The pesticides are said to be only mildly toxic, but one of them enhances pesticides known as pyrethroids that are already accumulated in the creeks, most likely from lawn and urban run-off. The combination, according to UC Berkeley environmental toxicologist, Donald Weston, is far more deadly than the individual chemicals.

Weston found that the mosquito spray effectively doubled the toxicity of the sediment in many Sacramento streams. Victims of the toxins include may flies and caddis flies, which are part of the fish diet.

?EUR??,,????'??These species are important prey for fish,?EUR??,,????'?? Weston said. ?EUR??,,????'??So it does mean that their prey will be less abundant.?EUR??,,????'??

Weston?EUR??,,????'???s work prompted the state Department of Pesticide Regulation to re-evaluate more than 600 products containing pyrethroids.

The department ordered 123 manufacturers to provide data on the products including how long they remain active in the environment, how they may be getting into creeks, and how the contamination can be controlled.