Bee-friended mistletoe gets needed help

Small enough to ride on the head of a honeybee and sometimes mistaken for a fly, each drab bee of the genus Hylaeus in the United States tends to "literally go unnoticed, just another of the forgotten pollinators," observes bee expert Stephen L. Buchmann of the U.S. Department of Agriculture's Agricultural Research Service in Tucson.

 Half a world away, however, other tiny bees of that genus are winning considerable respect for an amazing engineering feat: They can unlock the tightly sealed blooms of New Zealand's imperiled mistletoe.

 These parasitic plants spend their lives in the limbs of trees. Lacking roots, they bore through bark and make themselves at home, gently siphoning off their water supply from their host.

 New Zealand's giant mistletoes, which can live a century and grow to 9 feet in height and width, produce large scarlet buds at Christmastime. The buds cannot open without outside help, however. Last year, Dave Kelly and Jenny J. Ladley of the University of Canterbury in Christchurch, New Zealand, described how the flowers' tough petals are sealed together in the floral equivalent of a childproof cap.

 The researchers also observed some native birds that twist off the ends of the fingerlike buds during the roughly 5-day window when a flower is ripe. Severe predation by nonnative animals, including rats, ferrets, and cats, is cutting the population of such birds, leaving the future of New Zealand's mistletoes in jeopardy. If their flowers aren't opened and pollinated, the plants won't fruit 3 months later to produce seeds for a new generation.

 In the Dec. 19-26 Nature, the Canterbury researchers and their colleagues now report that native Hylaeus can wrestle ripe mistletoe buds open.

"They crawl all over the top, digging in and biting," Kelly observes. "Arching their backs, they push and pull and heave." Often, nothing happens. "But when they're successful, the flower explodes open, and they hit [pollen] pay dirt," explains Kelly, reached while he was videotaping the solitary bees in beech trees on the slopes of Mount Cook.

 Most flowers, with their distinctive sizes and colors, "have evolved to be opened by only one or a small group of closely related pollinators" -- usually either insects or birds, notes plant ecologist Thomas Hemmerly of Middle Tennessee State University in Murfreesboro. "For the same flower to be opened by both, that's novel," he says.

 Adds Buchmann, this finding "helps get rid of the belief that there is an almost lock-and-key, one-to-one matching between a pollinator and its favorite blossom." However, "there remains a question of how fully [bees] are taking up the slack," cautions ecologist William F. Morris of Duke University in Durham, N.C.

 The bees don't appear to be nearly as efficient as birds in opening mistletoe buds. "That difference," Morris says, "might tip the balance between a plant population that's viable over the long term or [one that's] declining."

 In fact, for all the help that the bees offer in pollination, they can't disperse the seeds that develop in autumn. For that, Kelly says, "you've got to have a bird that will eat the fruit and move its seeds" -- via feces -- to another tree. This argues, he says, for controlling the native birds' predators.