Asked September 22, 2021, 3:52 PM EDT
Jackson County Ohio
Thank you for writing in with this question, because I have learned some things I did not know before.
First, as I understand it, any control would have had to be applied to the plants in early to mid-May timed to coincide with the hatch of the eggs. This is a good fact sheet from Iowa State Extension explaining that: Lilac/Ash Borer | Horticulture and Home Pest News (iastate.edu). Their suggestion for wrapping the trunks of young trees from fall to spring would be difficult to carry out on a multi-stemmed lilac shrub.
Kansas State Extension goes into more detail about prevention through cultural means -- avoiding pruning after late spring and into summer, avoiding injury to the base of the plant from lawn mowers or string trimmers: How to Avoid Getting “Bored” by the Ash/Lilac Borer | Extension Entomology (k-state.edu)
The Home, Yard & Garden Pest Newsletter from the University of Illinois again emphasizes the timing of insecticides -- specifically when the lilacs have finished blooming. This also provides the types and trade names of the insecticides that can be used: Lilac/Ash Borer (illinois.edu).
Although you are going to lose blooms from the shrub if you prune now, you probably won't be getting any flowers on those branches affected. If I were you, I would cut out the infected branches and dispose of them in the trash -- do not try to compost them. Other than that, there is not much you can do at this time of year. But you can be prepared for next spring!
Best of luck.