October 26, 2005
Pyrethroid pesticides found at toxic levels
in California urban streams
By Robert Sanders
BERKELEY – A group of commonly used pesticides recently
found at toxic levels in stream sediments in many agricultural
areas around California is also a problem in urban streams, according
to a new study by researchers from the University of California,
Berkeley, Southern Illinois University and the Central Valley
Regional Water Quality Control Board.
Pyrethroid pesticides, widely used on crops like cotton, fruits
and lettuce, and the dominant residential pesticide now that organophosphates
have been phased out, were found in three streams in Roseville,
Calif., a largely residential community north of Sacramento, at
levels toxic to organisms that live in the sediment.
Though toxicity tests were conducted on tiny, shrimp-like animals
called amphipods, a species recommended by the Environmental Protection
Agency (EPA) when testing sediment toxicity, the pesticides could
be affecting other bottom-dwelling animals as well, according
to the researchers.
"Amphipods are a relatively sensitive group of animals,
so this study is telling us that pyrethroids at these test sites
are reaching levels where they can be lethal to sensitive species,"
said study leader Donald Weston, adjunct professor of integrative
biology at UC Berkeley. Future studies should focus on aquatic
insect larvae that commonly live in creeks, such as stoneflies
and mayflies, which also tend to be sensitive to pesticides.
Weston and his colleagues reported their findings in the Oct.
19 online issue of Environmental Science & Technology (ES&T),
a publication of the American Chemical Society. The article will
appear in the Dec. 1 print edition.
The California State Water Resources Control Board funded the
project to investigate whether the switch to pyrethroids from
organophosphates, which as of 2004 were eliminated for most urban
uses, may pose risks to aquatic life, according to state biologist
Robert Holmes, one of the authors.
Pyrethroids have been on the market for more than 20 years, and
only now are people looking at potentially toxic effects on sediment-dwelling
creatures, Weston noted. The new study demonstrates that a potential
toxicity problem only recently recognized in agricultural areas
is now spreading into urban areas as pyrethroid pesticides have
taken over the residential market. Bifenthrin,
the pyrethroid that contributed most to the toxicity measured
in the study, is available widely and sometimes mixed with fertilizer
to spread as granules over lawns.
"Pesticides are often cheap, but most people don't recognize
the environmental costs of using them," Weston said. "It
is my feeling that a lot of urban pesticide use may be unnecessary,
and the answer lies in changing practices and changing the mentality
of pesticide applications in the urban environment."
The findings come at a critical time, when the EPA is beginning
the process of re-registration for most pyrethroids. When pyrethroids
were first registered in the 1970s and 1980s, data indicated that
they bind to sediment and do not stay in the water column. Now,
more than 20 years after pyrethroids hit the market, the EPA is
asking for studies of sediment toxicity.
"The presumption was that if it binds
to sediment, that substance becomes unavailable to organisms and,
from a toxicity standpoint, irrelevant. And we're showing that
not to be a fair assumption," Weston said. "If binding
to sediment was a solution, we wouldn't be worried about DDT,
we wouldn't be worried about PCBs, and we wouldn't be worried
about a half dozen other organochlorine pesticides now banned."
He noted, however, that having the pesticide bound to the sediment
does offer some advantages. As compared to the organophosphates,
which are mostly dissolved in the water, having the pyrethroids
bound to the sediment may make it easier to minimize their release
to creeks, and may mean that toxic levels do not travel as far
Though pyrethroid pesticides are generally considered safe for
fish and other animals that live in the water, until last year
there had been little testing of pyrethroid toxicity on insect
larvae or shrimp-like amphipods that live in bottom sediments.
In the April 8, 2004, online edition of ES&T, Weston and colleagues
Michael J. Lydy and Jing You of Southern Illinois University in
Carbondale reported that some 20 percent of streams in California's
Central Valley contained sediments toxic to amphipods, a laboratory
standard for sediment toxicity.
In the current follow-up study, Holmes, Weston, Lydy and You
sampled three streams in a residential area of Roseville, a rapidly
growing community of homes and subdivisions with little agricultural
activity that could have confused the picture with respect to
pyrethroid sources. Nearly all the creek sediments proved toxic
to amphipods, the researchers found.
The main stream, Pleasant Grove Creek, was in pretty good shape,
Weston said, with contamination only where tributary streams draining
housing subdivisions entered the main creek. He and his team also
sampled two of these tributaries - Kaseberg Creek and the south
branch of Pleasant Grove Creek, which in the dry summers carry
mainly runoff from overwatered residential lawns. Nearly all sediment
samples from these tributaries were toxic to amphipods in lab
tests and contained pyrethroid concentrations known to kill these
Weston and Holmes chose these creeks in part because they are
in an area that harbors natural populations of the amphipod Hyalella
azteca, the same species as the researchers used in their lab
testing. While natural amphipods were found in the main creek
where there were low levels of pyrethroids, no wild amphipods
were found in areas of the tributaries with high levels of pyrethroids
and where they had found the sediments to be toxic in their lab
tests. This gives the researchers confidence that their laboratory
findings are directly relevant to the organisms in the field.
Weston said that the most toxic of the
pyrethroids - all of which are identified by the suffix -thrin
- was bifenthrin, which could have gotten into the streams as
runoff from homes treated by professional pest-control companies
for pest such as ants, or from lawns treated with pesticides or
popular fertilizer/pesticide combinations.
"Our work should be of broad public
interest," Weston said, "because the source of the toxicity
we are finding in the creeks is just residential pesticide use
in a typical suburban community. When people apply pesticides
to their yards, or hire exterminators to do it, they just assume
the pesticides will stay there. I think our work will increase
awareness of the possible environmental dangers of pesticide overuse
and maybe help people think twice before using pesticides 'just
to be safe' when they do not have a clear pest problem."
The study was funded by the Surface Water Ambient Monitoring
Program (SWAMP) of the California State Water Resources Control
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See also Science News:
... bifenthrin, was found at levels about 15 times
higher than those reported in areas of California with intensive