City trees host pest invaders
From the Spring/Summer 2009 edition of Bush Telegraph Magazine.
The invasion of a North American bug into some of Sydneys most attractive street trees is concerning local residents and city visitors, not to mention those responsible for ensuring our native trees are protected from destructive pests. NSW DI&I researcher, Dr Deborah Kent, reports.
A hybrid between the American and Oriental sycamore trees (Platanus spp.), the London plane tree has been a common street planting for nearly a century, especially in Sydneys inner suburbs. It features distinctive deeply lobed or palmate leaves, round burr-like flowers and clusters of fruit on a stem. The bark is usually flaking, but on some larger trees the lower trunk bark is thick and furrowed.
In recent years, the leaves of some of these city trees have turned a mottled white and become droopy, while in some cases leaves have turned brown and dropped earlier than in past autumns.
While some of these symptoms are a result of recent drought conditions, there is another more significant cause - the incursion of an exotic bug. The Sycamore lace bug (Corythucha ciliata) is native to North America but has also spread to Europe, Japan, South America and now Australia.
The bug was first identified in Sydney in late 2006 by an overseas scientist visiting the Australian Museum. Initial surveys found it in many Sydney suburbs, with most trees in the inner suburbs affected. Subsequently, it has been found throughout Sydney and regional NSW as far south as Gundagai, west to Orange and north to Newcastle.
It has not yet been found outside NSW, with other states maintaining border and airport surveillance.
London plane trees provide shade and beauty in many urban and rural streetscapes. They are popular as they are disease resistant, grow rapidly in poor soils and are tolerant of pollution.
The long-term effect of Sycamore lace bugs on the health of these trees is yet to be fully assessed, but severe leaf damage and reduced growth are already evident. Control methods including chemical and biological techniques are being investigated.
The risk posed by the incursion of this exotic bug to an exotic tree may initially appear irrelevant to the forest industry. But it highlights the pathway an exotic insect or disease can use to travel from the city, the most likely point of entry, to the bush, as well as demonstrate the speed and spread of colonisation.
The Sycamore lace bug incursion is a concern for all those managing forest health. It is also a reminder that everyone can play a part in safeguarding our forest trees. An exotic pest can reach native forests by first passing through urban trees - it is in the cities that we may see them first and have a chance to stop them.
It is not hard to imagine what would happen if an exotic pest or disease which attacks a greater variety of tree species, including eucalypts, should arrive in Sydney and spread into our native forests and plantations via urban trees.
Reports of unusual changes on trees can be reported to the national exotic pest hotline on T: 1800 084 881.
Dr Deborah Kent Forest Resources Research, Sydney
