The Other Pollinators
Nature has many wild workers. When watching insect activity around my flowers last spring, I missed the honeybees but hoped they would come buzzing around. I saw very few, because--as I found out later--their population had been devastated by a mite, but as I watched for them, I made a wonderful observation: plenty of pollinating was still going on. My flowers were blooming, my vegetables fruiting. What was doing it?
I saw many other kinds of bees: robust hairy bumblebees, small mining bees, and leafcutter bees. Metallic-hued sweat bees, bee-mimicking flies, butterflies, and beetles were also hard at work. I began to realize what was happening. Though beekeepers' colonies had been handling pollination, when mites afflicted the honeybees, nature sent in its second-stringers, and bountiful harvests still came in.
How Does Pollination Happen?
Pollination, a fortunate by-product of insects' nectar feeding and pollen collecting, is essential to the continued existence of many plants. When insects reach for the sweet juice of flowers that they need for food, they walk all over the flower parts, actively and passively collecting pollen and transporting it to other flowers.
Pollination is a necessary prerequisite to fertilization; without it, a plant will not set seed or fruit. It occurs when insects move pollen from the (male) anther of one flower to the (female) stigma of another flower. Two-thirds of all flowering plants depend on pollinating insects for this service. More than 3,800 species of bees exist in the United States, and most of them collect nectar and pollen. All adult bees eat protein-rich pollen and feed it to their young. In addition to insects, some birds, bats, and other mammals pollinate. Wind and water also transport pollen, but my focus here is on insect pollinators.
In recent years, the Varroa mite destroyed honeybee populations in many regions of the Northeast and central Midwest (Illinois, Iowa, Kansas, and Nebraska). Foulbrood, a bacterial infection that kills bees' larvae, and several other diseases and parasites also afflict colonies, despite the worker bees' attempts to remove infested brood in the hive (scientists call this hygienic behavior). When honeybee populations suffer, honey production goes down, and pollination of cultivated crops is in jeopardy. While it's important to care for and encourage honeybees, it's also important to understand that there are many other pollinators.
A Guide to the Pollinator Bees
Besides honeybees, pollinators include several other bee species, as well as insects that have filled the gap left by dwindling honeybee populations.
Honeybee (Apis mellifera)
Europeans who knew the honeybee as an effective, broad-dieted, relatively docile, and easily managed pollinator--as well as a provider of honey and beeswax--brought it to the New World. Since its introduction, the honeybee, also called the domesticated bee, has become common and its habitat widespread.
Honeybees are social. Many of them exist together to support the whole colony in which the queen is the central member. A colony consists of 40,000 to 60,000 honeybees. Sterile worker females do most of the colony's work, caring royally for the queen; her sole purpose is to lay eggs. "Busy as a bee" means busy indeed. The workers build the honeycomb, tend the queen (always turning to face her when she passes by!), feed pollen to the young, clean the hive, forage for nectar and pollen, and convert the nectar to honey in their honey stomachs.
Worker bees also guard the entrance to the hive, scout for new food sources, and communicate to other workers information about new flowers and the richness of the nectar supply. The drone males aren't effective pollinators. Their sole function is reproduction, and when their mating duty is complete, they are ousted from the hive or killed by workers.
Most honeybee colonies live in man-made hives that can be moved from crop to crop for specific, managed pollination. The high number of bees in a colony makes their work significant. If a colony becomes too crowded, workers will swarm, separating from the first hive with a new queen. Then, they will build a new hive in a different location. If that colony escapes into the wild, the colony is called feral. If the new colony chooses an empty hive provided by an alert beekeeper, the beekeeper will have added nicely to his or her population of domesticated bees.
Africanized Honeybee (Apis mellifera scutellata)
The recent incursion of the Africanized bee into the United States is another worry to beekeepers. In 1956, an African subspecies of the honeybee was introduced into Brazil. The African bee looks just like the honeybee we know, but in tropical regions it is an even more effective pollinator and honey producer than the domestic honeybee. South American beekeepers found, however, that the advantage they saw in the bees' aggressive manner meant that the bees were also far more difficult for humans to manage than domestic honeybees were. The African bees are apt to sting fiercely in groups at the slightest disturbance, and they've attacked people and livestock, earning them the name "killer" bees. These bees have moved north and are breeding with the domesticated European honeybee race in southern subtropical regions, causing difficulty for commercial beekeepers there. The northern limits of this invasion are not yet known.
This medium- to large-sized insect is covered with black and yellow or orange hairs. Like honeybees, they're social insects with a queen that rules a colony. In the spring, the queen selects a nest site in the ground for a colony of 100 to 200 bees. The site may be an abandoned rodent burrow or a formerly occupied bumblebee nest. The queen first raises workers who tend the hive and colony. They forage, collect and store nectar, and care for the young. Late in the season, males mate with females who will be the next year's queens. Bumblebees are abundant on late-flowering ornamentals. Fall cold kills all members except the new queens.
Bumblebees have long proboscises (tongues) that enable them to reach nectar in flowers with deep nectaries (plant glands that secrete nectar). Plants such as tomatoes have pollen hidden inside anthers that are like saltshakers, making pollen collection difficult or impossible for most bees. The bumblebee has a specialized technique to actively collect such pollen: It tucks its abdomen under and shakes the pollen out of the flower and onto its body hairs.
Bumblebees are widespread, common, and polylectic pollinators (they will visit many types of flowers). Bombus grisiocolus and B. occidentalis are found throughout the continental United States. Bombus impatiens is small and common in eastern states, as is B. ternarius. If you want to attract many species of bumblebees to your garden, leave a clay pot with a hole in the bottom upside down in a garden bed in the spring.
Squash Bee (Peponapis pruinosa)
These native wild bees are solitary: They live on their own without a colony, and females lay single eggs in protective cells. Squash bees are specialized: they collect pollen and nectar only from the flowers of cucurbits (squashes, pumpkins, and gourds). In the late 1970s, Vincent Tepedino, research entomologist at the USDA Agricultural Research Station Bee Biology Laboratory at Utah State University, compared the squash bee's pollination skills with those of the honeybee and found the wild bee to be more efficient, considering frequency of visits and amount of pollen deposited on the flower. Squash bees are up at dawn, well before honeybees are active, so they can reach squash flowers early in the morning. Other scientists have observed the squash bee in a wide range of activities within the flower: resting, grooming, and mating in the flower structure, in addition to feeding on nectar and pollen. All that activity means more pollen collecting but only for a life span of about 2 months, until the food source is gone. Squash bees are found in most regions of the United States, but not in the Northwest.
Leafcutter Bee (Megachile)
The squash bee isn't the only one with dining preferences. The most famous member of this large and diverse species is the alfalfa leafcutter bee, M. rotundata, which was introduced from Eurasia to pollinate alfalfa. Leafcutter bees prefer legume blossoms, and collects pollen on a brush of hairs called a scopa, on the underside of its abdomen, rather than on leg hairs.
More than 140 species of leafcutter bees are found throughout North America. Many nest in wood cavities, such as holes left in logs and branches by insects or birds. Some species nest in soil. Females cut circular pieces out of leaves, using their sharp mandibles and bending their bodies downward as they rapidly cut. These bees use the leaf pieces to line their thimble-sized cell. A female leafcutter bee packs a protein-rich mixture of pollen and nectar into the cell and then lays an egg. The larva develops in the cell while feeding on the stored food and pupates over the winter inside the lined cell. The adult emerges in spring or summer at just the right time for pollinating.
Growers who provide this bee with a specially designed, open-roofed structure partly domesticate it. Beekeepers place these structures with machine-punched holes in boards to make it easy for the bees to nest. Keepers can remove cells containing eggs and store them in a climate-controlled environment until the bees are needed for pollination.
Mason bee (Osmia)
Mason, or orchard, bees are solitary bees of the same family as leafcutter bees. A female builds nest cells that are often lined with mud or pieces of flower petals or leaves; she places them singly or in cell groups inside a wooden cavity such as a hollow plant stem or old beetle-emergence hole in dead wood. Osmia ribifloris is found only from the Rockies to West Texas, but the habitat of other mason bees ranges throughout the continental United States to Costa Rica. To attract mason bees, you can make nesting areas with boards.
Sweat Bees (Halictidae family)
Though most species of this small bee, found throughout the continental United States, are black or brownish, some, such as Agapostemon femoratus, are bright metallic green. All species nest in the ground. Halictids have a range of nesting habits, from dispersed solitary nests to densely situated ones with individual bees sharing common entranceways to primitive social arrangements. Lateral tunnels end in a single cell. Halictid bees are common insects and good general pollinators.
This bee takes its name from its habit of landing on people to lick the salt from their skin. This bee will sting only if you swat at it. (However, you shouldn't swat at any bee.)
Alkali bee (Nomia melanderi)
This native western bee nests in moist alkaline soils near natural seeps and springs. Also a halictid bee, the alkali is easily recognized by its metallic green abdominal stripes. Western scientists and farmers attract this wild bee by building nest sites that simulate natural in-ground nests in alkaline soil. Although alkali bees are solitary, individuals nest near each other. This efficient bee is adept at pollinating alfalfa, clover, mint, onions, and celery.
Mining Bee (Andrena)
Many species of mining bees, which are found throughout North America, are important pollinators for a variety of plants. They collect pollen on leg and body hairs and take pollen and nectar to their cells through underground tunnels. All mining bees are solitary, but some nest in dense aggregations. The A. milwaukeensis, a boreal (northern) bee, can be found generally in regions from the Yukon to Nova Scotia and in high altitudes from the Appalachians to the Rockies.
Not Just Bees: Other Pollinators
Hover flies, also called syrphid flies. Gardeners need practice to recognize these flies because they look just like bees. In fact, that's the key to their protection from predators. Eristalis tenax mimics a honeybee, and E. flavipes mimics a small bumblebee. If you look carefully when a fly is at rest, it has only two wings, usually held apart and slightly raised from the abdomen. But bees have four wings, sometimes held crossed and flat against the abdomen when at rest. Also, a fly has round eyes and short, thread-thin, barely visible antennae; a bee's eyes are more crescent-shaped, and its antennae are longer, thicker, segmented, and elbowed.
To further mystify predators, the syrphid fly attempts to imitate the bee's behavior. It hovers over a flower or buzzes in erratic patterns over a food source, giving this group their common name: hover flies. Once you recognize these impersonators, you'll have no fear of being stung, because they can't sting. They're effective transporters of nectar and pollen, and you should welcome them to the garden.
Of the 111 beetle families found in North America, 30 are considered messy pollinators because they tramp all over a flower, feeding and defecating while effectively moving pollen around by picking it up on their hard cuticles and body hairs. The soft-winged flower beetle, Anthocomus bipunctatus, is one of the many beetles that transport pollen.
Butterflies and Moths
Generally, butterflies seek nectar by day, and moths do so by night. They pick up some pollen on their body and leg hairs when they rest and reach into a flower with their long, recoiling hollow proboscises. Some skippers (day-flying butterflies distinguished by their vertical upper-wing position when at rest) jam their hairy faces into a flower and withdraw them covered in pollen.
The current but temporary decline of honeybee populations is a perfect time to learn more about other pollinators, particularly when you view pollinators as one of the first links in the food chain. Whether yours is a fruit, flower, or vegetable garden in a small spot or on many acres, your plants need pollinators. Nature is a dynamic system containing many species, including specialists and generalists, and diversity is an essential part of that system. Because of the decline of the honeybee populations, we may find a resurgence of other beneficial insects. They include thousands of species hard at work drinking, walking, buzzing, and stepping all over the flowers, working to meet their own needs as well as those of many plants--and our own, too.
Make a Hospitable Habitat
Although the honeybee is an extremely valuable pollinating species, wild bees and other insects not often recognized as pollinators are also vital to our well-being. Spraying broad-spectrum pesticides destroys all insects: harmful, beneficial, and pollinating.
Gardeners can encourage the proliferation of the beneficial and pollinating insects by providing nesting sites, especially for solitary bees. Turn flowerpots with bottom holes upside down, and leave cut garden plants' stems exposed. Blocks of wood or logs with holes that are drilled about 1/4 inch wide and 4 to 6 inches deep provide convenient nest sites when placed in a protected area.
Amy Bartlett Wright, Portsmouth, Rhode Island, is a scientific illustrator and writer. For his help with this article, she thanks Terry Griswold, research entomologist, Bee Biology Lab, Utah State University.
Photography by Michael MacCaskey (top and bottom) and the National Gardening Association (middle)