Dear Alissa, this moth feeds only on species of Populus, so poplars, cottonwoods, and aspen. Like most sphinx it feeds on the leaves. If the damage to birch is flagging and limb dieback, it is most likely due to the bronze birch borer, a buprestid. Here’s a link:
http://pnwmoths.biol.wwu.edu/browse/family-sphingidae/subfamily-smerinthinae/smerinthus/smerinthus-ophthalmica/

Helmuth Rogg
June 1, 2020, 1:18 a.m.