Wood-boring insects cannot be eradicated with insecticides once they are inside the wood. Preventative sprays applied to the bark might interrupt life cycles so new generations don't re-colonize the tree, but the borer responsible would need to be identified to species first so the proper timing of bark treatment was used. We recommend you have a certified arborist evaluate the tree, since they can look for sources of tree stress and also help to narrow-down a diagnosis. They cannot cure or treat every condition, but can assess a tree's current state of health or decline and make recommendations. By themselves, borers don't always kill a tree. Flowering cherries tend to be comparatively short-lived trees, doing well for perhaps 20-30 years before tending to decline.

Wood-boring insects usually target plants already experiencing stress, even if outward signs of that stress (like wilting, leaf loss, branch death) are not yet obvious. Cherries do not fare well in compacted or poorly-drained / over-watered soils, but they also don't want to get too dry for too long. (Last year, for example, much of Maryland was in drought for most of the growing season.) Injury from storms, improper pruning, or trunk dead zones from advanced girdling roots can also give such insects an opportunity to invade the trunk or major branches.

There are also beetles and other insects that tunnel into already-dead wood, and other than slowly hollowing-out a trunk, they are not causing any damage. This is because the only living layer of trunk wood is just under the bark, extending only a few inches into the wood; the center heartwood is already dead wood, which is natural and normal. If something gave such insects access to the inner trunk wood, like a wound or cut into the bark on the outside of the trunk, then they might be chewing into the wood there, leaving sawdust, without harming any live sapwood. This is something an arborist can help to investigate. Some have more extensive training and diagnosis experience than others, so you can always get a second or even third opinion before deciding on what action to take (if any action is needed). You could ask before hiring someone if they have experience with flowering cherry trees. (Probably...they're so widely grown.)

Miri