Elm leaf beetle
Description
Larvae are very small and initially black. As they mature, they can grow to 12 mm. ELB adults are 4 - 8 mm long, yellowish-olive green in colour with two black stripes. Although free flying, beetles are often spread by vehicles.
ELB larvae are the most damaging, causing skeletonisation of leaves where the whole leaf apart from the vein is eaten, causing the leaves to turn brown and fall prematurely. This significantly affects the trees ability to photosynthesise, consequently reducing energy reserves required for dormancy. Adult beetles produce holes in leaves, otherwise known as ‘shot holes’. If left untreated, ELB can rapidly defoliate a tree, particularly in warmer weather and several years of infestation can cause tree death.
During winter adult ELB adults take shelter and hibernate, emerging again in spring to feed. During spring, tiny eggs are laid in clumps on the underside of leaves. Eggs hatch 7 - 10 days later and larvae emerge, which commence feeding on leaves straight away. Larvae travel down the tree to pupate in cracks or the soil through December and January, emerging after 1 - 2 weeks as beetles. Depending on temperature, there are usually 1 - 2 generations per year.
Control
ELB can be effectively controlled through the use of preventative and chemical control tactics. Two natural parasitic enemies, including a small wasp (Oomyzusgallerucae sp.) and a small fly (Erynniopsisantennata sp.) are being evaluated for use in Australia. Additionally, a spray with Bt (Bacillus thuringiensis) has shown some promising results in controlling ELB. Healthy trees are less likely to be affected by ELB than trees under stress. Therefore, maintain good tree health by watering well, particularly during dry periods and apply slow release fertilisers in winter. Minimise compaction over the root zone and cover the root zone with mulch rather than lawn to encourage good soil conditions.Banding can help break the life cycle of ELB and involves placing a sticky band around the trunk of the tree prior to larvae migration. This method has shown to reduce damage to the leaves by up to 50 per cent in English elms. Timing is important as bands need to be applied at the right time of the ELB life cycle to be effective.
Canopy foliar sprays with insecticide can assist with the control of ELB, however this is only practical on younger, small trees. Trunk injection with insecticide is an effective control method, which is also environmentally safe. Soil injection with insecticides also provides control, however care should be taken when trees are suffering from moisture stress. Confidor® 200 is registered for the control of ELB in elm trees by soil injection.
References
ACT Government (2018), ‘Elm leaf beetle’, Access Canberra, https://www.accesscanberra.act.gov.au/app/answers/detail/a_id/2175/~/elm-leaf-beetleACT Government (2015), ‘Elm Leaf Beetle in the ACT factsheet’, https://www.environment.act.gov.au/__data/assets/pdf_file/0011/902279/Elm-leaf-beetle-in-the-ACT-fact-sheet.pdf
Department of Primary Industries, Parks Water and Environment (2013), ‘Elm leaf beetle’, Biosecurity fact sheet, https://dpipwe.tas.gov.au/Documents/Elm_leaf_beetle_fact_sheet.pdf
Elmsavers (Viewed Dec 2019), ‘Elm tree treatment’, https://www.elmsavers.com.au/treatment
Treeworks (Viewed Dec 2019), ‘Elm Leaf Beetle Treatment’, https://www.treeworks.net.au/assessments/elm-leaf-beetle/