New Zealand Sweet Stakes

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.

Researchers have a better understanding of honeydew's huge importance to other organisms that live in southern beech forests. Because these forests are less diverse than many other forest types and because few of the resident plants provide fleshy fruits or abundant nectar, many native birds, lizards, insects, and other invertebrates rely on honeydew for a high-energy food, sipping drops directly from the threadlike tubes. (The beech scale itself benefits from honeydew feeders; removal of the sticky honeydew stimulates the flow of sap through the insect's digestive system, preventing "constipation.")

In the Northern Hemisphere, more than 250 invertebrate species--ants in particular--have been recorded feeding on honeydew. In European forests, for example, ants consume about two-thirds of the honeydew produced by aphids and other sap-sucking insects. New Zealand has only a few native ant species; here it is birds, rather than invertebrates, that avidly feed on the honeydew. While some New Zealand caterpillars and beetles do rely on honeydew for food, invertebrates more often benefit indirectly from a fungus that grows on the sticky drops. The fungus provides invertebrates with food and with lodging in its spongy interior and fissured surface. Aptly named the sooty mold fungus, this organism coats any surface where honeydew lands after falling from the insects' anal tubes--tree trunks, roots, shrubs, saplings, and even the forest floor. It may cover beech trees so thoroughly that their pale gray bark turns as black as charcoal, with new honeydew drops shining on top.

Caterpillars, beetle larvae, and other invertebrates find a home in sooty mold and become a source of protein for foraging birds, but the honeydew provides the birds with essential energy. Bell-birds and tuis--forest birds in the nectar-feeding family called honeyeaters--have brushlike tongues that enable them to lap up honeydew drops easily. They hop up the beech trunks, licking as they go and occasionally pausing to clean the sticky sugars from their feathers. Most frequently they feed on the branches in the canopy, possibly because more honeydew is found in the higher limbs of the tree than near ground level. Honeyeaters spend more time in forests with honeydew than in forests without it. The birds will flock to beech forests during the winter, when fruit and nectar are scarce; tuis may spend more than 80 percent of their feeding time harvesting honeydew when it is plentiful. Without the scale insect and its sugary excretions, these birds would be much less common in beech forests, to the detriment of some resident plants. Honeyeaters are the primary pollinators for native mistletoes and certain other nectar-producing plants. If these birds decline, the plants will no longer be able to produce new seeds. (See "A Floral Twist of Fate," September 2000.)