Scale insects form the basis of an ecosystem in the native beech forests of New Zealand. Here is an explanation of this ecosystem and how introduced wasps are changing it.
This sounds like it could be the title of some horror movie – scale insects, honey dew, beech forests, and wasps! This is in fact a scary tale that is playing out in the native beech forests of New Zealand. If you’ve spent any time in the beech forests of the South Island you’ve probably noticed that many of the trees’ trunks are covered in a black mould. That black mould indicates the presence of tiny insects that provide an important food source for native wildlife.
A tiny honey dew drop at the end of a scale insects’ anal tube. Photo Landcare Research.
What Are Scale Insects?
Scale insects are very small and look like round, flat sacks with a long anal tube and no legs or wings. They live in the bark of beech trees, upon which they feed. These insects insert their mouth into the phloem of the tree (that is the structure inside the trunk of the tree that transports sugar within the tree).
The phloem gives the insects more sugar than they need to survive. In fact, they get too much and they secrete or get rid of that excess through their long anal tube. The tube looks like a thin stem sticking out of the bark of the tree and often has a drop of honeydew on it.
What Animals Use The Honeydew As A Food Source?
The high-energy honeydew is an important food source for native animals, such as birds, bats, insects, and reptiles. In particular, nectar feeding birds, such as bellbirds, tui, and kaka rely on the honeydew, especially during winter.
You can see the tiny anal filaments with honey dew drops here on this beech tree that’s covered in the sooty black mould. Photo Department of Conservation.
What Is The Sooty Black Mould?
Sooty black mould is a black fungus that grows on the bark of these native trees. The honey dew drips down onto the bark and the mould grows on that.
The mould often completely coats the tree in black. Many native beech forests on the South Island are covered in this black mould. These scale insects and the black mould live on beech trees, and tea trees (manuka and kanuka).
Generally, the mould is not harmful to the trees because it’s feeding on the honey dew excreted from the scale insect. However, if the mould grows on leaves it can prevent photosynthesis from occurring and thereby damage the tree.
How Are Introduced Wasps, Changing This Ecosystem?
Wasps have been introduced to NZ and now are abundant in the beech forests. They are altering this ecosystem by consuming up to 90% of all the honeydew produced. That leaves little for native animals.
The honeydew creates a plentiful food source for wasps, helping them to spread and become prolific in many native forests. Now when you go for a walk in many areas you need to be careful that you don’t get stung by wasps!