Scale Insects on Trees and Shrubs

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Euonymus Scale close-up

Obscure Scale

Unaspis euonymi

Lecanium on Silver Maple.

Although scale insects commonly affect trees and shrubs in Missouri landscapes, they are generally overlooked until they begin to cause obvious symptoms on host plants. Often, a scale infestation is first detected when honeydew, excreted from the insects feeding on plant stems, makes cars and sidewalks sticky and sooty mold begins to grow on honeydew-coated leaves and stems. However, some insects, referred to as “armored scales”, extract very little liquid from their hosts and, therefore, produce no honeydew. Trees and shrubs can live for years with scale infestations, but the insects take their toll. Heavily infested plants may lose vigor and show dieback of branches. In extreme cases, the insects can remove so much moisture from leaves or stems that leaves turn brown and die. In some cases, plants weakened by other stresses become attractive to scales.

Some scales are difficult to detect because they cover themselves with waxy material that blends well with the plant to which they are attached. Obscure scale on pin oak is a good example of this. Others, the “softscales”, look like small bumps on the stem than can be easily smashed with a finger. A scale spends most of its life stationary, with mouth parts (stylets) inserted into phloem cells in stems. Females may lay hundreds of eggs, which hatch to produce “crawlers”. The tiny crawlers (which look like dust) walk or move on wind to spread the infestation.

Scale insects that have been reported this year in Missouri include tulip tree scale, magnolia scale, lecanium , euonymus scale and oak kermes. Kermes sometimes concentrates its feeding at one point on the branch, causing tips to break off and fall to the ground. Others, such as obscure scale on pin oak are persistent problems and seem to be increasing with stresses such as pin oak chlorosis and excess soil moisture. While scale can be difficult to manage, keep in mind that a well designed program can maintain a certain level of natural enemies of scales, which will help in the long term. For example, a dormant oil application in early spring followed by horticultural oil when crawlers hatch will provide good control with little impact on beneficial organisms. Also, use of soil applied or trunk injected systemic insecticides may be less harmful to beneficials than aerial application of more conventional materials. The most beneficialfriendly approach is to determine which species of scale is present and determine when oil application would be most effective. Crawlers of euonymus scale are active in late May to early June and immature stages of oak kermes scale are susceptible in September.

For an excellent summary of scale insects on shade trees and shrubs and how to manage them, see: http://extension.entm.purdue.edu/publications/E-29.pdf