California Avenue's beleaguered street trees were the target of city concern late last week due to the appearance of hundreds of fuzzy tan growths on the underside of leaves of the young southern live oak trees.
The growths turned out to house larvae of a harmless "gall wasp."
The young trees made headlines in late November because of being "over-pruned" by a contract tree-maintenance firm, according to city staff. They were planted a year ago to replace the 63 street trees that were cut down in September 2009, triggering an angry community response.
But the fuzzy galls, spotted by an editor at the Weekly, caught city planning Arborist Dave Dockter by surprise and he called for reinforcements from the city staff and University of California, Davis, experts.
Steve Scott, a project manager in the tree section of the Public Works Department, did a quick inspection and solved the mystery.
"The woolly structures on the undersides of the leaves are galls made by a gall wasp," he reported in an e-mail.
"I noted the same thing on city Southern Live Oaks on the south side of the 800-900 blocks of Page Mill Road a few years ago and submitted them to the Santa Clara County Division of Agriculture. They sent them on to the state lab in Sacramento.
"The identification was live oak woolly leaf galls caused by the cynipid wasp Andricus laniger."
He said there are no controls recommended for the wasp, and the galls have little impact other than discoloring the leaves. He said the density of the galls on California Avenue is higher than those he saw on Page Mill. Each small tree has scores of the galls.
He said the wasp is specific to several species of live oaks and will not appear on other types of trees planted on the street.
He said the galls may have come in with the oaks, although the Page Mill oaks he saw earlier are less than a mile away.
Dockter said the city will be checking with the supplier.