SCALE INSECTS

Scale insects spend most of their life as an immobile bump on a leaf or stem, siphoning off fluids from the host plant. Some scale insects excrete a fluid called honeydew that attracts a variety of nuisance pests like ants, bees and wasps to feed on the sweet exudates. The honeydew also is a great growth medium for sooty mold, a black staining mold that can cause a sticky mess on a car or driveway or anything beneath it. 

           

There are two broad categories of scale insects, soft and armored scales.  Armored scales are, as one might expect have a rigid and hard body covering, like pine needle scale or oystershell scale. Soft scale insects are squishy and juicy and are best exemplified by the magnolia scale. These body coverings, coupled with their immobile nature, can make scale insects very hard to control.

Most scale control strategies revolve around trying to kill the vulnerable crawlers (newly hatched scale insects) and many insecticides are labeled for this purpose. The problem is that each scale insect species has a different time when the crawlers appear and some have more than one generation of crawlers per year. Dormant oils can help suppress over wintering adults and prevent some egg hatch, but the need for thorough coverage (as in every square inch) makes this hard for larger trees. Another factor is location and the pests can often be found in greater numbers when the plants are sheltered from the elements or surrounded by paving.  Stressed plants are also more often infested with scale insects. Even if you get control of the scale insects the other factors that make the plant vulnerable are beyond your control so they can become re-infested. 

           

In recent years some systemic products that target feeding adults have become available. Armored scale insects are susceptible to a product called Safari and soft scales can be controlled with Merit.  Acephate injections can provide a fairly quick kill but have a short residual in the tree.  An acephate injection followed by an imidicloprid (Merit) treatment will provide an immediate and more lasting control for scales like cottony maple scale.