Honeydew

Forest and Bird, Volume 18, Issue 4, 1 November 1987, Page 14

Honeydew

by

and

Henrik Moller,

Kay Clapperton,

Peter Gaze,

Graham Sandlant,

Bruce

Jocelyn Tilley

Life blood of South Beech Forests

Honeydew is the name given to the carpets of shimmering silver drops that clothe the trunks of beech trees. It is formed from sugary sap bled from within the trees by tiny scale insects. Native birds and introduced insects compete to sip the energy-rich drops, thus forming a crucial link in a complex web of life within South Island beech forests. In this article a DSIR Ecology Division research team outlines the influence of honeydew and wasps on our native trees and animals. Bs in the bark of South Island beech trees lives a fascinating native insect, the honeydew beech scale insect. It has long piercing mouth-parts which slowly draw off sap from the sugar vessels (phloem cells) of the tree. A hollow white thread, which hangs from the capsule of the scale insect, is a waxy extension of its intestine. This ‘‘anal tube’’ is the insect’s plumbing system, draining its wastes and unused sap to the outside. The scale insect uses only some.of the sap it takes from the tree, and the rest passes through to accumulate as sugary drops on the tip of the anal tube. We call these drops ‘‘honeydew’’ because they taste sweet and shimmer like dew. Honeydew- infested trees are black because a sooty mould (a type of fungus) lives on the sugar of the honeydew drops that have been blown or washed onto tree trunks. The sooty mould in turn provides a moist and energy-rich substrate for the many insects which live within it.

Friend or foe of the tree?

Many people believe that beech scale insects kill their host trees because they bleed them of their sugar. Certainly, you can find dead trees which are black and knobbled from previous infestations of the scale insect. However, the growth and seeding of trees may not always depend on the amount of sugar they have; the availability of nitrogen-rich nutrients may be more important. Overseas research has shown that honeydew dripping onto the ground probably nourishes the soil bacteria, some of which fix nitrogen from the air. More nitrogen compounds may be formed amongst the roots of infested trees, which absorb them for their own growth. Scale insects could therefore indirectly be conferring a net benefit to their host tree.

Honeydew as Food

The drops of honeydew have a high sugar content, and are an important energy source for a variety of animals living in the

forest. Nectar-feeding birds, such as the tui, bellbird, kaka, and silvereye, take honeydew drops in the same way as they harvest nectar from flowers. The drops are taken to a lesser extent by a variety of other birds — even by seed eaters such as chaffinches. Lizards have also been seen feeding on honeydew. The importance of honeydew to nectarfeeding birds is shown by the many tui and bellbirds which flock to patches of beech forest in winter from nearby pine plantations where honeydew does not occur. More birds are found in forests with more honeydew. Because the drops are rich in energy, the birds can fuel-up quickly in the short days of winter to survive the long, cold nights. As well as sipping the drops, kaka eat the insects living in the bark and in the sooty mould on trees. These insects, together with the sooty mould and its associated sugar, are eaten also by kea, possums, rats, and even sheep.

There are many more insects on honey-dew-infested trees than on nearby uninfested trees. Ants are particularly common, but small beetles, flies, bumble-bees, and particularly honey bees and wasps abound. On hot sunny days you not only smell the sweet heady scent as you approach a tree heavily infested with honeydew — you can also hear it buzzing with bees and wasps collecting drops.

Wasps, the new invaders

An important newcomer to the New Zealand forest is the wasp. The German wasp has been in New Zealand since the 1940s, and reached the South Island honeydew forests by the mid-1950s. In the late 1970s a second wasp species, the common wasp, was found to be present also. Common wasps have now spread throughout most of the bottom half of the North Island and the top two-thirds of the South Island (Fig.1). Of the honeydew forests, only those of the

West Coast and northwest Nelson have not yet been colonised by common wasps. This year, German wasps were more abundant in this uncolonised area than they were in areas with both species. The most likely explanation is that common wasps have not yet reached this last corner of the country and when they do, German wasps will decline because of competition with the new species. Despite this, there were many more wasps in honeydew forests this year, which suggests that the new species is adding to the total numbers of wasps in our forests. In Europe, good years for German wasps are also good years for common wasps. If the relationship is similar in New Zealand, these first-year results from our study will indicate the relative abundance of the two species in years to come. Wasps have become so numerous that they are now by far the greatest harvesters of honeydew drops in summer and autumn in Nelson forests. One tree in our study area near Nelson had 360 wasps per square metre of trunk — the equivalent of about 500 wasps crawling over an average sized door! Wasp numbers fluctuate so much that we do not yet know if these are typical or exceptional densities. We found many more drops, much bigger drops, and drops with higher sugar concentration on tree trunks that we had covered with mesh screens to prevent wasps from taking the honeydew. Because wasps removed drops soon after they began to reform, the drops remained small and had a low sugar concentration. This meant that bees and birds had to lap up many more drops to get enough energy. In the months when wasps were most numerous, the task became so unrewarding

that the bees and the birds gave up gathering honeydew altogether. Our bird banding programme showed that many bellbirds and tui left our study area, at least temporarily. The next step will be to determine whether they can find enough food elsewhere to survive this shortage of their preferred food, honeydew, in late summer and autumn. Wasps also kill many native insects to feed their developing larvae. Some of the insects would have been eaten by insectfeeding birds, so both these birds and the nectar feeders may be harmed by the competition for their food. We have heard of wasps killing chicks in the nest, and this has also occasionally been reported overseas, so the wasps could be harming our native birds directly as well. Several ecologists have mentioned the possible impact of wasps on bird populations, but so far there has been no research to prove it. The huge densities of wasps that we found lead us to suspect that their impact has been very much underestimated so far. As with the introduced mammals before them, the wasps have further altered a balance of nature established over millions of years where our plants and animals evolved in the absence of new ecological invaders. Scientists of the DSIR Entomology Division are attempting ‘‘biological control’’ of the wasps by releasing a parasite which attacks the pupae of the wasp. In the long run, this may impose a new balance as wasps decline to low numbers and have less effect on our forests. Honey Bees Another introduced insect in our forests is

the honey bee. Honeydew is the main substance in beech forests which bees use to make honey. Wild bees living in the forest build their honeycombs in hollow tree trunks, and beekeepers are putting more and more hives in the honeydew forests because the dark, strongly flavoured honey made from honeydew fetches a good price in Europe. The main commercial harvesting of honeydew by beekeepers is in the foothill forests of the Southern Alps in Canterbury. Beekeepers pay a small levy to the Forest Corporation for using forestry roads when placing the managed hives. The honey bees, oblivious to recent restructuring of government departments, fly across the boundary to collect honeydew from patches of beech forests administered by the Department of Conservation within the mosaic of the Forest Corporation's plantation forests. Beekeeping in forests is a huge potential money earner for beekeepers and for the owners of the forests — it is claimed by some that far more money can be made from beekeeping in our native forests than from chopping them down. Research is now needed to see whether keeping extra bees in forests would significantly reduce the amount of honeydew left for native animals. The Web of Life Everything in ecological communities is potentially interconnected, and no plant or animal lives in isolation from other organisms sharing its community. The honeydew is a crucial link in a complex web of life within South Island beech forests (Fig.2). The beech scale insect takes its food from the tree. The tree loses some en-

ergy in the form of sugar, but may gain nitrogen; the sooty mould is nourished from honeydew drops spread on the tree trunk by rain and wind; the wasps, the bees, the birds, and other animals lap up honeydew for food; and the beech scale insect may even benefit from the increased flow of sap through it when the drops are harvested by these animals. These complex relationships are important for the health of the forest and its inhabitants, but they are not yet fully understood. Unfortunately, cuts in government funding are now forcing DSIR Ecology Division to scale down the study of honeydew despite its economic importance and its value for conservation. The Department of Conservation has partly come to the rescue by contracting our research focusing on the impacts of wasps on endemic insects and birds. The native species of our forests have coevolved over millions of years and have adapted to depend upon each other. Recently, humans have cut down much of the honeydew forest, and have introduced new species which may compete with native species for honeydew. The most recent newcom1 are the wasps, which probably also greatly reduce the number of native insects in our forests. This has altered the balance of nature as yet we do not quite know how dramatically, but studies are urgently needed to find out. # Further Information A 24-minute VHS video on DSIR Ecology Division’s study of honeydew and its use by birds and insects is available from the Publications Officer, Science Information Publishing Centre, P'O Box 9741, Wellington (cost $50 incl. GST).