European elm scale

European elm scale appears as small, flat, reddish-brown bumps on American elm bark in the Puget Sound, particularly on trunks and older branches. You will see oozing sap and sooty mold accumulation beneath scale infestations. Heavy infestations can girdle branches and cause significant dieback. Scales overwinter on bark and remain relatively inconspicuous until populations build.

Apply horticultural oil in late dormancy before buds open. Encourage natural parasitoids and predatory beetles by avoiding insecticides. Prune heavily infested branches. Maintain tree vigor through proper watering and wound care.