Mistletoe - Part 2

Marion Jarratt

Note: This article originally appeared in 'Brigge', the newsletter of the friends of Burrendong Arboretum. The Arboretum is located in the central west of New South Wales, near Wellington.

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In Part 1 I looked at our cultural association with mistletoe and its occurrence in Australia and suggested that it was a species living on a knife's edge because of its dependence on external factors for seed spread and survival. How does it cope?

Pollination is not a problem - the flowers of most mistletoes are large and brightly coloured [often red], produce a copious nectar and are an important food source for birds which are the main pollinators. But - how can mistletoe seeds get to the right host?

Here in Australia one of the ways they do it is by mimicking the foliage of their host. One species which parasitises the inland casuarina is so like its host in the colour, texture and form of its foliage that even botanists can miss it! The ripe fruit itself is not very conspicuous but the mistletoe bird recognises the shape, colour and texture of the casuarina and searches for the fruit in this species. Because the inter tree movement of the birds is not influenced by the availability of fruit, the birds will select any casuarina to search and thus the seed is spread. In simple terms if you need to be dispersed by a fruit eating vector to a particular host species then it is a great advantage in looking like your host!!

Buds and flowers of a mistletoe (Amyema species) Photo: Brian Walters

The life of a mistletoe seed is fascinating.

The actual berry, called a pseudo berry - it actually lacks true ovules - is small, sweet and extremely sticky. Birds eat the flesh and what is left is basically an embryo with a sticky covering. Spread occurs when the bird voids the seed or wipes its beak on a branch to get rid of it. (Have a look at the ripe fruit next time you see a mistletoe and feel the stickiness. We had a mistletoe bird living in the garden some years ago and there were always 'seeds' adhering to the clothes line and even the sheets!).

The stickiness both stops the seed from drying out too fast and helps it to hang on to the branch while it gets its "roots" into the host. The root is called an haustorium, from the Latin haustor which means 'dragger of water', and of course that is exactly what it does, extracts water and minerals from the sap of the tree. It is just amazing that this little embryo can penetrate through the bark, the phloem, the cambium and right into the sap wood of its host. In fact the annual survival rate for seed of two species of mistletoe studied is as low as 19% and 31% so there is a bit of luck involved!

Having established a root in the host plant, most mistletoe species branch and develop leaves to carry on their own photosynthesis. Generally speaking a plant develops from the 2-4 leaf stage to a well foliaged plant approximately 1 metre in diameter in about three years and will act as a focus for infecting neighbouring areas in a radial manner for up to 10 years.

So - is mistletoe a noxious weed or a very important component of biodiversity?

On the one hand studies have indicated there is a positive relationship between mistletoe and diversity, especially in relation to birds. Ecologists suggest that, rather than being the cause of degradation , mistletoe is an indicator of landscape health. Presumably if mistletoe is present but not taking over then it indicates that the animal, bird and insect communities are keeping the parasite in check but still benefiting from the food and shelter it offers.

On the northern tablelands the flowering and fruiting of box and drooping mistletoe at different times of the year determines the movements and abundance of honey eaters in forest and wood land areas, while the foliage is dense and provides a safe nesting and resting site. The leaves are high in nitrogen, phosphorus and trace elements and the fruit rich in carbohydrates, protein and lipids. Both are also a favourite food of arboreal mammals such as possums and sugar gliders, while there is a unique range of insects and other invertebrates that benefit from mistletoe. In addition the leaf litter from mistletoe has a unique composition which is likely to have a positive effect on biodiversity. In fact it has been suggested that "it won't be too long before the strategic inclusion of mistletoe is a standard component of many revegetation projects". [P 10, Woodland Wanderings, Vol 4, Spring 2005].

Flowers of the mistletoe Dendrophthoe vitellina Photo: Brian Walters

On the other hand mistletoe has increased dramatically across our agricultural landscapes and in some areas is totally wiping out the remaining trees. It is thought that mistletoe has benefited from many of the changes made since European settlement; in particular suppression of fire, isolation of single trees, the creation of many additional bushland edges and the reduction in arboreal mammal populations would all seem to enhance its chance of survival and spread. The influence of fertilisers, especially superphosphate, and increased nutrient loads associated with livestock could also be factors - particularly in relation to tree health.

Few practical control methods are available. Insects and fungi do not seriously inconvenience Australian mistletoes so the development of biological control is unlikely. Lopping is an expensive operation (especially if a cherry picker is used) and virtually impossible on a large scale. Work has been done using chemical control, either by spraying (in some cases the mistletoe metabolises faster than the host foliage and will be more intensely affected) or by trunk injection (the results of this have been variable but in some cases on certain species of eucalypts 2,4-D proved to be an effective systemic poison) but work was confined to only a few species of eucalypts and little has been done lately in this area.

A very interesting long term study is being carried out in the Holbrook region in New South Wales by Charles Sturt University. The first focal resource to be studied is mistletoe. Study sites consist of 40 x 1-25 ha blocks of grassy box woodland and dry foothill forest. At 20 of the sites all mistletoe [5,000 clumps weighing over 40 tonnes] has been removed. The remaining 20 blocks will act as controls. Over the next two decades biodiversity, tree health, mistletoe spread and other variables will be monitored and it is hoped that a much clearer picture of the factors influencing mistletoe ecology will emerge.

References:

From 'Brigge', the newsletter of the Friends of Burrendong Arboretum Inc., July 2006.

Australian Plants online - 2008
Association of Societies for Growing Australian Plants