The special variety of aphid infests Chinese hackberry trees. They sucks a sugary liquid out of the leaves, then excretes it, making a sticky mess on cars, gardens, lawn furniture and the area.

Rockridge resident Norma Fox didn't know about the aphid until she recently noticed that the sidewalk on her block was all black and sticky.

"It's just like walking on dry soda pop," Fox said.

Asian Woolly Hackberry Aphids can be easily identified. They look like miniature white puffballs and congregate on sooty blackened leaves of once-green trees.

But they're not easy to get rid of.

Unlike most aphids, some members of this variety have wings in spring, summer and fall, making for a rapid spread of the pest. Their excretions, euphemistically called honeydew, grow mold and create a sticky black mess wherever they falls.

"If you park under a hackberry tree with a major infestation, you'll have to wash your windows everyday before you go to work," said Jim Ryugo, a parks and building manager with the Oakland Public Works Agency.

The aphids first emerged on Chinese hackberry trees in Florida and Georgia in 1998 and 1999, according to Don Shor, owner of Redwood Barn Nursery in Davis. They arrived in the Sacramento area in 2001 and 2002.

Ryugo estimates that they've been in Oakland for a couple of years.

The big question for Rockridge residents is what to do about the pest. The trees they infect are owned by the city -- and Oakland's policy is not to spray pesticides.

City officials, though, are expected to launch a community discussion on how to confront this issue over the next few years , Ryugo said.

"The city won't be doing anything about it in the short term, because it is believed that the aphid infestation in itself is not detrimental to the tree," Ryugo said. "It does create an inconvenience for residents who might be parking their cars under the trees or who might be picking the sticky stuff up on their shoes and tracking it into the house."

In the Davis area, Shor has had success with the pesticide Merit (imidacloprid) that, he says, has fairly low toxicity. Merit is the main ingredient in two products that have successfully controlled the problem there.

The first pesticide -- Tree and Shrub Insect Control Concentrate made by Bayer -- is mixed with water and then poured onto the roots of a tree. The aphids are killed when they feed on the leaves.

The second product -- Pointer Tree Injection made by Monterey Lawn and Garden Products -- must be injected directly into small holes drilled into the trunk of a tree, Shor said.

"I had one customer who tried both. He told us the injection moved faster, but they both worked equally well," Shor said.

The advantage of the injection method is that the pesticide is used where it is needed and doesn't get soaked into the ground, Shor said. Still, some gardeners discourage use of the injection method, because moving the drill bit from tree to tree could spread other diseases.

Shor recommends cleaning the drill bit with bleach before each use.

Drenching the area underneath a tree could harm other plants and nearby gardens, Ryugo points out. The city doesn't endorse the use of insecticides, he said.

Gardeners who don't want to use pesticides may just have to live with the sticky mess, he said. Soapy sprays or horticultural oil might help control the pest on a young tree -- but they wouldn't be effective on a larger tree.

On the bright side, Ryugo said, the aphid's sticky excretions are water soluble and can be washed off cars and lawn furniture.

The city planted the Chinese hackberry's in and around Rockridge in the late '70s and early '80s in place of Chinese elms when the elms started to get Dutch elm disease.

"At the time, we had no idea they would get infested with aphids," Ryugo said.