Natural History

Unsung heroines of pollination

Unsung heroines of pollination

Most of North America’s 3,500 species of bees go unnoticed, primarily because people expect all bees–apart from the big, conspicuous bumblebees and carpenter bees–to resemble the familiar honeybee. In fact, many are smaller, looking more like wasps, fuzzy flies, or flying ants than honeybees. Their behavior, more than their appearance, is what gives them away: if you see an insect on a flower deliberately packing pollen onto its hind legs or under its abdomen, it’s bound to be a bee.

Once you know what to look for, the abundance of bees can be impressive. On a warm day in June, a typical backyard garden in the northeastern United States may contain some thirty species and hundreds of individuals. Native bees (primarily the females, with a little help from the males) and other flower-visiting insects can usually meet the pollination needs of gardens and most small farms. Agribusiness, in contrast, depends on the use of additional pollinators, traditionally the introduced honeybee. With the recent, devastating decline in honeybee numbers, however–and the growing recognition that for some crops, other bees may do a better job–researchers are looking at other species, now commonly referred to as pollen bees.

Several factors are critical for developing a bee into a manageable pollinator for agribusiness crops: it must be willing to nest under crowded conditions with others of its kind; its brief adult life, which lasts just a few weeks, must coincide with the flowering of the crop, plant; it must prefer the crop flowers to those of other plants so it won’t wander away; its foraging style and body shape should result in efficient cross pollination; it must be able to raise its brood on the nectar and pollen produced by flowers; and its natural enemies must be controllable.

Whatever their commercial prospects, of course, all pollen bees are vital members of their natural habitats and deserve protection.

Bumblebees These big, fuzzy bees (genus Bombus, with forty-six species in North America) are important pollinators in cold climates. Most are social, forming small colonies with queens and workers and often nesting underground in abandoned mouse nests. Only young mated queens survive the winter to start new colonies in the spring. Some species make no nests but instead, like cuckoos, lay their eggs in the nests of other bumblebees.

Ever since methods of keeping bumblebee colonies year-round were developed ten years ago, the business of raising these bees–which are in demand to pollinate valuable greenhouse-grown crops–has been thriving in several countries. Bumblebees adapt to being confined inside a greenhouse, while honeybees try to get out, banging against the glass ceiling and generally winding up dead on the floor. Another commercial advantage of bumblebees is that they can pollinate certain plants, such as hothouse tomatoes, very fast, vibrating their bodies so rapidly that they literally “buzz” the pollen out of the flowers.

Carpenter bees Big and solitary, carpenter bees (genus Xylocopa) resemble bumblebees but are not closely related to them. Most are tropical; only eight species live in North America, where they pollinate many wildflowers and crops. The flowers of passion-fruit (including the North American maypop) are structurally adapted for pollination by the big carpenter bees; smaller bees simply fail to trigger the flower’s pollination mechanisms. Plants of horticultural interest pollinated by carpenter bees include our native catalpa trees and wisteria, which is native to China and Japan.

Male carpenter bees maintain territories around nests and flowers, where they hover, occasionally darting away to fight with an intruding male or to pursue a female. Mated females make their brood cells inside tunnels that they chew in soft, dry woods and often–unfortunately–houses. (Their nests are easily recognized by the neat round hole at each entrance; reaming and plugging up the holes may encourage the bees to nest elsewhere.) Sometimes, young adult daughter bees stay in their natal nests with their mother for a while, suggesting that, given enough time, these bees might evolve into social species.

Mason bees About 140 species of these solitary bees live in North America, primarily in the West. Most are blue or green in color and are active in spring. Females carry pollen beneath their abdomens, instead of on their hind legs as most bees do. Mason bees (Osmia) nest in preexisting holes–both natural ones made by beetles or other insects and artificial ones, such as nail holes. Their brood cells, made of mud and sometimes masticated leaves, are lined up in the holes. Several species of mason bees have been used to pollinate fruit trees and have proved very efficient: three or four females per tree will suffice. Also efficient and easy to care for, the hornfaced bee (O. cornifrons) has been used for about sixty years to pollinate apple trees in Japan, where it is often preferred over the honeybee.

Leaf-cutter bees These solitary, usually grayish bees (Megachile) live worldwide, with about 120 species in North America. Most are active in midsummer. Like mason bees, they usually nest in preexisting holes and carry their pollen loads under their abdomens. Their brood cells are made of bits of leaves and petals, cut by the nesting female. Agriculturally, the most important is the introduced Eurasian alfalfa leaf-cutter bee (M. rotundata), raised by the millions to pollinate alfalfa, the United States’ most valuable livestock forage crop.

Mining mud bees About sixty species of these rotund, fast-flying, solitary bees (Anthopora) are found in North America, mostly in the West..The females usually dig their nest tunnels in the dry clay of cliffs and adobe walls. Some species make a hollow chimney of dried mud that conceals the nest entrance. After making each brood cell out of mud, the female lines it with a fatty material secreted from a large gland in her abdomen. She adds this energy-rich, fatty substance to the nectar and pollen she provides for the larvae. To meet her own high energy needs and those of her brood, a female mud bee may visit 337,000 flowers during her six-week-long adult life (during the same time period, an individual honeybee might visit perhaps one-tenth as many flowers). Whether zooming from flower to flower or hovering in front of one while probing for hidden nectaries with their long tongues, these bees are reminiscent of hummingbirds and hawk-moths. Two mining mud bees–A. abrupta and A. pilipes villosula–are now being tested as possible pollinators of fruit crops.

Polyester bees These solitary bees (Collates) dig nests and cavities for brood cells in the ground, then line the cell cavities with an abdominal secretion that solidifies to form a transparent, waterproof, polyester membrane. Nests may be close together, numbering in the thousands in some locations. About one hundred species of polyester bees–each with its own flower preferences–live in North America; some are excellent blueberry pollinators. As yet, there is no commercial use of polyester bees, but preliminary trials suggest that they could be induced to nest in artificial “bee beds.”

Digger bees With some 500 species in North America, and many more elsewhere in the Northern Hemisphere, these usually drab and small solitary bees (Andrena) are rarely noticed but may be our most abundant bees. Digger bees make burrows and cells underground and often hide the entrances to their nests beneath leaf litter or in grass to evade their many enemies, which include parasitic flies and cuckoo bees.

Most digger bees are active in spring, when they pollinate various wildflowers, trees, and fruit crops. They are especially valuable in orchards because they carry large, loose loads of pollen, which are spread widely among the flowers as the bees fly from tree to tree. These bees are very vulnerable to agribusiness practices such as irrigation, clean cultivation, and pesticides. So far, efforts to manage them have failed, and the best way to preserve them–and continue to reap the benefits of their pollination services–is to preserve wild land (around orchards, for example).

Sweat bees These small bees get their common name from their attraction to hot, salty perspiration. When they land on human skin, they walk around, licking and tickling. Brushed away, they return; swatted, they sting. Sweat bees (several genera in the family Halictidae) live worldwide, with about 500 species in North America. They are usually dark, although some species are bright green. Most nest underground, often with many nests in one small area.

Sweat bees are unusual because, depending on the species, they may either be solitary or have various cooperative nesting arrangements, including nests with more than one queen and up to hundreds of workers. Cuckoo sweat bees lay their eggs in the nests of others. The alkali bee (Nomia melanderi), a turquoise-banded solitary species, is used commercially to pollinate alfalfa in the West.

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