The Blossoms that Buzz in the Spring
On a warm day of my first spring in Alaska many long years ago, a friend took me for a hike through a patch of early flowers. I spotted a bee working among the blossoms. It was the skinniest, silliest-looking honeybee I ever saw. I commented on the poor creature's condition, which made my Alaska-born friend laugh. That's not a bee, she said. It's a hoverfly. Those little guys and bumblebees do about all the pollinating that gets done in the north.
She was right, but my mistake was understandable. Hoverflies do look a little like small, scrawny bees, though they seem to fly more like miniature helicopters. They hover in midair, wings an invisible blur, between forays onto flowers.
She also was right about pollinators. Most of us believe that pollen gets carried from plant to plant by domestic honeybees, without which it would be a flowerless world. Yet the world of flowers needs a whole array of pollinators, from butterflies and hummingbirds to bats and carrion flies. The variety of pollinators drops off at higher altitudes and higher latitudes. We do have a fair number of butterflies, but in Alaska and the Yukon, native flowering plants depend mainly on bumblebees and hoverflies, just as my friend said.
Bumblebees, like butterflies, are nectar hunters. Their long tongues can reach deep into comparatively big flowers, sipping from nectaries near the flower's base. They often get smeared with pollen in the process. The bumblebees harvest some of the pollen, adding it to the foodstore back in the brood chamber, but inevitably some grains escape to reach the next flower and perform their fertilizing work.
Hoverflies have shorter tongues, more suited to shallow flowers. In gardens, Alaskans can usually find them on flowers such as sweet alyssum, daisies, and marigolds. In the wild, they often frequent plants of the carrot family, such as cow parsnip.
But nectar isn't the mainstay of their diet. They eat the protein-rich pollen, dining directly from the flowers. Since they're not social animals, as bumblebees are, they don't pack any pollen home to feed the kids and cousins. (In fact, the larvae of many hoverfly species are carnivorous, eating any other small insects they can catch in the wet earth where they live.)
Though pollen is nutritious, it isn't easily digested. The exine, or shell, of a pollen grain is tough and superbly resistant to attack by alkaline or acid substances, such as stomach acids. However, the contents of a pollen grain are released if the exine is soaked in a sucrose-containing solution, and sucrose is a common sugar in flower nectar. Thus, as long as they lap up a little nectar now and then, the hoverflies can digest pollen.
Fortunately for the continuation of the floral species, hoverflies are messy eaters. Some pollen grains adhere to bristles here and there on the flies, and can be carried to the stigma of another flower.
Because hoverflies do not show the fidelity that bees do for visiting only one kind of flower at a time, experts have generally considered them inefficient pollinators. Recent work by Colorado scientists Carol Kearns and David Inouye has improved that reputation, however. The researchers marked flies with brilliant fluorescent powders, then observed the pink, yellow, and orange hoverflies to see how they moved among the flowers. The flies turned out to be pretty lazy; each one usually moved to an adjacent plant of the same kind to continue its foraging. Thus hoverflies make perfectly good pollinators, at least for plants that grow in patches.
Kearns and Inouye have written up their research in the April 1993 issue of Natural History magazine. Even though they worked at high altitude, there's enough similarity to high latitude conditions to make their article informative for any northerner wondering why those yukky little flies are so necessary to our beautiful flowers.
