Honeydew

In the forests of the South Island, beech trees are host to tiny parasitic scale insects which produce droplets of a substance that is a vital part of the food chain – honeydew.

Early in their lives these scale insects (Ultracoelostoma assimile) bury themselves in the bark of beech trees and start sucking the sugary sap. Any sap they can’t handle is pumped out a hair-like anal tube together with other waste products to end up as a glistening drop of sugar-rich honeydew. So, honeydew is really a nice name for sweet, sticky bug poo.

Most sap-sucking insects (the bug family) excrete a similar product (aphids on rose bushes for example), but a single large beech tree may be home to hundreds or even thousands of scale insects all pumping out drops of honeydew. Happily they don’t seem to harm the trees much. Manuka are another species of tree which are also commonly afflicted by scale insects.

This honeydew is an important food source for birds such as kaka, bellbirds and tui, and for geckos and insects. The honeydew that isn’t consumed falls on the bark and the forest floor, promoting the growth of the black sooty moulds that coat the bark and nitrogen-fixing bacteria in the forest floor that in turn provide nutrients for the trees. Honeydew is thus a vital part of the forest ecology.

Unfortunately, millions of introduced German and common European wasps now compete with the native fauna for the honeydew, with the result that breeding rates for many native birds have dropped in the most wasp-infested areas.

In ‘mainland island’ areas where DOC has established intensively predator-controlled sanctuaries for native flora and fauna (such as the Rotoiti Native Recovery Project in Nelson Lakes National Park), wasp control is an essential aspect of restoring the natural ecology.