Mistletoe was once considered a pest that kills trees and devalues natural habitats but, in recent years, it has been recognised as a species that has a large influence over its ecological community.
About 1500 species of mistletoe are found around the world with 94 species native to Australia, of which 71 are endemic. Two Australian species are root parasites while the others are aerial parasites. These attach to the host plant using a connection called a haustorium. For aerial varieties, it’s a bond that can promote hollow formation on tree branches after the mistletoe dies, providing nesting sites for birds and mammals.
Mistletoe fruit is high in protein, carbohydrates and lipids; while the leaves are high in nitrogen, phosphorous and trace elements. A broad array of animals depend on mistletoe for food, consuming the leaves and young shoots, while also transferring pollen between plants. Birds and animals will also disperse the sticky seeds. These consumers include insects, possums, gliders, honeyeaters and, of course, the mistletoebird. A study at Gundaroo found there were fewer birds in a site in which mistletoe had been removed.
One of the most beautiful and interesting consumers of mistletoe is a butterfly, the Amaryllis Azure. Found throughout Australia, it has iridescent blue wings but can be difficult to spot as it flies high among the tree tops where the mistletoe grows.
The Amaryllis Azure lays its eggs on mistletoe which grows on a tree that has been colonised by little black ants of the genus Papyrius. When the eggs hatch, the ants provide shelter for the caterpillars in their nest at the base of the tree or under the bark. The caterpillars are shepherded by the ants to a clump of mistletoe at nightfall to feed and then led back to their shelter at dawn. The ants protect the caterpillars through encouraging them to feed at night, when predators such as wasps and birds are less likely to be active. In return the caterpillars provide the ants with food in the form of a sugary substance produced from glands on their back.
Dr David Watson of Charles Sturt University estimates that mistletoe is now ten times more abundant in south-east Australia than it was prior to white settlement. This abundance may indicate the absence and scarcity of natural consumers or decreasing tree health. He recommends the best way to control excessive mistletoe is by addressing the underlying cause, such as putting up nesting boxes to encourage possums and gliders.