No Sweat That Slime All Over Your Car Isn’t Tree Sweat; It’s Aphid Droppings

Ah, sweet spring: showers, flowers and that weird slime on the windshield.

Tree sweat is what some people call it, but they’re wrong. It’s bug, er, excrement - aphid excrement, to be more exact. And it returned in force this month to the Inland Northwest.

“It looks like you’re going to be in for a good aphid problem, at least for a while this spring,” said Kurt Schekel, a horticulturist at Washington State University. “Fortunately it all washes off. But it is a mess and it sure gathers the dust too.”

It’s unclear whether this year’s layer of “honey dew” - the scientific term for aphid spray - is worse than any other.

“I wouldn’t say that it’s a more active year,” said Richard Zack, a WSU entomologist. “I would say it is pretty much normal.”

Depending on what kind of tree is involved, normal can mean enough goop that a car parked regularly in one spot will leave behind a shadow of bright, relatively clean pavement. Before this weekend’s rain, dozens of cars in Pullman alone had turned into studies of murk.

At work is the aphid’s simple but industrious digestive tract, which must process huge amounts of tree sap for a little nutrition.

The resulting liquid is mostly sugar and it can be so sweet that ants will often protect aphids to keep up their food supply, said Keith Pike, research entomologist at the WSU Research Center at Prosser.

To avoid getting stuck in its own goop, the aphid sprays it clear of its body or kicks it away a droplet at a time with a rear leg, Pike said.

Feeding habits vary with different aphid species, and there are about 1,000 different aphids in the West. Some species like the Russian wheat aphid confine themselves to Pike’s specialty, grain crops, which are actually having an easy go of it so far this year. Among the tree-dwelling aphids, favorite hosts include oak, elm, linden and maple.

Tonie Fitzgerald, a WSU extension faculty member in Spokane, generally recommends that concerned homeowners spray with insecticidal soaps. But because that can miss aphids feeding on the undersides of leaves, she suggests having a pest control company inject a tree with a systemic insecticide.

Not that people need to be so concerned.

Aphid drippings can harbor different fungi but most experts agree the host trees are rarely worse for the wear.

“It’s usually an annoyance to people who put things under the tree that leads people to try to control them,” Fitzgerald said.

As for the damage done to cars, that can be fixed with soap, said Jim Kerr, owner of Palouse Audio, a local auto detailer.

“Nobody complains about it,” he said. “It just gets on cars and washes off. What can you do?”

, DataTimes