he decline of honeybee populations has brought the agricultural community to the brink of a pollination crisis. The best hope for the long-term survival of many American farmers may be the revival of native bees. Yet they, like their domesticated cousins, face some daunting obstacles.
California's Central Valley, one of the most productive agricultural regions in the West, is a forbidding wasteland for native bees. Endless acres of orchards, fields, and suburban ranchettes are kept clear of anything that's perceived as a weed -- and that includes wild shrubs like ceanothus, buckthorn, and redbud, whose flowers are rich pollen sources for bees in early spring.
A few dozen honeybees patrol the dazzling purple-pink blossoms of a redbud growing in a small arboretum, tucked along the banks of a creek that crosses the campus of the University of California, Davis. Flying among them is a single native bumblebee, insulated on this chill afternoon by her thick lemon-yellow fur. She's loading pollen onto her corbiculae, hair-rimmed, basket-like structures on her back legs that hold the bright grains for transport to her colony.
Robbin Thorp waves his net like a magic wand and extracts the bee from the tangle of twigs where she's been foraging. With expert movements, he guides her into the pointed end of the net and pops her, unhurt, into a glass tube where I can study her up close. Thorp, an emeritus professor of entomology, has devoted a long career to native bees. As a graduate student in the 1950s he identified five species previously unknown to science. Now, though his beard has turned snow white and he's been officially retired for a decade, Thorp continues to spend time in the field. But these days he's more likely to be tracking the decline and disappearance of once-abundant bees.
When Claire Kremen, a conservation biologist at the University of California, Berkeley, set out in 1999 to study the contribution of native bees to crop pollination in the Central Valley, she called on Thorp to train her research crew in the art of identifying bees on the wing. Under his tutelage they learned to tell the common local species of bumblebees apart, to recognize the narrow yellow-black striping and streamlined shape of a squash bee and the blue-green iridescence of a metallic bee.
Kremen's study focused on watermelon, because the blossoms need a lot of pollen -- about 1,000 grains per flower -- to produce a marketable fruit. If native bees can do well pollinating watermelon, they're likely to succeed with just about any other crop. She and her field assistants spent long summer days walking transects in watermelon fields, counting the numbers of each kind of native bee they saw working the flowers. Kremen's research, among the first to examine the status of native bees on agricultural land in the United States, produced dramatic results.
Farms with no nearby oak woodland or chaparral have too few native bees to succeed without the services of rented honeybees. But those near remnants of wild habitat host native bees of many species, in numbers high enough to pollinate even a demanding crop like watermelon. The farms that fell into this category were all organic operations set on smaller plots of land tucked into hillsides where native vegetation survives. By contrast, conventional farms not only use a variety of pesticides but are set in the midst of the Central Valley's hostile landscape.
"Pollination is a valuable service that we're destroying through our land management practices," says Kremen. But she points out that there are many ways conventional farming could change to support bees. One is to grow cover crops like rye and clover, which aren't harvested but instead plowed under to enrich the soil after they've flowered. Farmers could also use roadsides and ditches to restore native plants and create bee-nesting areas. They could reduce their use of pesticides or apply them at night, when bees aren't flying. Growers ought to do these things, Kremen believes, not out of selfless concern for threatened bees but because, in the end, it will protect their own bottom line. Since honeybees -- which now pollinate up to $14 billion worth of crops annually in this country -- are in steep decline, native bees are needed as a backup. The costs of managing bee habitat could be offset by reductions in the amount a farmer spends on renting honeybees, a cost that continues to increase for many crops. In 1999, for example, U.S. plum growers paid about $6.4 million for honeybee pollination.
Kremen was able to discover which species are most efficient by "interviewing the bees." This involved shrouding watermelon blossoms in bee-proof veils, uncovering them just long enough for a single bee to visit, and measuring the pollen left behind. Some of the natives, including two species of bumblebee and the squash bee, do a far better job of delivering pollen than do honeybees. Kremen also noted that over the two years of her study, the numbers of native bees shifted. In one year, a few types of high-efficiency bees accounted for most of the pollination. The next, many species contributed. That finding argues for the need to maintain a diversity of bees, leaving enough flexibility for crops and their pollinators to survive shifting conditions.
"We need to have a balanced pollinator portfolio, and we don't right now," says Stephen Buchmann, founder of The Bee Works, an environmental consulting firm specializing in pollination issues. "Just like in investing, we need to have a balance between short- and long-term risk."
Buchmann acknowledges that honeybees are indispensable to modern agriculture, but he points out that they can't carry the burden of crop pollination alone. He believes that protection of native bee habitat and active management of native species must also be part of the solution to the pollinator shortage. The rise and fall of the alkali bee, recounted in his book The Forgotten Pollinators, coauthored with Gary Nabhan, illustrates both the great agricultural potential of native bees and the threats they face.
The native alkali bee, a solitary creature that digs its nest near seeps in the alkaline soil of western deserts, is a champion pollinator of alfalfa (alfalfa hay is a staple food for dairy cattle and other livestock). The plant's flowers are typical of legumes: The sexual parts are held under tension, and to gather pollen a bee must trigger their release, receiving a smack on the head in the process. Alkali bees, which are particularly well suited to pollinate wild legumes like lotus and locoweed, are unfazed by this experience. European honeybees avoid it.
In the 1950s, alfalfa farmers in Nevada, Idaho, California, eastern Oregon, and Washington began to create artificial nesting areas for alkali bees, seeding them with plugs of soil from natural bee beds. One result was a lasting expansion of the alkali bee population in the places where humans needed them most. Another was a boom in alfalfa seed production in the American West. With alkali bees working the blossoms, the yield of seed used to replant hay fields skyrocketed.
Then, during the 1970s, managed alkali bee populations began to crash. Some alfalfa growers believe that a shift in pesticide use on neighboring crops did the bees in: Alkali bees will fly a mile or more in a day and forage on a variety of flowers, so they could easily have picked up poisons beyond the alfalfa fields. Or perhaps alfalfa farmers' own use of insecticides to combat a common pest, the lygus bug, destroyed the bees. In any case, the lion's share of the U.S. alfalfa crop, worth $5 billion a year, now depends on the alfalfa leafcutter bee, a nonnative species. Canadian farmers produce the bees, which feed on the pollen of canola and alfalfa, and sell loose cocoons by the gallon to growers in the United States. About $30 million worth of leafcutter bees are purchased each year; efforts to raise them in this country sputtered when infectious diseases wiped out the young bees. One such disease, chalkbrood, is now hitting the Canadian industry. A collapse in the leafcutter bee population could wipe out most alfalfa production in the United States, with serious consequences for the dairy industry.
The alkali bee is not the only native with impressive agricultural talents. The blue orchard bee, an opalescent creature native to the western United States, can pollinate almonds, cherries, and other orchard crops far more efficiently than honeybees can. Since it's adapted to local conditions, it's hardier, too: It will fly at lower temperatures than the honeybee and work blossoms in the rain. In a four-year experiment at a cherry orchard in Utah, William Kemp of the USDA Northern Crop Science Laboratory in Fargo, North Dakota, found that fruit production doubled when blue orchard bees (affectionately known to bee fanatics as BOBs) were used in place of honeybees. Kemp and his colleagues are encouraging fruit growers to nurture BOBs on their land. A few entrepreneurs have begun to trap the bees in the wild and rent them to growers.
There can be risks in the commercialization of native species, however. When alfalfa farmers began to manage for alkali bees on their land, they were working within the bee's natural range. But when tomato growers discovered the power of bumblebees to increase their yields, the bees were treated like any other product in a globalized economy and were shipped from continent to continent, with disastrous consequences.
For decades, hothouse farmers used electric vibrators to pollinate their tomatoes, an expensive and sometimes labor-intensive process. The flowers of certain crops -- notably tomatoes and members of the tomato family including potatoes, eggplant, and peppers -- hold their pollen inside chambers with tiny openings. The grains are trapped, like salt in a saltshaker. For a bee to release the pollen, she must vibrate her body like a violin string, grasping the flower while using rapid-fire contractions of her flight muscles to produce a high-intensity buzz. Honeybees don't do this.
But bumblebees are masters of buzz pollination -- a fact that farmers did not think to exploit until Roland de Jog, a Belgian medical doctor and bumblebee enthusiast, came up with the idea of placing his pet bees among a friend's tomatoes. The experiment was a huge success, and in 1987 de Jog founded Biobest, a company based in Westerlo, Belgium, that rears bumblebees and sells them to tomato growers in both Europe and the United States.
In the early 1990s, a U.S. breeder shipped American bumblebee queens to Biobest. The resulting colonies were shipped back to the United States, carrying with them an infectious disease to which native American bees had no resistance. "That exotic disease wiped out Bombus occidentalis," says Robbin Thorp, who has documented the disappearance of the species, which was once commonly found everywhere from central California to British Columbia. The USDA restricts the use of bumblebees shipped from Europe, in an effort to keep them inside greenhouses and out of the wild. But the bees still escape, carrying infectious diseases with them. A recent study in Canada showed that levels of infectious disease are much higher among wild bumblebees near tomato greenhouses. Mistakes like these could cause other species to follow B. occidentalis into oblivion.