TreeWorks is taking bookings now for Elm Leaf Beetle treatments.
The Elm Leaf Beetle (Xanthogaleruca luteola) is about 4-6mm long, green and brown colour with two stripes. The beetle and its caterpillars can totally destroy an elm tree in one season. As elm trees come into leaf with the arrival of spring the adult beetle emerges from its winter hibernation. The beetles feed on the leaves in the canopy of the tree where they lay yellow eggs on the underside of the leaves that then hatch as black and yellow caterpillars or larvae. The larvae feed on the green tissue of the underside of the leaf. The larvae then crawl down the trunk and pupate in bark cracks or around the base of the tree. The pupate emerge as adult beetles and return to the foliage to continue the cycle of destruction.
- March – September: Hibernation
- October: Beetle emerges
- November: Eggs laid
- December: Larvae
- January: Pupae
- February: New adults
- Leaf damage to elm trees occurs between August and February.
- Shot holes in the leaves caused by the adult beetles.
- Skeletonisation of the leaves caused by larvae.
- Defoliation where leaves are falling off out of season around August to November.
- Discolouration can be another sign around December to February.
Keep elm trees free of dead limbs and suckers as well as minimizing drought stress to give them better resistance. Keep elm trees watered in dry periods. Fertilise in late winter with slow release fertiliser. Avoid compaction of soil over root zone. Encourage good soil conditions by covering the root zone with mulch rather than lawn.
Treatment The Elm Leaf Beetle will never be eradicated but it can be managed.
Early treatment is essential and can include:
- Trunk injection Trunk injection with Imidacloprid* is considered to be the most effective and environmentally sound option. The insecticide is injected directly into the trunk of the tree. The tree transports the chemical to the leaves where it kills the beetles. Tree trunks must be >200mm diameter. Beetles only need to ingest a small amount of leaf material for the chemical to kill them; you will see dead and dying beetle on paved areas indicating that things are proceeding as planned. You will see dead and dying beetles on paved areas indicating that things are proceeding as planned. Trunk injection is safe around children, pets and riparian situations.
- Soil injection Soil injection involves injecting the soil under the drip line of the tree and can be applied to trees of any diameter. Soil injection requires up to 100 L of water per tree and is not recommended when soil moisture is low. The chemical is taken by the feeder roots and translocated through the tree’s vascular system to the shoots and leaves where it will kill the adult beetle and the larvae. As the movement of the chemical is slow the injections need to be carried out around 6-8 weeks prior to the beetles emerging from hibernation. Soil injection is detrimental to soil flora and fauna, including earthworms and mycorrhiza. Applications near creeks and rivers or near food gardens pose particular risks for invertebrates and human health. Soil injection can be used on trees of any trunk diameter.
- Canopy spraying Foliar sprays can be used on young trees towards the end of October but are often impractical on older trees due to the size of large elm trees. Foliar sprays need to be applied annually.
- Non-chemical control Banding is a safe non-chemical control used to trap larvae that migrate down the trunk between December and early February. Adhesive tape can be wrapped around the trunk in a strip about 20cm wide, with the sticky side facing out. This breaks the beetle lifecycle. The bands need to be applied before the larvae start to crawl down the trunk, which can vary depending on the weather each season, so close monitoring is essential. Banding treatments may need to be reapplied several times each season if beetle numbers are high. These methods will not reduce damage in the initial year of application, but should reduce beetle numbers and damage in subsequent years. This method, while environmentally safe, may not be effective against heavy infestations.
Timing – The best time to treat elm trees is just as it is about to come into leaf. – It can be done later if necessary. – Stem injection can be done any time but ideally before the beetles have laid their eggs or just after the eggs hatch. – Trees should be treated every three years.