Lilac in big trouble!
Help! I noticed my lilac bush's leaves turning black a month or so ago. I thought it may be because it wasn't getting enough sun. On closer inspection I found holes at the base of the bush as well as soft, flaking wood. And then this menacing looking bug flew around and then landed. I suspect it and its brethren are making the holes. What do I do? This lilac was transplanted from my grandmother's property a number of years ago so it has sentimental value.
Carroll County Maryland
Hi - The symptoms you are observing on the leaves are quite common on lilacs at this time of year. Lilacs are susceptible to bacterial and fungal diseases, but it is too late in the the season to use any type of treatment. At the end of the growing season, clean up and discard in the trash (not compost) the affected plant material.
Lilacs are susceptible to borer damage, which is apparent in your second photo on the older trunk. Prune out all dead and dying canes as close to the ground as possible. https://extension.umd.edu/hgic/topics/borers-shrubs
The insect in your third photo appears to be a European hornet (Vespa crabro). These do strip bark off some plants in order to feed on the sap or other insects attracted to the sap, and they use the bark material for their nests.
You can and should prune old wood from lilacs on a regular basis to stimulate healthy new growth and flowering. The rule of thumb is to cut back no more than 1/3 of the plant at any one time, removing the oldest wood first. Wood that is too old ceases to flower well. The younger shoots that regrow will flower better (once mature enough) and are not as attractive to pests such as lilac borer and European hornets.
Below are links to a video demonstrating the procedure and a description of how to prune:
Once the lilacs have reached a more youthful, vigorous state, such drastic pruning will not be regularly needed except to periodically remove aging stems here and there. Maintaining full sun exposure (6+ hours a day in summer) also will help your lilac flower to its fullest potential. If you are able to prune any nearby plants that might be shading the lilac too much, that can also help to improve the conditions for your plant.
Thank you for the detailed information. I am hopeful I can save the lilac. I saved a dogwood tree infested with dogwood borer by applying diatomaceous earth with a turkey baster inside the borer holes. Can I do the same with the lilac or would it be better to just prune the damaged wood? Thanks again for your comprehensive answer!
You're very welcome. It would be better just to prune out the damaged wood.