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

As I mentioned last week, we have already had people coming in with leaves from their elm trees showing signs of elm leaf beetle attack.

This seems to be a bit earlier than usual, so if you have an elm in your garden, it might be worthwhile checking it over now.

That way, you can be on the front foot when it comes to the control of this significant pest.

Originally from Europe, this small insect has spread widely across our area and can cause severe damage to European elm species if left unmanaged.

American elms and aelkovas can sometimes also be affected, while Chinese elms are very rarely damaged.

Adult elm leaf beetles are small, about 6-9 millimetres in length, and easily recognised by their bright yellow-green bodies with distinctive black stripes across their wing covers.

The early signs of elm leaf beetle are the damage they cause, which is multiple tiny shot holes in the leaves.

These are caused by the beetle feeding on them after they have emerged from their winter hiding places.

This damage will progressively worsen as they move on through their lifecycle stages.

The elm leaf beetle develops through four life stages over the warmer months, going from egg to larva, pupa and adult.

It is the adult beetles from last year that survived the winter that are doing the damage now.

If you can interrupt the cycle at this stage, you will really limit the damage done as summer progresses.

The stage of their lifecycle and the time of year determines the best way to treat the problem.

During November, the main population of beetles will have emerged, and the female will have started laying eggs which are bright yellow, lemon-shaped, and grouped in a double row on the leaves.

After seven to 10 days, the larvae will hatch out of the eggs and will look like very small caterpillars about three millimetres long and black.

The hungry emerging larvae will feast on the leaves, sometimes skeletonising them to leave almost just the veins.

Treatment at this stage is by spraying the canopy foliage with a systemic insecticide like Imidacloprid, which controls both the beetle and the larvae.

You should also start preparing the control treatment for the next stage of their lifecycle.

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 this stage of the lifecycle 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 one metre-and-a-half or more above the ground.

This catches the larvae before they have a chance to pupate and once again interrupts their lifecycle, 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 lifecycle, the population of future generations should be greatly diminished.

Other treatment methods that can be used to control elm leaf beetle involve tree injection by an arborist or soil injection around the drip line of the tree canopy.

Both these methods, however, require quite a lead-in time to allow for the insecticide to make its way up into the tree canopy and should be used in conjunction with trunk banding.

Elm leaf beetles should not be ignored, as if left unchecked, they can completely defoliate a tree, with secondary generations compounding the damage even more.

Over several years, repeated defoliation weakens the tree, slows growth, and increases susceptibility to other pests and diseases.

This makes these stately trees unsightly and, in extreme cases, may even cause them to die.

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 in late winter or early spring.

Having an elm tree in your garden comes with the responsibility of being vigilant about the control of elm leaf beetles and preventing their spread throughout the area.

If you are unsure of how to best treat your elm tree, call in to the Garden Centre with a few photos of your tree and its trunk system, and we will run through your best options.

We can also show you the trunk banding we have done on the elms in our Golf Gardens.

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Friday, 11th February 2028

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