The Effect of a Heat Wave on Urban Tree Pests in Melbourne, Australia: Examples that May Inform Climate Change Tree Management

Abstract

"Climate change will have profound deleterious effects on many trees in urban environments; however, as in any biological system undergoing change, there will be benefits. On 7 February 2009, the Australian city of Melbourne experienced its hottest day on record (46.4 °C [115.5 °F]) after a heat wave. In the days that followed, the foliage of native Australian mistletoes, Amyema miquelii and A. pendula, growing on Eucalyptus camaldulensis were observed to lose their green color and turn gray. In large numbers, the mistletoes can cause significant stress, leading to tree death. In the aftermath of the record hot day, large numbers of mistletoes died, and 5 years later the level of mistletoe infestation remained low. On the afternoon of 7 February 2009, tens of thousands of elm leaf beetles, which heavily graze the mature elms of Melbourne (Ulmus procera and U. × hollandica), were found dead under the canopies of street trees, and numbers remained low for at least 5 years thereafter. Similarly, psyllids, Mycopsylla fici, and infestations of Ficus macrophylla, which can seriously defoliate trees, fell from high to undetectable levels in the month following the heat wave. The effects of heat waves and very high temperature days have significant implications for those managing pests in urban forests. Pest control programs were unnecessary in the immediate aftermath of the heat wave and hot days and for up to 5 subsequent years. This has positive implications for tight tree management budgets, but could also lead to a discontinuation of pest monitoring and control programs. Such an approach could see a return to high levels of infestation.