Elm leaf beetle treatment and management advice
Elm leaf beetle is a pest of Elm trees in Europe and America. It was detected in Victoria, Australia (Mornington Peninsula) in 1989. Elm leaf beetle has been present in Adelaide from 2011.
The Elm leaf beetle’s life cycle can be summarised as follows:
- The beetles hibernate in winter in sheltered places in the garden and around buildings.
- The adult beetles emerge in spring, feeding on leaves causing shot-hole damage.
- The adult beetles lay their first round of eggs in October and November (double rows of very small yellow eggs on the underside of leaves).
- The eggs hatch and go through 3 larval stages, each feeding on the leaves, leaving a skeletisation pattern. The larval stages cause most of the leaf damage.
- The final larval stage migrates down the trunk in summer to pupate and emerge as new beetles.
- The Elm leaf beetle’s life cycle repeats and 2-3 generations are possible in a year.
How does the Elm leaf beetle affect my tree?
The level of damage can vary from minor (only a small number of leaves affected with some chewing) to major (all leaves extensively damaged). The damage to the tree is largely aesthetic. Light Elm leaf beetle infestations are unlikely to have a significant impact on the long term health of your Elm tree. Heavy infestations on a regular basis have the potential to lower energy reserves with the potential to predispose your tree to other pest or disease agents over time. Maintaining good tree health is key in minimising the impacts of Elm leaf beetle on your trees health. The beetles have been known to invade the home and become a nuisance.
What trees are affected?
Elm leaf beetle affects European elm species including English elm (Ulmus procera), Dutch elm (Ulmus x hollandica), Golden elm (Ulmus glabra ‘Lutescens’), Wych elm (Ulmus glabra) and others. It has also been identified in Japanese zelkova (serrata). The Asian elm species such as the Chinese elm (Ulmus parvifolia) and Ulmus ‘Sapporo Autumn Gold’ have been less affected, with fewer cases of damage by the beetle reported.
As with any pest or disease issue, an integrated approach to managing tree health and the pest is required. This includes a combination of cultural practices, physical control and chemical control, as described below. Greater success is best achieved where all three strategies are used together.
The following principles should be observed in maintaining the health of your tree:
- Avoid any root disturbance within the dripline of the tree.
- Reduce the extent of hard surfaces, paving and plant competition around your tree.
- Maintain adequate levels of mulch around your tree. (75mm thick layer of organic mulch)
- Maintain adequate irrigation around your tree during dry or hot weather.
- Avoid heavy pruning and poor pruning practices.
Chemical control - Trunk injection
Trunk injection is our preferred method of treatment at present.
A systemic Insecticide (containing Imidacloprid) is injected directly into the trunk.
There is less ‘off target’ damage, where only insects living and feeding in the tree are affected (including beneficial insects). This is in contrast to spraying or soil injection where more ‘off target’ insects (and other organisms) are affected in the soil and surrounding area.
Smaller amounts of chemical are used in this method.
There is minor trunk wounding at the injection sites.
Physically sweeping up, or vacuuming the yellow pupal cases from around the base of the tree at regular intervals greatly reduces the population of insects that can emerge and fly back up into the tree.
Interrupting the larvae’s migration down the trunk can also provide some assistance in reducing insect numbers. A range of trunk banding methods may assist, such as horticultural glues and sticky banding.
What should I expect?
The uptake of the chemical depends on the tree’s transpiration rate, which in turn is affected by weather, soil moisture levels, level of existing leaf damage, tree health and other various factors. It can take up to 10 days or longer to spread throughout the crown. Branching habit can affect distribution of the chemical, particularly in the Golden elm.
Given the voraciousness of the insect, its ability to fly from tree to tree, its ability to be transported by people and vehicles and the fact that not all trees in your area will be treated, it is expected that adult insects will reach your tree, feed and lay eggs. The chemical treatment is expected to reduce the insect population to low levels, keeping tree damage below an acceptable threshold. This treatment is not expected to eradicate the pest or prevent all insect damage.
We anticipate that the treatment should be effective for about 2 years. We believe regular treatments every two years should provide suitable protection from the insect. More frequent treatments may be required, depending on the presence and level of reinfestation, and your tolerance to leaf damage. Some minor damage to treated trees is unavoidable and normal, and should not be a cause of concern. Please call our office if you have any questions.