How is Witch Hazel Pollinated?
While I was preparing my recent column on witch hazel, Dick Christensen, president of the Niagara Frontier Botanical Society, called my attention to an inquiry by Donald W. Stokes in his delightful book, The Natural History of Wild Shrubs and Vines. Stokes describes the two species of witch hazel in eastern North America: ³One blooms in late fall after the leaves have shed; the other [more southern species], sometimes known as Springtime Witch Hazel, blooms in late winter and early spring before the leaves appear.² He then asks, ³Who pollinates the flowers? Both species bloom at times when very few insects are out collecting food. I have watched the flowers when they are in bloom and the only visitors I have seen are ants.²
That question intrigued me, because it is hard to imagine a plant blooming at a less hospitable season for pollination. By the time witch hazelıs delicate yellow blossoms appear in mid-November, all the other fall flowers — the asters and goldenrods, the everlastings, the burdocks, teasels, and thistles — have faded to brown. More important, the plummeting temperatures are killing off the few insects remaining abroad. For example, all male wasps have already died, and only the few queens are now quietly aestivating in subterranean passages.
In The Book of Forest and Thicket John Eastman nominates as witch hazel pollinators fungus gnats, parasitic wasps, hover flies, and tachinid flies; Arthur Craig Quick in Wild Flowers of the Northern States and Canada suggests late flying bees. But Stokes has watched and never seen these insects; neither have I.
Now by quite remarkable chance unrelated reading has led me to a different answer to Stokesı question. It turns out that he and I have been looking at the wrong time of day.
Bernd Heinrich is one of my favorite authors. His most entertaining books are Ravens in Winter, about his experiences with these crow relatives in the sub-zero New England winter woods, and Bubo, about his fierce pet, a great horned owl. But Heinrich is also a serious scientist and like the very best of them, he asks significant questions, addresses those questions in creative ways, and provides evidence for interesting answers. One of his central research concerns is how insects adapt to temperature.
In a 1987 Scientific American article, he describes how a few species of owlet moth remain active when winter temperatures drop to near freezing. When they are at rest, the mothsı body temperature reduces to near that of their surroundings and they enter a state of torpor. In daytime and when temperatures are below freezing they hide on the ground under leaves that provide excellent insulation.
But when night comes, these moths have to fly to find food and that requires them to raise their body heat as much as 50° to activate their flight muscles. To do this, they shiver, the same response to cold that our human body applies in far less efficient form as what we call chills. When the moths fly, they rapidly lose this necessary 86° temperature: on long flights they must repeatedly stop and shiver to warm up again.
It is in this interesting article on the thermoregulation of owlet moths that Heinrich provides the definitive answer to our question about witch hazel pollination: ³Adult winter moths generally feed on the sap of injured trees, although on late-fall nights a few years ago I saw many of them feast on the blossoms of witch hazel, Vermontıs latest-blooming plant.² And as they fed, the moths inadvertently fertilized the flowers.
He concludes, ³Until that time no one knew just how the plant was pollinated.²