Cottony Maple Scale
The Cottony Maple Scale is an insect pest of maples, especially silver maples and locust trees, but also will infest several other species. The pest is very common in the tri-city area of Michigan in June.Although the insect does not usually cause much damage to the tree, it is truly a pest to the humans who live below because of the copious honey-dew excretions of the insect. The honey-dew falls onto surfaces below and sooty mold proliferates in the sticky sweet substance.
Systemic insecticides applied by trunk or soil injection, or soil drench are the methods of choice but are more costly than surface sprays. The increased cost is justified because systemic insecticides are more effective against scale insects. The best thing about systemic treatment is that it does not kill the beneficial predatory insects (twice-stabbed ladybugs and tiny parasitic wasps).
Surface sprays are often ineffective and can worsen the condition if done in May and June, because beneficial predatory insects are killed while the adult scales are largely resistant to treatment by sprays. The white cottony egg sacs burst open in July as the eggs hatch and the immature scales crawl out to the leaves. Sprays coating the underside of the leaves at this time can be effective, but thorough coverage is difficult.