Now is the time to treat for scale insects. Two weeks ago I sprayed my Dogwood tree, Redbud tree, and Red Twig Dogwood shrub for scale. If you have a Lilac shrub or a whole row of Lilacs like I do, they also will need to be treated for scale now.

          The preferred (and organic) treatment this time of year is horticultural oil. You may find it labeled as“dormant oil” or as an “All Seasons Oil”. The dormant oil is heavier than the All Seasons Oil. I like the All Seasons Oil as I can use it whenever the air temperature is warm enough.  Make sure what you use is “horticultural oil” and be sure and read the label to make sure that the plant you are treating is listed. If the tree or shrub has not started to leaf out, you can apply the oil at the dormant spray rate.  If the tree or shrub has started to leaf out then apply at the summer rate.  Spray using a small pump up sprayer and thoroughly coat all of the branches, and also the leaves if any. According to Don Janssen, Extension Educator at University of Nebraska Cooperative Extension in Lancaster County, “Oils are not poisons. Instead, the thin film of oil covers the target insect or mite and plugs the spiracles or pores through which it breathes.  The cause of death is primarily suffocation.”

          Horticultural oils are a good preventative.  Spray must be applied when the air temperature is warmer than 40 degrees F. but no hotter than 90 degrees F. Oil should not be used on glaucous (blue) evergreens such as Colorado Blue Spruce, and Koster Spruce as it will remove the blue color. The tree will not recover the color. As with all pesticides, be sure and read the label before application.

          As a dormant application, (before the leaves start to appear on the plant) the horticultural oil in addition to scale will also help to control aphids eggs, certain caterpillars, mite eggs, sawfly eggs and larvae, and whitefly during immature stages. For some plants, such as fruit trees and roses, lime sulfur should be mixed with the oil. Lime sulfur is an organic fungicide and helps to prevent some disease problems. As a caution, do not apply lime sulfur and horticultural oils separately, unless it is 30 days before or 30 days after application of the other. However, you can mix them together and apply at the same time. Two weeks ago I sprayed all my fruit trees with “Oil & Lime Sulfur Spray” that was already mixed. A second application 6 to 10 weeks later may be applied and is quite often recommended.        

          If the scale has hatched and is in the “crawler” stage, use a contact insecticide such as acephate (Isotox, or Orthene), permethrin (Eight, or Super Eight), carbaryl (Sevin), or Malathion. Make sure the label says it works on scale and is appropriate for the plant you are spraying. Always follow label directions. The crawler stage is usually from mid May to early July depending upon the kind of scale and the plant.


For more information go to In the top box scroll down to “Lancaster County”. In the bottom box type in “scale”. Entry #6 is Don Janssen’s article “Horticultural Oils for Pest Control”, published March 10, 2006 (printer version).

Or go to In the search box type in “Scale”. A list of publications will appear and you can click on the one you want.

Copyright 2006, April 1 ******************************************************************