Natural History, May, 2001 by Laura Sessions
Sugar was a shared resource in a forest community until a greedy newcomer moved in.
Biologist E. O. Wilson has called invertebrates “little things that run the world,” because of their numbers, variety, and influence on larger organisms and even entire ecosystems. New Zealand is home to “little things” that, while each only a few millimeters long, have benignly modified about 250 million acres of the country’s beech forests. Known as sooty beech scale insects, these agents turn the resources of the beech trees into a substance crucial to their own survival and to that of other forest dwellers, from fungi to birds. The association of the insects and the trees is an ancient one, and the expansive food web in which they are actors was, until recently, intact.
Sooty beech scale insects (Ultracoelostoma assimile and U. brittini) are sap suckers, or homopterans, that grow in the furrowed bark off our species of southern beech trees (Nothofagus) in New Zealand. During its complex life cycle, the beech scale insect goes through several developmental stages called instars. The females pass through four stages, the males five. Second- and third-instar females insert their long mouthparts into the cells of a beech’s phloem–the tissues that carry nutrients through the tree–and suck up sugars. After satisfying their appetites, they excrete the excess sap and wastes through a waxy anal tube. A sweet liquid, called honeydew, accumulates one drop at a time at the tip of this tube, which looks like a thin white thread.
Homopterans are common and widespread. Most of the world’s 33,000 species produce honeydew, but few can match the beech scale’s enormous and constant output of the substance. In the Northern Hemisphere, honeydew producers such as aphids are active only seasonally, but beech scale insects draw off and convert energy from beech trees year-round, and they do so copiously during the austral summer. From January to April, the tree trunks in a southern beech forest often shimmer with a thick coat of honeydew, and the droplets’ heady, sweet smell fills the air.
In some forests, ten and a half square feet of tree trunk (think of the top of an average card table) may support as many as 2,000 scale insects. More than 40 percent of the food the trees have produced through photosynthesis may be lost to sooty beech scale insects. These beeches do not appear to be harmed, although for most plants, losses of much less than 40 percent of their energy reserves would be insupportable. Currently, scientists can only guess how the trees are able to withstand such a drain, but various theories are being explored. Possibly only the more vigorous and faster-growing beech trees are tapped by beech scale insects. Fallen drops may recycle sugars to the soil and thence to trees, or the insects may promote extra photosynthesis in host trees.
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