The southern beech forests of New Zealand are home to a unique resource – honeydew – delicious to birds, fungi and insects. This sweet sticky substance forms the basis of an important ecosystem.

What do the insects look like?

Honeydew is produced by small scale insects that live within the bark of beech trees (Nothofagus spp.). These insects are very simple in body structure and are related to aphids. Rosa Henderson from Landcare Research describes them as looking like a little flattened sac with no wings or legs. They are just mouthparts with a very long anal tube, which is the one part of the insect that you can see protruding as a thin waxy stem from the bark of the tree. The tube often has a small drop of sweet sugary honeydew on the end (and, yes, you can eat it).

How do the insects make honeydew?

The scale insect inserts its mouth into the phloem of the tree – the internal tube that conducts sugar from the leaves throughout the tree to the roots. The fluid in the phloem provides the scale insect with all it needs to live, but the fluid has so much sugar that the insect takes in far more than it needs. Instead of using all this food and getting fat, it secretes the excess sugar through its anal tube, forming small droplets that are eaten by birds or fall to the forest floor.

What eats the honeydew?

These droplets are a rich source of food for a number of organisms. For tūī and bellbirds, it is an easy energy source. Tūī have been observed working their way up and down beech trees eating the small droplets of honeydew as they go. This food resource is available all year round, so in the winter months when nectar is scarce, the honeydew is an important food source for the birds.

Other organisms also benefit from this sweet sugary syrup. As the honeydew drops from the end of the anal tube, it covers the bark and ground surrounding the tree. This promotes the growth of black sooty mould fungi that eventually grow to cover the bark of the tree. These dense black fungi, which are very obvious in these southern forests, are an important food source for a range of animals, including several species of beetles and moths. These small insects also provide food for birds, and a complex web builds up – all sustained by the small scale insect lodged inside the bark.

Why are scientists interested?

Rosa Henderson studies scale insects throughout New Zealand. Her research has identified a number of new species found in the high alpine regions as well as in the bush within easy access of Auckland city. All of these insects are part of a food web supporting a range of native florafauna and fungi. Research suggests that the scale insects that inhabit beech trees actually help the tree to survive, but how this occurs is not yet known.

Nature of science

Knowledge about the sooty mould fungus ecosystem is another example of how scientists work together to find the answers to these complex situations. No individual scientist can possibly find out all that is needed to be known, so lots of scientists need to contribute their little bit of specialised knowledge, their piece of the puzzle, to explain the bigger picture.

Unfortunately, many of these insects are being threatened by habitat destruction or introduced species. Foreign wasp species have discovered honeydew, but instead of supping the drops as they form, the wasps have found that they can access more honeydew if they eat the end off the anal tube that is protruding from the tree branch. Temporarily this causes an increase in the amount of the honeydew that the scale insect produces, which the wasps eat voraciously. The wasps eat more and more off the end of the anal tube until they kill the scale insect.

In forests where wasps have been found, there can be up to a 90% reduction in the amount of honeydew that is available, which means no year-round food for tūī or bellbirds, no sooty beech mould, no beetles and moths, and less birds in the bush – just one more thing that is contributing to the loss of the dawn chorus in New Zealand native bush.

    Published 30 April 2009