Be on the look for oystershell scales in the landscape. The following is more information.
Oystershell scales feed on ash, cotoneaster, dogwood, maples, willow, popular and many other trees and shrubs found in the landscape. This scale attaches to twigs, branches, and trunks and feeds on surrounding cells sucking out sap and killing them. Heavy infestations may encrust branches and cause stunting, yellow foliage, bark cracking and branch dieback.
Pennsylvania reportedly has one generation a year, but research conducted by the Penn-DEL research group found two generations st present: 1 generation crawlers at 305-799 [445 peak] GDD base 50 and 2 generation crawlers at 2113-2259 GDD Base 50. The scale is light to 50 dark brown or gray and oyster-shaped in appearance. Adult females are white, crawlers are white to light yellow, and eggs (overwintering stage) are white.
Lady beetles, predatory mites, and parasitoids attack crawlers and adult oystershell scales. Prune out heavily encrusted branches to reduce infested plant material and reduce future crawler populations. Some chemical control options include: horticultural oil or insecticidal soap during crawler activity, acetamiprid, dinotefuran, azadirachtin, acephate or a pyrethroid (e.g., deltamethrin) are other products available for effective scale management.
Oystershell scales. Photo from the United States National Collection of Scale Insects Photographs Archive, USDA Agricultural Research Service, Bugwood.org
Information from Brian Kunkel, Ornamental IPM Specialist, UD.