Cottony psyllids (Psyllopsis discrepans) are soft-bodied insects closely related to aphids. Originally from Europe, they have killed thousands of trees in Edmonton since 2000, and were first found in Grande Prairie in August 2015. As of summer 2018, 500 city owned black ash trees have died and been removed due to this invasive alien pest. There are approximately 1200 more public black ash trees infested and being injected with insecticide every two years to keep them alive as long as possible.

Plants Affected

  • Black ash
  • Manchurian ash
  • Hybrids of black and Manchurian ash


The first generation of cottony psyllid hatches early in spring and enters the bud as soon as it breaks. They feed on the newly forming leaves, which become twisted, curled and deformed as they emerge. The nymphs inject a toxin during feeding which contributes to deformation of the new leaves.

The second generation hatches in mid July and feeds on the new leaves that the tree produces after the first generation of psyllids damage or kill the spring leaves. This combined feeding pressure kills the tree much quicker than pests who only have one generation per year.


Cultural or Non-chemical Controls:

  • Keeping your tree well-watered will help it withstand feeding pressure.
  • Plant a replacement tree as soon as possible before your black ash dies completely. American or Brandon elm and green ash are both not recommended as replacements due to approaching fatal pests. See for a list of trees that grow in the Grande Prairie area, or else contact Parks Operations for tree recommendations.

Chemical Controls:

  • Due to the insects being sheltered underneath the curled leaves, tree injection with a registered herbicide is the most effective chemical treatment.
  • The City recommends homeowners contact local tree service companies to ask if they provide tree injection services for privately owned trees.


Twisted leaves on black ash
Psyllids shelter in this white fluff
Black ash tree dying from cottony psyllid infestation
Heavily damaged leaves
Infested tree