The chemicals, pyrethroids, are man-made versions of natural compounds in chrysanthemum flowers. Their use has skyrocketed in the past few years as U.S. consumers and professionals search for less-toxic alternatives for dangerous insecticides already banned.

The compounds, particularly one called permethrin, are prevalent in lawn products and household and pet sprays, as well as in insecticides sprayed by exterminators and farmers. Also, many cities and counties spray a pyrethroid for mosquito control to prevent the spread of West Nile virus.

Other studies have raised concerns about runoff containing the insecticide imidacloprid and glyphosate-based herbicides like Roundup.

Last fall, a UC Berkeley scientist reported that pyrethroids are polluting streams in Northern California suburbs, wiping out crustaceans and insects vital to ecosystems.

Mary-Ann Warmerdam, director of the state Department of Pesticide Regulation, said that notices are being sent to manufacturers of about 600 pyrethroid products informing them that the state is reevaluating their use. That kicks off a process that will probably culminate in new regulations, and perhaps bans of some products in California.

Although they poison nerve cells of invertebrates, the compounds are among the least toxic insecticides for humans and other mammals as well as birds. That is why they have replaced the organophosphate insecticides diazinon and chlorpyrifos, which were phased out by the EPA because they are particularly hazardous for children.

The most toxic compound found in the California creeks is bifenthrin, which is sprayed around houses by exterminators and is found in some products that are spread on lawns.

Consumers can identify pyrethroids in products by checking labels for compounds that end in “thrin.” They are broad spectrum insecticides effective against a wide variety of flying and crawling insects.

Stay tuned to landscapearchitect.com for additional developments about pesticide regulation.