Oak lecanium scale

Meet the pest

Oak lecanium scale (Parthenolecanium quercifix) has been around a long time. Because it faded away each season, it has been thought not to need treatment.  The normal treatment is to spray the trees; while sometimes effective, the spray can reach understory trees and shrubs and have side effects.  Another reason to not treat this pest is that it has natural enemies.

Quecus phellos, Oak lecanium scale, William Eck, Bartlett

Preparing the RMA project, Bill showed us what scale damage looks like. Photo: RMA

But Bill Eck of the respected firm Bartlett Tree Expert Company in Gaithersburg, Md., is seeing signs of bad infections in mature willow oaks (Quercus phellos) in Washington.  After RMA hired Bartlett’s to fight the scale in the only threatened trees on Embassy Row — which fortunately were just 12 — we are publishing his warning:

“This scale infection has become a serious problem around the district each year. Many of the grandest trees along MacArthur Boulevard, Loughboro Road and Lowell Street are in trouble.  They are heavily infested and many are already in severe decline.“ He also reports bad infestations on M Street and adjacent streets near the Navy Yard.

Learn more in our September 28 Press Release.

Oak Lecanium Scale, crawlers, Bartlett Tree Expert Company, willow oak, Quercus phellos

Look closely at willow and pin oak leaves for the scale insects in the crawler stage feeding on leaves.
Click/tap image to view full size.
Photo: Bartlett

Along twigs, the lumpy-looking adult females hide thousands of eggs, which hatch in early summer. The newly emerged crawlers settle on leaves in late summer and fall and extract sap from the tree’s vascular system.  In late fall the crawlers move to the twigs and continue feeding through the following spring. Severe scale infections weaken the trees so they die prematurely of other causes.

The issues are whether this scale has been doing more damage than in the past  and whether it has mortally harmed some mature DC trees already.

Willow oak is a desirable species which the Urban Forestry Administration (UFA) has planted on many streets for decades. Many of the mature trees Eck has said are badly infected are “street trees” under UFA jurisdiction. So Restore Mass Ave asks the UFA to inform the public whether the threat from this pest is getting worse.

Injection more selective than spray

Injection targets this scale in the tree’s internal system like a flu shot in the human body.  So it can be more effective than spraying badly infected trees. In the RMA project Bartlett injected acephate (Lepitect infusible). Eck said the “commonly accepted treatments for street trees, soil drench of imidacloprid or bark spray of dinotefuran have been resulting in failures, especially on large trees.”

As for the trees treated in the RMA project, Eck says, “The injection should suppress the scale through the growing season.” The scale would take several years to build up damaging populations again.”

Meanwhile, the best way to protect willow and pin oaks from scale damage is regular watering and mulching, he advises.  Go to our Street Tree page for steps for growing big, healthy street trees.

City response

RMA requested that the city Urban Forestry Administration (UFA) in the Department of Transportation give the public its own assessment about this pest.  Read UFA’s response in an article about our warning which ran in The Current October 7.

Residents wondering about the threat to willow oaks and pin oaks on their streets should contact the Mayor’s 311 service line online or telephone 311 to request an UFA arborist inspection.

 Useful links

Some links we used in our research:

North Carolina Extension


Abundant nature close-up photos