Cottony Maple Scale
Cottony maple scale is always present in Iowa but in most years is too low in abundance to attract attention. However, in some years, the scale population increases above “normal” and becomes large enough to get noticed. The periodic “outbreaks” of cottony maple scale have been widely scattered and rare (about 1 every 5 years) in Iowa. This scale prefers maples, particularly silver maple, but may occur on many other hardwood trees and shrubs.
Cottony maple scale is most easily noticed in late spring. During June, the inconspicuous female scales that spent the winter on the stems expand and begin producing a large, white, cottony egg sac. Egg sacs may grow to as large as 1/2 inch in diameter and may contain up to 1000 eggs. Large numbers of egg sacs look like popcorn strung along the stems and branches. “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.
Scale insects are sap feeding insects. Newly emerged scales crawl to the leaves and settle on the undersurface and begin a sedentary existence of feeding on the tree’s sap. The scales grow to adulthood on the leaves and mate in August or September before the females return to the twigs to spend the winter.
Damage from cottony maple scale insects is usually very limited. Some premature leaf drop may result but small to moderate populations cause no harm to trees. 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. Annoying, large quantities of honeydew, a sugary solution excreted by the scale insects, may drip 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. Only a tiny proportion of the eggs ever hatch and few of the new scales survive. Most are eaten or attacked by a complex of natural enemies, especially lady beetles. Because 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. Small, newly transplanted, high value specimen trees may benefit from treatment.
The time to treat cottony maple scale is just after the eggs have hatched in early July. Two treatments 10 days apart are usually needed. 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.
Homeowners are encouraged to spray with insecticidal soap if they decide to spray in spite of the absence of any direct threat to tree health and vigor. Most tree and shrub insecticides can also be used, but the destruction of natural enemies caused by these insecticides should be carefully weighed against the benefit of reduced annoyance.
Insecticidal soap is available from garden center, hardware and department stores under various trade names. Look for a product that lists as the active ingredient “potassium or sodium salts of fatty acids.” Mix and apply commercial products according to label directions.
You may substitute ordinary dishwashing detergent for the commercial products with little risk of injury to maple trees. To make your own insecticidal soap spray solution, mix 5 tablespoons of liquid dishwashing detergent in 1 gallon of water.
Soap sprays have no residual activity and only control insects that are contacted directly. Thorough spraying of leaf undersides is important for control.