Any recommendation for an insecticide would necessitate knowing what insect is causing the problem. It is pretty much essential that you locate and identify the pest first to know what sort of product might be effective.

Having said that, insects munching on plant leaves is pretty much a gardening fact of life. Unless it is a complete defoliation, it is pretty much just a cosmetic issue and most often not worth treating. And even total defoliation is very recoverable for most plants.

Although deer are not known to favor Cotinus, they can and do sample anything, especially the youngsters. "Clusters" of leaves being eaten would lead me to suspect something larger than a insect......like deer or bunnies.