Pest profile

Leafcurl Ash Aphid

Prociphilus fraxinifolii · Hemiptera: Aphididae[3]

Leafcurl ash aphids are small, woolly aphids that feed on the undersides of new ash leaves, twisting the branch tips into tight, gnarled coils; the damage is cosmetic on healthy trees.[1][2]

Curled ash leaf opened to show small aphids coated in white waxy threads
Curled ash leaf opened to show small aphids coated in white waxy threads. (c) giantcicada, some rights reserved (CC BY) — via iNaturalist, CC BY 4.0

How to identify it

  • Aphids are small, light green and covered with off-white waxy threads, hidden on the undersides of curled leaves.[1][2]
  • Clusters of tightly coiled leaflets at twig tips are noticeable through the second half of summer.[1]

Where it lives

Iowa State calls it a consistent pest of green ash; Utah State covers it among woolly aphids of Utah landscapes.[1][2]

US map: public records from 21 states since 1990 (GBIF).

Biology and life cycle

Aphids arrive on new growth in mid to late May; only leaves that emerge after they arrive curl, so expanded leaves are not affected.[1]

Woolly aphids like this one may overwinter as eggs on bark or as immatures on roots, moving between parts of the plant or between hosts.[2]

Damage and signs

  • The curling is unsightly but confined to a small part of the canopy and has no significant impact on otherwise healthy trees.[1]
  • Honeydew or sooty mold may be present.[2]

Health risk

None to people or pets.[1][2]

Control

Traps and physical control

Prune off and discard the deformed tips if they bother you.[1]

Biological control

Syrphid fly larvae and lady beetle larvae are often abundant in the colonies and usually bring them under control.[1]

Pesticides

Spraying is generally not warranted: by the time curling shows it is too late, curled leaves won't flatten even if the aphids die, and sprays harm the natural enemies.[1]

Common questions

Why are the leaves at my ash branch tips curled into knots?
Leafcurl ash aphids feeding on new leaves cause tight coils at the twig tips in early summer.[1][2]
Will the curled leaves recover?
No, but the damage is cosmetic and limited to a small part of the tree.[1]
Should I spray?
Usually not; it's too late once leaves curl, and sprays kill the predators that control the aphids.[1]

Sources

  1. Iowa State University Extension and Outreach (Yard and Garden), “Leafcurl ash aphid.” yardandgarden.extension.iastate.edu/encyclopedia/leafcurl-ash-aphid (accessed 2026-10-15)
  2. Utah State University Extension, “Leafcurl Ash, Woolly Apple and Woolly Beech Aphids (Ornamental Pest Guide).” extension.usu.edu/planthealth/ipm/ornamental-pest-guide/arthopods/aphids-adelgids/leafcurl-ash-woolly-apple-woolly-beech-aphids.php (accessed 2026-10-15)
  3. iNaturalist, “Prociphilus fraxinifolii taxon page (order and family).” www.inaturalist.org/taxa/494593 (accessed 2026-10-06)

Updated 2026-10-15. Built from sourced records. Spot an error? Tell us.