Nature Matters, with Mary Ralston

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At this time of year many people go for walks and picnics in our reserves and national parks. Beech forests are lovely cool places to visit on a hot day and they are often good places to see some of our native birds, such as bellbirds, fantails and tomtits.

About a quarter of New Zealand is covered in native forest, and about half of these native forests contain nothing but beech. Beech tree trunks are often covered with a soft black coating – this is sooty mould, a fungus, not part of the tree itself. The sooty mould occurs because scale insects, which are a bit like aphids, live within the bark of beech trees. They are very simple insects, consisting of of just mouthparts and a long anal tube, which is the part you can see protruding as a thin filament from the bark of a beech tree. It often has a small drop of sweet sugary honeydew on the end.

The scale insect inserts its mouth into the bark of the tree and survives on the tree’s sugary fluid. But more syrup comes out than it needs, so it secretes the excess sugar through its anal tube. These small droplets are a valuable food for native birds, and unfortunately, introduced wasps. For tui and bellbirds, honeydew is an easy energy source which is available all year round.

Some of the honeydew drops from the end of the anal tube and covers the bark and ground near the tree, providing food for the black sooty mould fungi that cover the bark of many beech trees. And the interconnections don’t stop there: the fungi are an important food source for a range of animals, such as beetles and moths which in turn become food for birds.

But this wonderful food web has been disrupted in many forests by the wasps, which steal the honeydew. This puts pressure on the native birds which already struggle because of introduced predators such as rats and stoats. Wasp control operations are conducted in many of our foothills beech forests and will begin again in late summer – this benefits human visitors and the resident native birds and insects. Have a look for the droplets of honeydew when you’re next visiting a beech forest.