Lilac/Ash Borer

Scientific name Podosesia syringae, lilac/ash borer is a clearwing moth. Typically attacking green ash trees in the Cheyenne area. Younger trees with a trunk diameter of 6 inches and less are most commonly attacked.

Two green ash trees on W. 15th Street.
Both are affected by lilac/ash  borer. The top has been killed in the tree on the left by lilac/ash borer.

    Pupal cases left in the tree trunk after adult emergence.
 Photos: Cheyenne Urban Forestry

The ash borer overwinters in the tree in a pupal stage (changing from a larva to an adult moth). The adult emerges in late May through early June in Cheyenne as a clearwing moth. The lilac/ash borer adults mimic paper wasps. Which means you may see one flying around and think it is a wasp, when actually it is a moth, harmless to humans, but who wants to take a chance and swat at it. In the adult moth stage mating occurs, eggs are laid on the outside of the bark, with the eggs hatching in about 10 days. The newly hatched larvae then burrow into the cambium and phloem layers under the trunk or limb bark. The larvae feed in the phloem (food conducting tissue), the cambium layer (growth cells that form the phloem and xylem tissue) and into the xylem (the water conducting and food storage) layers in the trees. The feeding process can girdle the tree trunk or limb. Girdling disrupts the flow of food sugars produced by the leaves down into the trunk and root system and it disrupts the flow of water and nutrients from the roots into the trunk and leaf crown of the tree. Feeding deep into the xylem or wood part of the tree can weaken the structure of the tree allowing easy breakage. If the feeding of the lilac/ash borer larvae doesn't kill the tree or limbs, it leaves the tree susceptible to trunk or limb breakage.


                                      
                            Adult lilac/ash borers. On the left is a western variety and on the right is an eastern variety.  
                                                             Cheyenne may have either one or a mix of both.
  
Controlling the lilac/ash borer is difficult. Spraying the trunk and susceptible limbs with an insecticide that has the property of a long residue period is the most effective means of control. However, trees in a highly used area of town with an insecticide residue on the trunk is not acceptable. The Cheyenne Forestry Division used a systemic insecticide injected with water into the ground around trees threatened by lilac/ash borer. The little amount of surface soil available for the soil injection of the insecticide resulted in poor to no positive results. The next year we tried an injection of insecticide directly into the trunks of trees threatened by lilac/ash borer. This process also had very little to no effect. Spraying the trunk with an insecticide that has a long residual period seems to be the best control method. More info on the lilac/ash borer: from Wyoming, Colorado, and Kansas.

Links:

Colorado State University Cooperative Extension

University of Wyoming - Cooperative Extension Service, publication B-1035 is particularly helpful for tree care. This online publication contains information on: Aphids, Borers, Cottonwood blotch leaf miners, Cytospora canker, Fireblight, Gall makers, Aspen leaf spots, Oystershell scale, Pear slugs, Powdery mildew, and Spider mites.