Shade Trees and Scale Insects in Hamilton County Landscapes

High populations of scale insects are drawing attention in Hamilton County yards this spring. The insects, commonly called lecanium scales, feed on the sap of a variety of trees including maple, oak and many others. This week alone we have had two samples come into our Educational Center for problem diagnosis. One sample was found on oak while the other was a mature maple.

The first signs of scale infestation are often noticed before one sees the insect itself. Most noticeable is the sticky, wet substance on under-story plants, yard furniture, or automobiles. This material, known as honeydew, is excess fluid that is excreted as the insects feed. Under-story leaves and other surfaces may be lightly speckled or heavily coated with the shiny drops of honeydew. Eventually a sooty mold fungus grows on this substrate, blackening the foliage, twigs, and bark, and other objects beneath scale-infested trees. Dieback of twigs and branches and premature leaf drop may result as heavily-infested trees compete with scale insects for necessary moisture.

Lecanium scales can vary in size and color but are generally oval in shape. Females molt several times before reaching adulthood. With each molt, the female’s body grows bigger, while her legs and antennae become proportionately smaller. By the time of the second molt, the female scale, with her hardened, brownish, hemispherical body, is incapable of moving from the spot on which she has become fixed. Male scales are delicate, flat and nearly transparent.

Normally, scale populations are kept in check by environmental factors and natural enemies such as ladybird beetles. Heavily-infested trees may show some dieback of twigs and branches and there may be some early leaf drop. Landowners can reduce other stress on trees to help them recover. For landscape trees be sure to water in times of drought and avoid disturbances such pruning and root damage. When populations are heavy on specimen trees, horticultural oils and other insecticides can be applied.  For more information, click here for a great publication from Michigan State University Extension.

Excerpts were taken from Vermont Dept. of Forestry Lecanium scale fact-sheet.