"We don't know yet if the source is a product a homeowner would use, such as lawn insecticides or other outdoor insecticides, or if they may be coming from applications by professional pest controllers. We just know they are somehow finding their way to urban creeks," Weston said.
Weston does not foresee a dramatic change in mosquito control strategies as a result of this study's findings, particularly when weighing the risks of mosquito-borne diseases. "Public health risk still trumps everything, and pyrethrin and PBO exposed to sunlight degrade easily into innocuous substances after a day or two," he said.
However, based upon these study results, Weston said regulators should not only consider the toxicity of individual active ingredients in a product, but also how ingredients may interact with other chemicals in the environment. While a decidedly challenging task, such interactions currently are not considered when risks of pesticides are evaluated by regulators, he noted.
Changes to the instructions on how the pesticides are used may also help, the researchers said. Weston pointed out that authorities advised residents in Sacramento to hose down their outdoor furniture after the mosquito spray applications. That may have actually washed more PBO into the creeks, adding to the PBO coming from the spray that settled directly on the creek surface.
Considering this study's findings, said Weston, authorities may want to consider advising residents to leave their outdoor furniture unused in the sun for a day or two to let the pyrethrin and PBO residue degrade rather than send the runoff into the creeks.