A question on lecanium scale

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

The following question to the Landscape Alert Team by email this week:

Dr. Cregg:  I am a resident of Troy, Michigan and have found that I have scale on my Locust and Sycamore trees.  I obtained your name through the MSU Extension. I have had 4 companies to the home who have confirmed that scale diagnosis and have proposed a variety of treatment.

  1. Dormant oil in the spring and/or fall, their recommendations vary.
  2. The use of Merit in a ground injection in the spring and/or fall, again recommendations vary.
  3. Tree injection with an insecticide, spring and/or fall.

They don’t indicate what type of scale this is. I am confused as to the appropriate treatment and timing.  Can you offer any suggestions? I would appreciate any information you can provide.

My answer 

Dear Homeowner:  The insect problem on your trees is most likely a Lecaniumsp. scale.  These soft scale insects go in cycles and rarely cause any harm to landscape trees.  The scale infestation sometimes becomes worse when the trees are sprayed for other reasons (like the mosquito control program in Saginaw County) because the insecticide kills predators and parasites that keep the scale insects under control.  

Since this insect is unlikely to harm your trees, I recommend that you do not have your trees treated.  One exception is when excessive honey dew (sugary scale excretions) drips down on cars parked below the trees.  This can cause some aggravation.   In those rare situations, I recommend using an imidacloprid (Merit is one product) basal soil injection or an imidacloprid trunk injection in the spring.  But previous observations with large trees show that it takes two years of basal soil injections to reduce the scale infestation.

Dr. Smitley’s work is funded in part by MSU‘s AgBioResearch.

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