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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved
For many years, the ash trees were a near ideal tree for Utah conditions. The trees are strong, fast growing, drought tolerant and tolerant of alkalinity. Many thousands were planted throughout the state as excellent shade trees. There choice as a quality shade tree was badly compromised with the introduction of ash borer to the state.
Ash borer in Utah is not the true ash borer but in reality is the lilac borer. 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.
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 and they are usually found on the trunk and the main branch areas.
Timing for chemical control is done by pheromone traps that attract adult males. Spraying should begin within two weeks after large catches in the pheromone traps. Peak catch is usually May 1-15 in Salt Lake.
Check local nurseries for registered products for borer control. Thiodan is still available from some sources. Spraying large trees might require that you hire a commercial pest control company that can reach the large trees. Direct sprays at the trunk and where the main branches attach to the trunks.
Privet and lilac are less susceptible to the damage, but may show problems. The pest is easily controlled in lilac by pruning. The borer attacks only wood that is more than 4 years old. Renewal pruning prevents the need to spray for borer in lilacs.