Melbourne has been said to have the world’s most significant population of Elm Trees, due to the Elm Leaf Beetle already wiping out most of Europe & America’s Elm trees.
The Elm Leaf Beetle (Pyrrhalta luteola) is here in Melbourne and is responsible for major damage to our significant population of Elm Trees (Ulmus sp.) It is destructive and eats into leaves which can cause a decrease in the tree’s ability to photosynthesize light into energy creating food. This results in stress & decline as energy reserves dwindle and may lead to the eventual death of the tree.
To manage the insects’ infestation, it helps to understand a little about its life cycle.
The beetles spend the cooler months in sheltered places such as wood piles, house eaves, cars & under the bark. As trees come into leaf, they emerge and fly onto the new growth where they begin to feed. They damage the leaves by eating small holes that look as though someone let off a shotgun through the canopy.
In about November, the beetles lay small yellow eggs usually on the underside of the leaf. Approximately eight days later the eggs will hatch larvae (looks like a tiny shaved caterpillar). The larvae will feed on the leaf leaving a skeletonized appearance. They can ruin a tree in just a few weeks. The mature larvae will then migrate down the trunk and pupate into mature beetles. You may see this at Christmas or soon after. In southern Victoria, there are usually 1 or 2 cycles per year.
The most appropriate control from this problem depends on the season, degree of insect infestation, tree size and it’s location.
Elm Leaf Beetle infestation can defoliate a tree which might ultimately cause the tree to die. Other factors such as drought, poor pruning/lopping or soil compaction will also contribute to a decline in tree health. However, one or two years of beetle damage is very unlikely to kill the host tree.
Currently, the most effective control measures for this infestation is chemical application. Either trunk/stem injection or soil injection. Both methods have advantages and disadvantages. It is also possible to do a canopy spraying. However, this is only suitable for some trees at certain times of the year and is a more expensive option in the long term.
If you are considering having your tree treated it is worthwhile discussing with your neighbours (who own Elm trees) if they would like to have theirs treated at the same time as there are reasonable savings available when all get treated at one visit.
The insecticide Confidor is injected around the root zone where it is taken up by the roots and translocated to the foliage. This is a safe and effective method of treating large trees. Based on our experience the tree can be protected for more than two seasons. The best time for this treatment is July to November although later treatments are effective provided soil moisture is not limited.
This treatment is surprisingly affordable, yet the results are invaluable.
Natural and environmentally friendly sprays include Entrust and natural Pyrethrum. Timing is very important. Entrust is effective against larvae, and should be applied in early December for best results. Natural pyrethrum can be applied from November to February.
Stem Injection involves injecting the treatment directly into the vascular system. This treatment is quickly effective and can be applied at any time of the year. However, we recommend timing treatments for September to November before beetle activity occurs. The chemical used for stem injection is SilvaShield and has the same active ingredient as used in soil injection.