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W. Nile Spray Study Prompts Praise, Caution

Posted on: Wednesday, 22 February 2006, 18:00 CST

By Edie Lau and Chris Bowman, The Sacramento Bee, Calif.

Feb. 22--Pesticide residues turned up in six out of 10 Sacramento County waterways sampled in the wake of aerial spraying last summer to suppress the mosquito-borne West Nile virus.

A $60,000 study, sponsored by the Sacramento-Yolo Mosquito and Vector Control District and released Tuesday, found low concentrations of the pesticide in creeks and sloughs in northern and southern Sacramento County. Both zones were treated during separate periods in August.

For the most part, the concentrations declined within 20 hours of spraying, often to undetectable levels, but in four samples, concentrations rose over that time. The study authors didn't know why, but suggested urban runoff may have contributed to the spikes.

Overall, the results show minimal consequences for the aquatic environment, said Eric Ziegler, a senior scientist with the consulting company that did the study, Larry Walker Associates.

"It appears to indicate with the limited comparisons that we have done that we shouldn't expect significant mortality to freshwater aquatic species with these types of (pesticide) applications," Ziegler told the mosquito board in a presentation Tuesday.

Mosquito district board members appeared largely satisfied with the results, but outside scientists who reviewed the 22-page study at The Bee's request said the analysis left many questions unanswered.

"It's a decent study, but it could be improved," said Ray Chavira, an environmental scientist in the pesticide program of the U.S. Environmental Protection Agency's regional office in San Francisco.

For example, Chavira said the study would be stronger if sampling had been done for a longer time.

The consulting researchers took three "grab" samples of water from each location: before spraying, immediately after spraying and 15 to 20 hours after spraying.

Each sample was assessed for the presence of pyrethrins and piperonyl butoxide (PBO), the active ingredients in the pesticide used.

Overall, the highest reading was 20 parts per billion of PBO found in Elder Creek at Franklin Boulevard immediately after spraying. That reading declined to 3.5 ppb the next afternoon.

Chavira said the study would be more informative if sampling had continued for four days after spraying. Four days is key because the EPA has toxicity data for the two compounds on a variety of fish and aquatic insects based on exposures of up to 96 hours.

And whether a fish is exposed to a toxic substance for 24 hours or 96 hours - or somewhere in between - can mean the difference between life and death.

For example, according to the EPA's "Ecotox" database, the level of PBO that can kill half of a test group of rainbow trout is 4,000 parts per billion over 24 hours. But if the trout are exposed for 96 hours, the level of PBO that can kill half the group drops to 2.4 ppb.

In five instances, including at Elder Creek, the study found levels of PBO above 2.4 ppb in the last samples, which were taken 15 to 20 hours after spraying. It's presumed the levels continued to drop, but impossible to say definitively based on the results.

Ziegler said the decision to stop sampling within 24 hours was a practical one: Because the spray zones were treated for three nights in a row, the researchers were limited to taking samples within a 24-hour period between treatments.

However, he acknowledged that he could have extended the monitoring to four days after the last treatment.

Independent scientists also said the study would be more telling if it had included analyses of waterway sediments and if it had measured the combined toxicity of pyrethrin and PBO. In the pesticide, PBO is added to boost the power and persistence of pyrethrin.

Regarding the decline in pesticide concentrations in the water over time, Donald Weston, a UC Berkeley environmental toxicologist, said the data don't necessarily show that the compounds quickly degraded, as the district's consultant concluded.

"They may have just gone into the sediment," said Weston, who recently tested the sediment of Sacramento area creeks for a similar insecticide and found levels high enough to kill a shrimp-like species that lives in the mud.

Pyrethins quickly bind to particles suspended in the water and settle in the muck, where they can kill worms, clams and other creatures in the food chain.

And while the concentration of pyrethrins was below lethal concentrations for several species, the comparison is misleading, Weston and another scientist said.

The study fails to account for the much higher toxicity that would come from the combination of pyrethrins with PBO, they said. PBO can render the pyrethrins 10 to 150 times more toxic, said Inge Werner, director of the aquatic toxicity laboratory at the University of California, Davis.

Of the waterways sampled, those that showed pesticide residues were Arcade Creek, Chicken Ranch Slough and Coyle Creek in northern Sacramento County; and Elder Creek, Laguna Creek and Morrison Creek in southern Sacramento County.

Waterways that showed no residues were two sites on the American River, Cripple Creek in the north and Land Park pond in the south.

Despite the study's shortcomings, Chavira said, the mosquito district deserves credit for doing the study at all.

"They went out on a limb and did the sampling, and I applaud them for it," he said.

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Copyright (c) 2006, The Sacramento Bee, Calif.

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Source: The Sacramento Bee

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