Sometime in the next couple of weeks, if not already, the adult ash-lilac borer will be laying eggs on ash trees, lilac bushes and occasionally even privet bushes. The ash-lilac borer (Podosesia syringae) can be a very destructive pest. So much so, that if you have an ash tree on your property, it is recommended to treat it even though the damage may not be noticeable at this time.
The Pest:
Ash-lilac borers overwinter as larvae in already infested host trees and shrubs. Then the pupation stage occurs in the spring. It’s the adult stage that displays Moth emergence from the infected plants, which begin in mid to late-April, peaking in May. By mid-to late-June this wave subsides, usually ending by the first week of July. Thankfully there is only one generation of these critters per year.
The moth has clear wings and resembles a wasp in appearance. It is about 1 inch long and has a wing span of about 1½ inches. It likes to deposit its eggs in cracks and crevices of the bark. These eggs then hatch 10 to 14 days later. Larvae immediately bore into wood. Frass (insect excrement), which is a telltale sign, can be seen accumulating around the bore holes.
These pests often attack plants that have been under stress. Providing water during dry periods in the weather will help plants resist attack. Weakened trees may need protection until they are healthy enough to resist attack on their own. If there is swelling and cracking of the bark, it is because of repeated infestations. Branches may be severely weakened at these feeding sites. Also during dry periods late in the summer, terminal shoots of infested plants will wilt.
The Mitigation:
In order to address this problem, pesticide treatment should be applied within 10 days of egg laying to be effective. Among various insecticides that offer control for this insect, look for the chemical Permethrin for best control. Although this chemical can be found in a number of products, make sure the treatment for ash-lilac borers is detailed on the package before you purchase it.
Apply Permethrin when the VanHoute spiriea is in full bloom, which in most years, can range from the third or fourth week of April, to about May 1 in central Kansas area. However, with the fluctuating temperatures we have had this year, the bloom time will most likely be delayed somewhat.
Apply pesticide mostly on the lower two to three feet of the branches of lilacs and privets. On the trunk and larger branches of the ash, thoroughly treat the area where branches intersect with the trunk. Since most of the eggs will be laid within 10 feet of the ground, satisfactory control can usually be achieved by concentrating sprays in this area. Heavily infested ash should be cut and burned during the fall and winter. Infested stems of lilac or privet should be removed as well.
Rip Winkel is the horticulture agent in the Cottonwood District for K-State Research and Extension. Contact him by email at rwinkel@ksu.edu or call 620-793-1910.
