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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved
One particularly troublesome tree pest is the lilac borer, which is erroneously called the ash borer in northern Utah. While there is an ash borer, it does not live here. Even more confusing is that the lilac borer attacks ash trees.
Adult lilac borers are clear winged moths similar to common paper wasps in color, size, and shape. Lilac borers feed primarily on the trunks and larger limbs of lilacs, ash, and privet with the most serious damage to ash.
Ash trees are very tough trees that withstand alkaline soils, dry climates and hot temperatures. The wood is hard and resisted storm breakage. Unfortunately, the lilac borer invaded and changed the ash from one of our best trees to a tree that requires yearly attention to survive and flourish.
The borer is actually a clear winged moth that resembles the peach tree borer. The female lays its eggs on the trunk or larger branches, but the preferred site seems to be where the branches join the main trunk.
The insects then hatch, burrow underneath the bark and live for the next year eating the wood of the tree. In April of the following year, the insects emerge and mate and start the cycle all over again.
Preventing the insects from getting into the tree is the only successful method of control. Controls must be timed precisely to destroy the pest before they invade the tree. Timing these sprays is based on pheromone traps that attract the male moth to determine when the peak flights are occurring. Those traps are now catching borers. Consequently, homeowners have several days to apply sprays for the most effective control.
The adults start emerging from infested hosts in early May and continue to emerge for six weeks. After mating, the female lays the egg on the bark. When the larvae hatch, it bores into the plant. Feeding creates tunnels that may girdle the tree. In the spring, the larvae cut holes through the bark as they emerge. These holes range from 1/4 to 3/8 inch in diameter.
Check with your local supplier for available products. Dursban, Lindane and Thiodan are no longer sold.
Spray the trunk and main branch areas within the next 10 days, prior to the time the insects reinfest the tree. Thorough coverage of these areas is essential, but sprays on the leaves and twigs are not needed.
Unfortunately, there does not seem to be an effective organic control for this particular pest. Pruning, fertilization and adequate water reduce the stress on the trees and consequently help reduce the damage from the insect.
On lilacs, do a good job of renewal pruning and keep the wood young and vigorous. The borer only attacks wood that is more than four years old.
Similar sprays also controls the pests on lilacs. On lilacs and privet, the insect bores in near the base of the stem about six inches off the ground. Once again, the stem is killed by the feeding of the insect. Fortunately, the pest only attacks lilac wood that is 4 years old or older.