Oystershell Scale

Oystershell scale is a hard or armored scale. It is a very destructive insect on commonly found on aspen, ash, poplar, cottonwood, cotoneaster, and lilac.  Chemical control is difficult, because the insect lives underneath a hard shell for most of its life.  The shell is small, about the size of a pin head.  Many shells of the insect grouped together appear like bark on a tree or a limb.  The insect underneath the shell feeds on the phloem tissue of the tree.  The tree's phloem tissue provides the transport and storage of food manufactured by the leaves.  When thousands or hundreds of thousands of the oystershell scale insects are feeding on a tree, the health of the tree can decline significantly, often to the point of death.  When a tree is declining in health, other insects and disease organisms can easily attack the tree and finish it off.  The best way to get rid of the scale is to scrape it off of smaller trees using a plastic scouring pad designed for use on pots and pans.  Insecticides sprayed on the tree do not readily penetrate the hard shell covering.  Sprayed insecticides, Carbaryl®, and Orthene®, to name some common ones, have to coincide with the crawler stage, which occurs at different times each year, but usually from late May to late June.  Do not use Orthene® or other insecticides containing Acephate on poplars, aspens or cottonwoods.


                                                   
                                                Photo: USDA Forest Service, Rocky Mountain Region archives through                                                                                       Forestry Images.

                                                               
                                            Photo: William M. Ciesla, Forest Health Management International, through                                                                                         Forestry Images.

                                                  
                                                  Photo: Whitney Cranshaw, Colorado State Forest Service, through
                                                                                   Forestry Images.

Colorado State University Cooperative Extension

University of Wyoming - Cooperative Extension Service, publication B-1035 is particularly helpful for tree care. This online publication contains information on: Aphids, Borers, Cottonwood blotch leaf miners, Cytospora canker, Fireblight, Gall makers, Aspen leaf spots, Oystershell scale, Pear slugs, Powdery mildew, and Spider mites.