Cottony Ash Psyllid in Bozeman Black Ash

We have recently identified a new non-native pest, the Cottony Ash Psyllid (Psyllopsis discrepans), in many of the City's Black Ash trees. Adult insects are small, only 3mm in length. They have slowly moved from the east coast, where it was first detected in the early 1920's and in Minnesota in 1993. There are current outbreaks in North Dakota and it has been present in Calgary and Edmonton since 2000.

As with many insects it is host specific, which means it attacks only limited species, in our case this is the Black Ash (Fraxinus nigra). Reports from other cities (in North Dakota) say that it will also attack the Mancana Ash (Fraxinus mandshurica), but we have not seen that here at this time. It does not attack Green Ash.

There are two Cottony Ash Psyllid generations per year. This psyllid over winters as an egg, hatching about the time of bud break when it begins to feed on the new leaves causing them to curl along the edges. The curling is caused by piercing sucking mouth parts that inject saliva into the leaf surfaces resulting in a "cauliflower" effect. Another common Ash pest, the leafcurl ash aphid (wooly ash aphid) causes a tightly balled up leaf, or pseudo-gall, different from the Psyllid damage. Aphids are less damaging to Black ash as their saliva is less toxic and beneficial insects prey on them more readily.

Dormant oils are not effective for control of this psyllid because they over winter as an egg. As the nymphal stage progresses chemical control of this insect becomes less and less effective due to the production of cottony material and the increased leaf curling.

Practically speaking chemical control of the psyillid is difficult because the control window is small. Other regions hit hard by the Cottony Ash Psyllid have opted to manage the over all health of the trees and replant with more resistant varieties instead of committing to repeated applications of pesticides. Improving tree health will be the most effective long term solution.

If chemical control is deemed necessary, a soap/permethrin mix works well when applied during the early nymph stage, mid to late June. So start monitoring at bud break for the appearance of the nymphs and the cottony material that accompanies them. Then in mid July monitor for the second generation and repeat the soap/permenthrin application as nymphs re-appear. Acephate (acephateOrthene®) has also been applied with success. Always read, follow and understand label directions if chemical control measures are pursued.

Usually, a healthy tree will withstand an attack, but repeated attacks in successive years can weaken and even kill trees stressed by poor growing conditions and this insect. Well watered trees have been found to better tolerate an attack by any pest. Black Ash is naturally found in wet places, and we have been experiencing many years of below normal rainfall. So, if you have a Black Ash that is exhibiting this type of leaf curl be sure to water it throughout the summer. In order to collect as much rainfall as possible keep turf and weeds away from the base of the tree in about a 16" inch circle. A mulch of bark or other suitable materials will help keep plant growth in this area down.

We will be monitoring regions in which the psyllid is more established and provide information regarding the long term impact of this pest.

We would like to thank: Justin Knott, North Dakota Department of Agriculture, Cheryl Moore-Gough, Extension Horticulturist, Will Lanier IPM/Diagnostics Dept. of Animal and Range Science, and Nina Zidack Plant Disease Diagnostician, for their help.