Hawthorn aphid

In spring as hawthorn buds expand, small reddish or dark-colored aphids gather on new growth and tender leaves, causing them to curl inward and distort. The affected leaf curling creates shelter for the aphids as they feed throughout spring and early summer on Crataegus species common to Puget Sound landscapes. You notice the distinctive curled, thickened leaves in April through June. Populations diminish as leaves harden.

Prune out heavily curled shoots during early leafing to remove colonies. Monitor new growth starting in April for curl symptoms and spray with water to dislodge aphids. For recurring concerns, apply dormant oil in late winter to suppress overwintering eggs. Once leaves fully harden, populations typically self-regulate through natural enemies. Horticultural oil or insecticidal soap can help.