Mistletoe
Marvellous mistletoe
by Helen Moss
Continental Australia is home to at least 84 species of mistletoe. Most are found in tropical areas, but 12 species occur in Victoria. Amazingly, none occur naturally in Tasmania.
Mistletoe can be found across all types of native forest and shrubland communities from the tall Ash forests of the Victorian highlands to the coastal mangroves, from mallee to mulga. Most people encounter mistletoe on suburban street or park trees or in roadside trees along country roads.
Mistletoes are an intriguing and important element in Australia’s remarkable biodiversity and play a crucial role in providing a food supply for several native birds, possums and insects. Their dense growth also provides shelter and nesting or resting spots for birds and small mammals.
Human association with mistletoe extends back for millennia and modern medicine is now investigating its potential as a cancer treatment.
Certainly mistletoes can occasionally cause damage to trees and they have been at least partially blamed for tree decline and death in rural Australia, but there are invariably other contributory circumstances, such as land clearing. Although some people regard the evergreen mistletoes as unsightly on deciduous trees during winter, they rarely do any harm and are an important source of nectar for some native birds.
Once people put aside the popular image of mistletoes as harmful parasites and understand their ecological significance, the beauty of these fascinating plants becomes obvious.
Mistletoes in Human History
Mistletoes have been both revered and vilified during millennia of human association. The Gauls thought mistletoes were sent from heaven as a cure for infertility and sterility. The Druids, who worshipped the Oak, thought them to be very powerful because one species grows on oaks, and used them in sacrificial ceremonies, first cutting them from the tree with a golden hook.
The Celts called mistletoes ‘l’herbe de la Croix’, believing that the cross upon which Christ died was made from mistletoe wood. Thereafter, mistletoe was banished from the earth and could only exist through the goodwill of its hosts.
In Europe and North America, mistletoe plays a part in Christmas festivities. Almost everyone is aware of the tradition of kissing under the mistletoe, but few are familiar with the origins of the tradition. Mistletoe is evergreen, while most European trees are deciduous, and European species produce fruit in mid-winter, while almost everything else is dormant. Thus, the mistletoe was a symbol of fertility, and kissing under the mistletoe was thought to ensure conception and childbearing. This tradition was transferred to North America, where similar conditions prevail, with European colonisation, where, likewise, the evergreens mistletoe, holly and ivy are associated with Christmas
Australians, in their land of evergreen trees, have never taken to this tradition, but there is no evidence to suggest that Australians are any less fertile than their northern hemisphere counterparts!
Mistletoes have been widely used in herbal remedies for everything from St Vitus’ dance to heart disease, whooping cough and even the stitch. Aborigines ate the sweet fruit and used the foliage of Amyema species, usually steeped in water, for various medicinal purposes.
In Australia, many people regard mistletoe as a pest to be eliminated. Mistletoe was proclaimed a pest plant in Victoria in 1904. In 1927, the then Forest Commission initiated a program of lopping off affected branches in forest areas. This was an exceedingly expensive exercise and was completely ineffective, because the prevailing ecological conditions which encouraged mistletoe growth remained unchanged.
In 1941 the Victorian School Paper contained and article by Edith Coleman suggesting that school students could form a ‘mistletoe army’ to destroy the ‘menace’. In 1948 the Mayor of Port Augusta sent possums to the Pichi Richi Pass area of the Flinders Ranges, where it was said that mistletoe was killing valuable trees. 1949 saw the first use of flamethrowers to kill mistletoe, in South Australia.
During the 1980s, experiments were conducted with the aim of controlling mistletoe by injecting herbicides into the trunks of the host trees. Not surprisingly, it was found that the method used to achieve 100% mistletoe mortality also killed most of the trees.
Mistletoe biology
The main agent for dispersal of mistletoes is the Mistletoe Bird, Dicaeum hirundinaceum. As the name suggests, bird and plant evolved together and are dependent upon one another for survival. The Mistletoe Bird has a specially modified simple, tubular gut so the mistletoe seed is not destroyed during digestion. Mistletoe seeds may remain inside the bird for as little as 25 minutes.
Mistletoes produce berries (below) which have a leathery outer skin and sticky flesh surrounding the seed. The thick, leathery skin restricts the amount of oxygen available to the embryo, and germination commences immediately skin is ruptured. The embryo produces a small shoot which has a bulbous tip. This bulbous tip expands to form a ‘holdfast’ as soon as it touches a solid base. As it expands, the holdfast adheres to the branch and its internal tissues grow into the host to reach the sapwood, from which the mistletoe plant extracts moisture and soil minerals.
Mistletoe birds are said to perch along branches, rather than across them like most other birds, so that when the seed is voided, it falls onto the branch upon which the bird is perched. Mistletoe Bird droppings consists of three to five mistletoe seeds with enough of the sticky flesh still attached so they stick to branches.
Seed germination is most successful on small branches because the bark is thinner, allowing easier penetration by the germinating mistletoe plant. Until the young mistletoe’s connection to the host’s sapwood has been successfully accomplished, it relies for its survival upon rain and dew. Once established, the young mistletoe will grow and branch, producing flowers after about three years. Although a eucalypt host may live for more than 150 years, a mistletoe’s life is limited to only two or three decades.
Mistletoe plants usually kill the host branch beyond their point of attachment, and while the presence of a few mistletoe plants will have little effect on the host, infestations comprising more than 50% of the crown will cause a significant decline in the health of the host. The occurrence of mistletoe increases on isolated trees, on partially cleared land, on roadsides and at forest margins, so it may be that mistletoe establishment is aided by the greater availability of light.
Although many native host species survive bushfire, mistletoes are fire sensitive and almost always die. This raises the possibility that mistletoes are more common than they were prior to European settlement, both because bushfires caused by lightning were not suppressed and because the fires started by Aborigines for hunting or for regeneration of grassed areas had the secondary effect of reducing mistletoe numbers.
Brushtail Possums are known to eat mistletoes, but during the 1930s, millions of Brushtails were killed for their skins. A decrease in possum numbers in rural areas, coupled with the destruction or fragmentation of their habitat, may also have resulted in an increase in mistletoe numbers.
Silvereyes, small fruit and insect-eating birds also eat mistletoe berries and disperse the seed. It is suspected that some introduced birds disperse mistletoe seeds, which could account for the frequency of mistletoes on exotic trees in urban areas.
Other Mistletoe users
Although Mistletoe and Mistletoe birds are dependent upon each other, many other creatures are reliant to a greater or lesser extent on mistletoe. The Painted Honeyeater, Grantiella picta (left), a species considered rare in Victoria, is dependent on mistletoe fruits for survival. While the Mistletoe Bird feeds almost exclusively on mistletoe berries and leads a migratory life flying along regular migration routes which correspond with the ripening of the berries, Painted Honeyeaters only nest where mistletoe is common. These honeyeaters migrate to the Box Ironbark forests in northern Victoria in spring and now have greater protection due to the very recent declaration of new Box Ironbark reserves.
Altogether, some 41 species of birds have been recorded feeding at mistletoe flowers and 33 species eat the fruit but, because they digest the seed more thoroughly, do not disperse viable seed. Mistletoes flower abundantly annually, while other nectar sources like their eucalypt hosts flower irregularly, and have a wide geographic distribution. This makes them an extremely important nectar source for birds, butterflies, Brushtail Possums and Yellow-bellied Gliders.
About 25 species of native butterflies and moths feed on mistletoes during their laval, or caterpillar, stage. Of those, eight species of Delias (‘Jezebels’ or ‘Union Jacks’) are entirely dependent on mistletoe. In southern Queensland alone, more than 30 species of butterflies and moths feed on mistletoe. Several, like the Genoveva Blue, are also dependent upon the presence on particular species of ants, which help guard the caterpillars from predators in return for a sweet secretion produced by the caterpillars.
Mistletoes are, without doubt, an important part of the fascinating and complex tapestry of life.
Thank You to Helen Moss Dip Hort. and CCS member for this article
