Mount Prospect identifies suspected culprit behind mysterious soot substance

Mount Prospect officials believe they have identified the culprit that has been leaving a sticky, sootlike on houses and cars in the village.

The public works department’s forestry division inspected reported areas and trees, and believe the black film and stickiness was left by the Cottony Maple Scale, a common insect pest.

According to The Morton Arboretum, the insect, which is native to North America, is usually hosted by apple, oak and maple trees — particularly silver and boxelder maples.

Dave Hull, Mount Prospect Public Works forestry and grounds superintendent, said the sap-feeding insects excrete a sticky, sugary material known as “honeydew.”

Hull said a sooty mold grows on the honeydew, appearing on leaves and branches.

“If there is a car underneath or furniture, it will get on that too,” he said. “When it’s on something hard like that, it can be washed off. It’s not always easy to wash off.”

This year has seen something of a population explosion for the insect, caused by milder temperatures, encouraging it to overwinter.

“We have seen that with other insects, too, such as bagworm, which typically was never a problem this far north,” he said.

Maple is one of the primary species it targets, especially silver maple. Hull said silver maples comprise about 10% of the village’s inventory.

“These are really large, mature trees with broad, spreading canopies,” he said. As a result, they can hang over a driveway or the eave of a house.

Hull said the conditions have not been right for treatments like soil injections at the base of the tree with Imidacloprid. He said it has been a dry summer. Nor is it practical to treat the trees this late in the season.

“It can take two weeks for any sort of insecticide to work, and the leaves are probably going to be on the tree for another three weeks or so,” he said. “They’re getting their sap from the leaves. Once the leaves are gone, the bugs are gone.”