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Description and Life Cycle
Adult Elm Leaf Beetle
The beetle hibernates in sheltered places during winter. In spring the beetles emerge and feed on the young leaves of most elm species grown in Australia. Their presence is made known by small ‘shot-holes’ appearing in elm leaves. The beetle is the size of a small pea, and is yellow or olive with black stripes. Elm Leaf Beetles can fly up to approximately two kilometres in search of Elm trees to feed on. They are also known to be ‘hitchhikers’ where they may hide under windscreen wipers, in tyre rims or other crevices in vehicles or trailers/caravans being towed and emerge when the vehicles stop traveling to search for Elm trees in the area.
Adult Elm Leaf Beetle damage
The adult Elm Leaf Beetle chew on the leaves creating ‘shot holes’ in the foliage. The damage is usually minimal (except in heavy infestations) and doesn’t inhibit the trees ability to photosynthesize. Depending on the level of damage sustained, and the overall health of the tree (i.e. if the tree is struggling through a drought), will determine the impact to the health of the tree.
Elm Leaf Beetle eggs
As the season advances, the beetle lays tiny eggs on the underside of elm leaves. The eggs are bright yellow and laid in a double row on the underside of the leaf. They hatch in 7 to 10 days. At first the larvae are very small and almost black. They feed on the leaves by eating the green matter, leaving a skeleton of leaf veins. The larvae are a ‘sap sucking’ pest whilst the adult beetles chew the leaves. (this image shows the eggs laid, the images below show the various larvae stages). It is possible to have larvae damage even after treatment as the adult beetles can fly into a tree and lay the eggs prior to eating the leaves, the treatment is only effective when the adult beetles or larvae physically ingest the leaves and the treatment. This is to protect all the beneficial insects that may inhabit the tree, only pests that eat the leaves are affected by the Elm Leaf Beetle treatment.
Elm Leaf Beetle larvae
The larvae increase in size in two more stages (called instars) reaching a length of 12 mm, when they are described as almost black or black and yellow and ‘caterpillar-like’. The image shows the larvae at the final instar before the pupae stage.
Elm Leaf Beetle larvae damage
The larvae are responsible for most of the damage sustained to an Elm tree. While the adult beetle chew the leaves, the larvae suck the leaves creating a ‘lace'-like’ effect. This reduces the trees ability to photosynthesize. In severe infestations, the tree will be defoliated (losing all its leaves) and use energy reserves to to grow new foliage. This can result in the tree being under stress and, particularly in times of drought, this can result in dieback or premature death of the tree if the defoliation is severe across several seasons.
Elm Leaf Beetle pupae
At this stage, depending on the regional climate, the larvae migrate down the trunk of the tree and turn into pupae (see the yellow pupae in the photo to the right), either on the ground around the trunk, or in crevices in the lower part of the trunk.
Elm Leaf Beetle cycle
After approximately two weeks, new beetles emerge from the pupae and in warm summers or in warm climates the new generation continues the breeding cycle. The new generation of beetles continues to feed on the leaves of elms for several weeks, but with the onset of cooler autumn weather seek shelter for winter in various places, including wood-heaps, sheds, houses and cars and re-emerging in mid Spring to continue the cycle for the next season.
When should I treat my trees?
The adult Elm Leaf Beetle emerges from hibernation usually during October, depending on the seasonal conditions. This coincides with the Elm trees breaking dormancy and the new seasons foliage growing. We recommend that the provision of the Elm Leaf Beetle treatment be provided from early Spring, before, or as the adult beetles are emerging from hibernation but preferably before the larvae emerge typically during late Spring/ early Summer. Treatment can be provided into Summer but assessment of the health of the tree/s should be undertaken to ascertain whether the level of infestation has negatively impacted the health of the tree, placing the tree at risk of premature decline or death.
