Two counties have called to report outbreaks of cottony maple scale on silver maple trees (Kossuth and Sac Counties). Other counties may eventually notice this problem also and are urged to report any major problems.
Cottony maple scale is always present in Iowa but in most years is too low in abundance to attract attention. When populations do increase above normal they are easily noticed during June as large, white, cottony egg sacs 1/2 inch in diameter line the stems and twigs like strings of popcorn. Normal populations usually have one, or at most, a few egg sacs at each twig crotch, while severe infestations may have enough eggs sacs to completely cover most twigs. This scale prefers maples, particularly silver maple, but may occur on many other hardwood trees and shrubs.
Damage from cottony maple scale insects is usually very limited. Even the occasional heavy populations will do little more than stunt tree growth with no lasting effect. Severe infestations that go on for several years may cause twig dieback and only under extreme conditions will entire trees be killed.
The biggest pest factor of cottony maple scale is the annoying, large quantities of honeydew, a sugary solution excreted by the scale insects, that drips from infested trees onto porches, sidewalks, cars, windows and people. Black sooty mold fungus thrives on the honeydew accumulations and further adds to the aesthetic disruption.
Weather and natural enemies team up to keep the cottony maple scale populations low in most years. Because these natural controls are usually effective, applied chemical controls are not recommended. Insecticide applications have the chance of upsetting the natural balance of biological controls present in the trees (by killing predators and parasites) and prolonging the occasional outbreak into a more serious problem. Large, healthy, established trees should be watched but do not need to be treated except in cases where honeydew dripping from heavily infested trees may be unacceptable.
The time to treat cottony maple scale is when the eggs hatch in early July. Two treatments 10 days apart are typically applied in those cases where control is warranted. Treatments as late as late July are effective if thorough application to lower leaf surfaces is achieved. Early treatments, that is sprays applied in June before the eggs hatch, are not effective. Most lawn and garden insecticides can be used. Read and follow label directions.
This article originally appeared in the June 8, 1994 issue, p. 86.