Leafcurl ash aphid
The leafcurl ash aphid (Prociphilus fraxinifolii) is a consistent pest of green ash trees. Leafcurl ash aphids are small light-green insects covered with off-white waxy threads. They remain on the underside of the leaves, inside the curled leaves. Biological controls, especially syrphid fly larvae and lady beetle larvae are often abundant among the aphids.
Presence of the aphids on the undersides of the foliage causes a distinctive gnarled deformation at the ends of the twigs. The clusters of tightly coiled leaflets are noticeable throughout the second half of the summer. Damage is annoying and aesthetically displeasing but has no significant impact on otherwise healthy trees.
Damage occurs only on new growth that emerges after the aphids arrive in mid to late May. Expanded leaves are not susceptible to leaf curling. Consequently, the damage remains confined to a very limited proportion of the tree’s total foliage.
Spraying insecticides is generally not warranted for this problem. By the time damage is apparent it is too late for effective treatment. Spraying would have to precede deformation, as curled leaves will not return to normal even if the aphids are killed. Damage is aesthetic and of no consequence on healthy, well-established trees and spraying will be harmful to biological controls. If preventive chemical treatment can be justified use a foliar systemic insecticide such as Orthene or Dimethoate.
As a practical consideration, prune off and discard the deformed terminals that can be reached and ignore the rest.