The larvae of sawfly species, oak slugs, get their name from their resemblance to the legless molluscs, slugs, but are actually in the insect family Tenthredinidae in the order Hymenoptera which includes bees and wasps. Like similar species of sawflies on rose or pears, the larvae can be found skeletonizing leaf surfaces, usually feeding on the underside of leaves. Although the species in this image is yet to be confirmed, it is thought to be either Caliroa distincta or Caliroa labata, two species confirmed in Washington state.
Feeding damage from the larvae has been spotted in early July in the North Willamette Valley. Look on lower leaves for first damage as these sawflies probably overwinter in the soil below the trees.
USDA-Forest Service, Northeastern Region. Scarlet Slug Oak Sawfly
Ohio State University. Scarlet Oak (Slug) Sawfly