Integrated Pest Management

Oystershell Scale

Oystershell scale on a willow branch. Oystershell scale on willow branch.
Oystershell scale on willow. Oystershell scale on willow.


  • Apple
  • Ash
  • Aspen
  • Beech Birch
  • Cottonwood
  • Dogwood
  • Elm
  • Lilac
  • Linden
  • Maple
  • Poplar
  • Willow
  • And many other deciduous trees and shrubs


Adult oystershell scales are immobile insects enclosed in a shell made up of its own shed skins and waxy secretions. Under the covering is a sac-like organism that is quite different in appearance from most insects. It has no legs, eyes, or antennae. They feed by sucking the fluids from within plant cells (mesophyll) located just under the bark. Because these are armored scales, they do not excrete honeydew. Populations can build quickly, causing reduced plant vigor.


In Utah, there is one generation per year. Eggs are deposited in late summer and early fall and overwinter under the female scale. Estimates of egg production range from 40 to 150 eggs per female. Egg hatch begins in late May or early June and continues for about two weeks. The newly-hatched, tiny white to light yellow nymphs are called "crawlers" and have functional legs. They move about over the bark for a short period of time, and eventually insert their long, thin mouthparts into the host plant, and begin to secrete the protective waxy coating, taking on the "oystershell" appearance as they go through successive molts and growth. There they will remain for the rest of their lives.


  • Branch dieback
  • Loss of plant vigor
  • Chlorotic (yellow) and abnormally small foliage


Monitoring is important to identify when crawlers are active. Ways to monitor include:

  • Wrapping a piece of double-sided sticky tape around an infested twig in mid-May. The tape should be checked with a magnifying lens every other day for the presence of crawlers.
  • Hold a piece of white paper or a white pan under the branches and tap the branches with a stick. Any crawlers can be seen with a magnifying lens on the white surface.

In some cases, heavy accumulations of scale can be removed from the bark with a stiff scrub brush or a plastic scrub pad.

Natural enemies of oystershell scales may provide some degree of control. These include certain species of lady beetles (such as twice-stabbed and sevenspotted), parasitic wasps (Aphytis mytilaspidis and Encarsia citrina), and predatory mites (in the genera Neophyllobius and Rhyzobius).


Oystershell scales are most susceptible to insecticides during the crawler stage.  

Residential options include: pyrethroids, insecticidal soap, horticultural oil, or Safari

Commercial oprions include: abamectin, buprofezin, pyriproxyfen, dinotefuran, spirotetramat,  oils

Merit and its generics (imidacloprid) are not effective on armored scales.

Precautionary Statement: Utah State University and its employees are not responsible for the use, misuse, or damage caused by application or misapplication of products or information mentioned in this document. All pesticides are labeled with ingredients, instructions, and risks, and not all are registered for edible crops. “Restricted use” pesticides may only be applied by a licensed applicator. The pesticide applicator is legally responsible for proper use. USU makes no endorsement of the products listed in this publication.