This appears to be caused by the Lilac borer, a native species that is a common pest of lilacs across North America. It is clearwing moth and adults resemble a wasp in appearance. There are some control strategies you can take which include pruning and chemical controls. Here are some links:

https://agsci.colostate.edu/agbio/ipm-pests/lilac-borer/

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/insects/caterpillars/lilac-borer-and-ash-borer

Wyatt Williams
April 3, 2024, 1:51 a.m.