Cottony maple scale was discussed this time a year ago when a localized outbreak occurred in northwest Iowa (between Kossuth and Green Counties). Another very localized outbreak has been reported this year in Black Hawk, Polk, Jasper, Marshall and Poweshiek Counties.
NOTE TO COUNTY EXTENSION OFFICES: Detailed information on cottony maple scale has been sent to the counties named. Other county offices that have reports of scale outbreak are encouraged to contact Extension Entomology for a copy of the electronic file on this insect.
Cottony maple scale prefers maples, particularly silver maple, but may occur on many other hardwood trees and shrubs. Now is the time the scale is most easily noticed. During June, each female scale produces a large, white, cottony egg sac up to 1/2 inch in diameter. 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.
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, especially lady beetles, keep the cottony maple scale populations low in most years. Because insecticide applications have the chance of upsetting the long term natural balance of this insect, spraying is not recommended.
See last year's newsletters ( June 8 and June 17 , 1994) for additional information.
This article originally appeared in the June 23, 1995 issue, p. 95.