Growing for Success | Stop elm beetles, save your trees now!

By Riverside Gardens

Oct 23, 2024

*A serene moment beneath the elm's leafy canopy.* Photo by Contributed

Larry Smith and the Riverside Gardens team talk all things pots, plants and pruning in their weekly gardening column.

*Golden leaves, golden moments with a golden elm tree.* Photo by Contributed

As the temperature slowly rises over the spring months, we also see an increase in the number of little critters wanting to feast on our plants with all their lush foliage offering a smorgasbord of treats to choose from. Hoards of aphids and thrip come in waves and hungry caterpillars seem to appear from nowhere, all content on enjoying our gardens. In a lot of cases, these critters can be allowed to run their course without too much concern but when it comes to pests like the elm leaf beetle, it requires a concerted effort to try and minimise the damage they can cause.

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Elm trees are beautiful stately trees that look magnificent in the right place, but if left to the ravages of these small beetles they can become very unsightly, defoliated and unhealthy.

Elm leaf beetles are from Europe and feed pretty much exclusively on the European elm species. American and most Asian elm species are less severely affected with most Chinese elm cultivars, Zelkovas, and many newer elm cultivars very seldom touched.

The elm leaf beetle develops through four life stages over the warmer months, going from egg to larva, pupa and adult. The stage of their life cycle and the time of the year determine the best way to treat the problem.

Adults commonly survive over winter in bark crevices, woodpiles, or nearby sheds and buildings. In spring, they fly to nearby elm trees and start chewing on the leaves. This usually occurs in early October, and you may notice some shot-hole damage start to appear on the leaves. Treatment at this stage is by spraying the canopy foliage with a systemic insecticide like Imidacloprid.

By November the main population of beetles will have emerged, and the female will have started to lay eggs which are bright yellow lemon-shaped and grouped in a double row on the leaves. After seven to 10 days, the larvae, which look like very small caterpillars about two millimetres long and black in colour, will hatch out of the eggs. The emerging larvae will feast on the leaves, sometimes skeletonising them to leave just the veins. Treatment at this stage is still foliage spraying, but you can also start preparing the control treatment for the next stage of their life cycle.

As they grow, the larvae will change to yellow and black and start to migrate down the trunk to pupate in the ground or in the textured bark at the base of the tree. Trunk banding can now be used to break the life cycle of the beetle. This is done by applying two sticky gel bands like ‘Go Natural Tree Guard’ around the trunk at least a hand span apart and about a metre and a half or more above the ground. This catches the larvae before they have a chance to pupate and interrupts their life cycle, preventing the next generation.

Larvae that do make it down the trunk will pupate for about 10 days, then emerge as adult beetles and fly back to the canopy to start the cycle again. In Northern Victoria, at least two cycles can occur each year if left untreated, but by interrupting the life cycle the population of future generations should be greatly diminished.

Other treatment methods that can be used to control the elm leaf beetle involve tree injection by an arborist or soil injection around the drip line of the tree canopy. Both these methods require quite a lead-in time to allow the insecticide to make its way up into the tree canopy.

The elm leaf beetle is a significant pest to elm trees throughout most of Victoria and can completely defoliate a tree if heavily attacked. If left untreated over a couple of years, this damage can lead to the demise of the tree, so early treatment is the best policy. It also requires people with elm trees to take responsibility and treat their trees to stop it from spreading to other neighbouring elms.

A healthy tree is more likely to bounce back quicker from elm leaf beetle damage, so keep the water up to the tree in dry periods, have a good layer of mulch over the root zone, avoid compaction under the tree and give them a good fertilise during late winter to early spring.

If you are unsure about the best way to look after your elm tree, call in and have a chat. We can run through your options and show you how we have treated the large weeping elm in our golf gardens.