The pesky critters are biting with a ferocity that officials said they haven't seen since the 1990s.
The Mayfield tidal gate, which is located east of U.S. Highway 101 and controls the flow of bay tidewater to the entire flood-control basin, has eroded. A gaping hole is allowing flooding of about 150 of the basin's 640 acres that are usually dry, said Russell Parman, Santa Clara County Vector Control acting district manager.
"We're in a rather unusual situation," he said, noting that the gates have successfully managed the water for 20 years.
The ebbing and flowing of salt water has activated the hatching cycle for the salt-marsh mosquitoes, which specialize in laying eggs above the water line at the edge of marshes. In just four days, an adult mosquito is on the wing.
So far, the mosquitoes have cycled through numerous "fly-offs" — hatching, going through their larva and pupa stages and turning into full-grown biters.
These particular mosquitoes, also known as floodwater mosquitoes, are robust fliers, traveling up to 5 miles from their hatching site to feed on a blood meal and return to the marshlands to lay hundreds of eggs.
Most of the complaints from bitten humans have come from businesses along the Baylands and the Palo Alto Golf Course. But residents further inland at Seale and Greer parks said they have also experienced the voracious insects.
"My daughter has been doing soccer practice every day this week at Greer. All of the girls on her soccer team were covered in bites. I dropped her off this morning and got three bites just getting her bag out of the back of the car," Palo Alto resident Michelle Cale said.
On Monday she took her dog to the park and was immediately attacked.
"It's particularly bad on the grass. Even the ones with bug spray on them were bitten. They are not the West Nile-carrying type of mosquito, but they are especially bitey ones," Cale said.
Parman said the floodwater mosquito is not a West Nile carrier. It feeds on mammals rather than birds. But it will aggressively bite during the daytime as well as at night. The attack of the mosquitoes could last through October if breeding conditions remain in their favor, he said.
Since the discovery of the eroded gate, vector-control staff have been applying weekly ground treatments of larvicides.
The city's parks and golf-course workers have reduced irrigation to combat the problem. Parks crews are inspecting and fixing areas that have standing water, department officials said in an email to concerned residents.
The Santa Clara Valley Water District is working to fix the tidal gate.
Vector-control officials are looking into possibly applying aerial spray to the water over the affected portion of Baylands, but they must get permission from regulatory agencies such as the California Department of Fish and Game. Parman said the situation is somewhat complicated due to the several environmentally sensitive and endangered species that inhabit the Palo Alto Baylands.
A contractor is waiting for approval of a proposal. Aerial sprays, if used, would be restricted to the marshes, he said.
Parman said it's unlikely that any fogging would be done, as has been applied in Mountain View and Los Altos. Fogging is used when birds and mosquitoes in the area are infected with West Nile virus, he said.
Vector-control officials recommend the public wear mosquito repellant on top of clothes, rather than under clothing. The mosquitoes will readily bite through clothing as thick as denim jeans and thicker. Spraying or wiping a repellant containing DEET on top of clothing will significantly help.
Updates on the floodgate repairs will be available through the Santa Clara Valley Water District, at ValleyWater.org.