bee nutrition; pollen provides the protein. When bees are rearing large quantities of brood (beekeepers say hives are "building"), bees deliberately gather pollen to meet the nutritional needs of the brood.

Good pollination management seeks to have bees in a "building" state during the bloom period of the crop, thus requiring them to gather pollen, and making them more efficient pollinators. Thus, the management techniques of a beekeeper providing pollination services are different from, and to some extent in tension with, those of a beekeeper who is trying to produce honey. Millions of hives of honey bees are contracted out as pollinators by beekeepers, and honey bees are by far the most important commercial pollinating agents, but many other kinds of pollinators, from blue bottle flies, to bumblebees, orchard mason bees, and leaf cutter bees are cultured and sold for managed pollination.

Other species of bees differ in various details of their behavior and pollen-gathering habits, and honey bees are not native to the Western Hemisphere; all pollination of native plants in the Americas and Australia historically has been performed by various native bees.

Other insects

Many insects other than bees accomplish pollination by visiting flowers for nectar or pollen, or commonly both. Many do so adventitiously, but the most important pollinators are specialists for at least parts of their life cycles for at least certain functions.

Prominent among Hymenoptera other than bees are predatory aculeate wasps (especially Crabronidae, Sphecidae, Vespidae, and Pompilidae). The term " pollen wasps", in particular, is widely applied to the Masarinae, a subfamily of the Vespidae; they are remarkable among solitary wasps in that they specialise in gathering pollen for feeding their larvae, carried internally and regurgitated into a mud chamber prior to oviposition. Also, males of many species of bees and wasps, though they do not gather pollen, rely on flowers as sources of energy (in the form of nectar) and also as territories for meeting fertile females that visit the flowers.

Many bee flies, and some Tabanidae and Nemestrinidae are particularly adapted to pollinating fynbos and Karoo plants with narrow, deep corolla tubes, such as ''Lapeirousia'' species. Part of the adaptation takes the form of remarkably long probosces. This also applies to empidine dance flies ( Empidinae) that visit a wide range of flowering plants, some species of which can pollinate the woodland geranium ( ''Geranium sylvaticum'' L.) as effectively as bees.

Carrion flies and flesh flies in families such as Calliphoridae and Sarcophagidae are important for some species of plants whose flowers exude a fetid odor. The plants' ecological strategy varies; several species of ''Stapelia'', for example, attract carrion flies that futilely lay their eggs on the flower, where their larvae promptly starve for lack of carrion. Other species do decay rapidly after ripening, and offer the visiting insects large masses of food, as well as pollen and sometimes seed to carry off when they leave.

Hoverflies are important pollinators of flowering plants worldwide. Often hoverflies are considered to be the second most important pollinators after wild bees. Although hoverflies as a whole are generally considered to be nonselective pollinators, some species have more specialized relationships. The orchid species ''Epipactis veratrifolia'' mimics alarm pheromones of aphids to attract hover flies for pollination. Another plant, the slipper orchid in southwest China, also achieves pollination by deceit by exploiting the innate