Oystershell scale

You will see small brown or gray oyster or mussel-shaped scales about one-eighth inch long covering trunks, branches, and twigs of maples, birches, elms, and other deciduous trees. Scales may be elongated and slightly curved. Heavy infestations create a silvery, crusty appearance on bark. The pest is common on stressed trees throughout Western Washington and the Puget Sound landscape.

Scrub branches lightly with a plastic brush to remove scales manually. Apply dormant oil before bud break in late winter to target overwintering eggs beneath scales. Monitor crawler emergence in early summer and follow with contact sprays if populations are heavy. Natural enemies including parasitoid wasps, mites, and birds control many populations. Maintain tree vigor to reduce susceptibility.