Look Out for Elm Leaf Beetle

Anyone who owns an elm tree will know what beautiful and valuable trees they are. Often a feature in any garden, they are also planted widely in avenues and public gardens throughout temperate Australia. In fact the elm was often used in Avenues of Honour, planted in memory of Australia’s fallen soldiers. These trees cannot be taken for granted though. Pests and disease have wreaked havoc on elms across the Northern Hemisphere. The most serious of these, Dutch elm disease, is not found in Australia. Strict quarantine has allowed Australians to enjoy perhaps the finest population of disease free European elms remaining anywhere in the world. In 1989 however, the elm leaf beetle Pyrrhalta luteola, was discovered damaging elms on Victoria’s Mornington Peninsula. This pest has since spread to the Melbourne metropolitan area and large parts of regional Victoria. Based on the experience in North America, it is anticipated that the elm leaf beetle will eventually spread to all parts of Australia where elms are grown.Will elm leaf beetle kill my tree?The elm leaf beetle will never be eradicated from Australia, however control measures are available which will reduce beetle numbers and prevent serious damage. There is no reason for a well managed elm to be lost due to elm leaf beetle. The elm leaf beetle is sometimes confused with Dutch elm disease but is not nearly as serious, unless of course it’s ignored.Elm leaf beetle. Why is it a threat?The elm leaf beetle has the potential to cause severe defoliation of susceptible elms, which can weaken mature trees and reduce their aesthetic and amenity value. Elms suffering from repeated elm leaf beetle attack are also more susceptible to other pests and disease. Elm leaf beetles commonly overwinter in and around buildings and can cause a nuisance when emerging inside buildings in large numbers during spring.Which elms are affected?Most of the elms planted in Australian streets, parks and gardens are European species including the English elm Ulmus procera, Dutch elm U. x hollandica, golden elm U. glabra "Lutescens", variegated elm U. minor "Variegata", and the weeping elm U. glabra "Camperdowni". Unfortunately all of these elms are susceptible to elm leaf beetle attack. Asian species however such as the Chinese elm U. parvifolia and the closely related Zelkova serrata are relatively resistant to elm leaf beetle. What do I look for?In late summer damaged foliage becomes obvious as adult beetles and larvae feed voraciously on leaves. Look for small holes in the leaves called "shot-holes". Shot-holes are caused by the adult elm leaf beetle. The beetle itself is about 6mm long, oval shaped, with characteristic black and yellow (sometimes olive) stripes. Another type of damage observed at this time of year is known as "skeltonisation". This is caused by the larvae (ie. the grubs) who strip the foliage, leaving only the skeleton of the leaf behind. Skeletonised leaves will turn brown and drop prematurely. A heavy infestation can cause a mature elm to be defoliated by mid-summer.What can I do to protect my elm?Several options are available to the home gardener wishing to protect an elm from elm leaf beetle damage.Non-chemical controlA safe non-chemical control is to trap larvae that migrate down the trunk between mid December and early February. Wrap adhesive tape around the trunk with the sticky side facing out (suitable only if the bark is smooth) or smear sticky gel (available from some nurseries) around the trunk of the elm. Larvae trapped on these bands will be unable to pupate and reach adulthood. These methods may not reduce damage in the initial year of application, but should reduce beetle numbers and damage in subsequent years.Chemical controlMore immediate control can be achieved by applying carbaryl insecticide to the foliage of elms in danger of defoliation. Apply a product containing carbaryl registered to control elm leaf beetle. Apply to the leaves whenever the pest is present. Remember to always read the label first.If your tree is too large to safely spray the canopy contact a pest control operator or tree surgeon (also called an arborist) licensed to apply pesticides. These professionals can discuss the range of services they offer for elm leaf beetle management, and can achieve very good results.Biological controlA spray made of the naturally occurring bacterium Bacillus thuringiensis subsp. tenebrionis has been trialed and found to be effective against elm leaf beetle larvae. This spray should become available in the near future. A long-term control for the pest may be the introduction of the elm leaf beetle parasitic fly. This parasite has been tested at Agriculture Victoria’s Keith Turnbull Research Institute against native and beneficial insects and was found to only attack elm leaf beetle larvae. Research is now directed towards importing and establishing this biological control agent in Victoria.