Lilac borers are insects whose larvae bore into stems usually during May and June. A sawdust-like material called frass is often seen around the base of stems after it has been pushed out the hole made by the borer. Canes often wilt and die during late summer especially if the summer has had a dry period. The larvae passes the winter inside the dead canes and pupates the following spring, usually during April. The adult, clear-winged moth resembles a wasp and often emerges during May through June though there is a great deal of variability. Eggs are laid on the stems of lilac, and the cycle starts over again. There is one generation in Kansas.
Though it is too late to spray for lilac borer this year, removal and destruction of dead canes will help reduce populations next year. You may also want to spray for the insect next spring. The first spray for ash/lilac borer should be applied when the Vanhoutte spirea is in full to late bloom, probably about May 1. A second spray should be applied four weeks after the first.
Thoroughly treat the lower portion of the stem of lilac or privet. Permethrin (Hi-Yield 38 Plus and Hi-Yield Garden, Pet, and Livestock Insect Control) are labeled for control. Though there are a number of other homeowner products that contain permethrin, the products listed above are the only ones I've found that specify on the label how the material should be applied for borer control. (Ward Upham)