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The Garden Plot for July 11, 2008
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: "Ash Psyllid"
Green Ash leaves are curling and sticky now. The culprit is our all-too-common Ash leaf-curl aphid which feeds on ash tree leaves each spring. Then, they move to the soil to feed on roots for the summer and we can forget about them until they return to ash trees in the fall. Unfortunately, we are seeing a new pest on Black Ash this year in Missoula. The new invader is called the cottony ash psyllid. This insect, a cousin to aphids and leafhoppers, was reported in Bozeman last year and the Dakotas in 2005. So, we may be seeing more of this powdery, white insect curling Black ash leaves in our area. The good news is that this pest appears to be quite host specific. It attacks only Black ash, specifically Fall Gold, Manchurian ash (Mancana), Northern Gem ash, and Northern Treasure ash. It does not attack green ash.
Cottony ash psyllids spend the winter on ash tree branches as eggs. In the spring at just about budbreak, eggs hatch into aphid-like young psyllids that start to suck sap from ash tree leaflets. This feeding causes leaves to shrivel, become distorted, and discolor. Leaves curl under from the sides, enclosing the psyllids and the white, waxy substance they produce. Adult psyllids are 1/10 of an inch long flying insects that seem to hop rapidly from leaf to leaf.
According to scientists in Minnesota who have been watching this pest since it arrived there in 1993, psyllid damage is not necessarily live-threatening to trees. A healthy, mature ash seems to be able to tolerate a single season of heavy leaf-curling damage. They suggest that repeated attack over several years will weaken and possibly kill trees. Psyllids are more damaging than our more common ash leaf-curl aphids because their saliva causes greater leaf distortion and browning, fewer beneficial insects feed on them, and there are two, rather than one, generations per season. We may see a second generation of psyllids hatching on Black ash mid to late July. Look for these second generation psyllids along the midrib of unaffected leaves as well as inside leaves already curled-up by the first generation psyllids. Canadian researchers report that well watered ash trees suffer less damage from the cottony psyllid. They suggest applying at least one inch of water to your ash tree each week.
Unfortunately, insect-preventative horticultural oil sprays were found to have little effect against cottony ash psyllid. Montana State University researchers suggest a spray mix of insecticidal soap plus permethrin when psyllid babies hatch in the spring and again in mid to late July.
For more information on cottony ash psyllids on Black ash call your local county extension agent or your city forester.
Helen Atthowe's new short program of gardening tips
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