One of the insects we see on rare occasion in Iowa is the cottony maple scale. When we do see it, however, the show is spectacular. Every 5 to 10 years we experience an "outbreak" of cottony maple scale somewhere in the state. The rest of the time the scale is present but in numbers too low to attract attention. I mention all of this because we have received a few calls and samples already this year. I'm not yet convinced there will be an outbreak over a large area, but here is what you need to know if someone asks.
First, the cottony maple scale prefers maples, particularly silver maple, but may occur on almost any hardwood tree or shrub. Second, the scale is obvious and distinctive (a rare combination when talking about insect pests). During June, inconspicuous female scales on the twigs begin to produce a large, white, cottony egg sac that may grow to the size of a dime (up to 1/2 inch in diameter). During an outbreak, large numbers of egg sacs look like popcorn strung along the stems and branches. "Normal" populations consist of 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.
The eggs within the expanded egg sacs (up to 1000 per sac!) begin to hatch in early July. The new scale nymphs crawl to the leaves where they 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 caused by the sap-feeding cottony maple scales is usually very minor. 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.
Unfortunately, while the trees are not suffering, we might be. 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. Most of the scales are eaten by a complex of natural enemies, especially lady beetles. Because natural controls are usually effective, applied chemical controls are not recommended. Insecticide applications may upset the balance of nature present in the trees; that is, sprays will kill the predators and parasites and prolong 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 (a waste of time and effort!).
Most tree and shrub insecticides can be used when control is warranted, but the destruction of natural enemies caused by these insecticides should be carefully weighed against the benefit of reduced annoyance. Homeowners are encouraged to spray with insecticidal soap ("potassium or sodium salts of fatty acids") if they do decide to spray in spite of the absence of any direct threat to tree health and vigor.