Science News (Vol. 169, No. 5, pg. 74)  [Printer-friendly version]
February 5, 2006


[Rachel's introduction: "The Environmental Protection Agency needs to
take a closer look at pyrethroids" with an eye toward changing how
those 22 compounds are marketed and used, argues Michael J. Lydy, an
environmental toxicologist at Southern Illinois University in
Carbondale. Ample and growing data, he says, challenge "the
suggestion that in the environment, pyrethroids will be innocuous."]

By Janet Raloff

Rachel Carson turned the pest-control world upside down in 1962. In
Silent Spring, she documented how long-lived organochlorine
pesticides, most notoriously DDT, were not only ridding croplands of
insects, streets of mosquitoes, and homes of spiders but also exacting
a high toll on songbirds and other nontargeted species. The chemicals'
broad-spectrum potency and resistance to breakdown, advantages in
their use against pests, emerged as hazards.

Shortly after the publication of Carson's book, industrialized
countries began phasing out such persistent organic pollutants, or
POPs. There's now a United Nations treaty aiming at their global
elimination (SN: 11/8/03, p. 301: Available to subscribers at

In the wake of organochlorine pesticides came organophosphate agents.
Although these agents are highly effective, their toxicity to
nontarget animals -- including people -- echoed the perils of DDT.
Regulators responded, and by the middle 1990s, once-popular members of
this class of agents -- such as dursban, malathion, and chlorpyrifos
-- were being phased out or severely restricted in their uses.

In recent years, farmers and others have increasingly turned to
products based on pyrethrins, chemicals made by certain members of the
chrysanthemum family. Farmers in various parts of the world have for
millennia used preparations from these flowers to protect crops from
insects. Since the 1960s, manufacturers have produced synthetic
analogs -- called pyrethroids -- of the herbal products' active

Although pyrethroids have greater toxicity to insects and somewhat
more resistance to breakdown than their natural counterparts do,
studies have demonstrated that these synthetic chemicals pose little
risk to most vertebrates, from songbirds to people.

Pyrethroids stand poised to overtake organophosphate insecticides for
farm use and are already the leading insecticides sold to homeowners.
However, emerging data show that even pyrethroids can pose serious
environmental hazards. At concentrations found in streams, the
chemicals can kill beneficial insects and crustaceans and may even be
acting -- below the radar screen -- to poison fish and lizards.

Most of these findings came to light in some dozen presentations in
Baltimore last November at the Society of Environmental Toxicology and
Chemistry (SETAC) annual meeting. The research described there
suggests that, at least where the mum-based pesticides might enter
streams, these compounds should be used sparingly.

"The Environmental Protection Agency needs to take a closer look at
pyrethroids" with an eye toward changing how the 22 such compounds
that it has registered are marketed and used, argues Michael J. Lydy,
an environmental toxicologist at Southern Illinois University in
Carbondale. Ample and growing data, he says, challenge "the suggestion
that in the environment, pyrethroids will be innocuous."

Hunting thrins

"Walk down the pesticide aisle of your local hardware store and read
the active ingredients in insecticides. Nearly every one ends in
'thrin,"" a dead giveaway that it is a pyrethroid, observes Donald P.
Weston, an environmental toxicologist at the University of California,
Berkeley. Only a few pyrethroids -- most notably esfenvalerate -- lack
that suffix.

Although many of these compounds have been used for decades,
especially on farms, "no one had looked for them in the environment,"
Weston notes. In the past few years, he and his colleagues launched
several surveys to check whether pyrethroids were causing harm in
streams. Because these pesticides don't readily dissolve, but instead
glom on to particles and quickly settle out of water, his team focused
its analyses on sediments.

Their findings proved eye-opening, Weston told Science News.

In one study of creeks adjacent to farmlands across a 10-county area
in California's Central Valley, researchers looked for five
pyrethroids and found one or more in at least three-quarters of the 70
sediments sampled.

The researchers then tested two stream dwellers: the amphipod Hyalella
azteca, which is a small, shrimplike crustacean, and a larval midge of
the species Chironomus tentans. Ecologists use these tiny "lab rats of
the sediment-testing world" for toxicity assessments, Weston explains.

At 42 percent of the sampled sites, the sediment proved deadly to at
least one of two species, his group reported 2 years ago.

In a follow-up study, the scientists spiked sediment samples from
clean sites with six common pyrethroids to compare their toxic effects
on H. azteca. They measured each compound's LC50 -- the concentration
lethal to 50 percent of animals exposed in a test.

In the April 2005 Environmental Toxicology and Chemistry (ET&C), the
team reported that permethrin's LC50 was 60 to 110 parts per billion
(ppb), depending on how much organic carbon the sediment contained.
The LC50 for the remaining pyrethroids was far lower, indicating
greater toxicity. The most toxic: lambda-cyhalothrin and bifenthrin,
which have an LC50 of 2 to 6 ppb.

The crustaceans' growth was significantly retarded at concentrations
just one-third of a pyrethroid's LC50.

Lawn pollution Farm runoff isn't the only -- or perhaps even the most
important -- way in which these agents get into streams. Weston and
his Berkeley colleague Erin L. Amweg reported data at the SETAC
meeting showing that pyrethroids are washed into waterways from
suburban yards by rain and lawn watering.

RUNAWAY RUNOFF. Lawn-watering runoff at this home in Roseville,
Calif., illustrates how pyrethroids used on the yard would be washed
into storm drains, which are a direct conduit to neighborhood streams.

In one recent study, Weston, Lydy, and others surveyed streams in
Roseville, a suburb of Sacramento, Calif. Only a decade earlier, land
along these creeks had been arid grassland. Since then, much of it has
been converted to subdivisions sporting four homes per acre, most with
manicured lawns.

Roughly 90 percent of the stream sediments sampled contained
bifenthrin, and the majority of them had bifenthrin concentrations
toxic to Hyalella, the scientists report in the Dec. 15, 2005,
Environmental Science & Technology. Often, one to five more
pyrethroids were present.

In contrast, the pesticides didn't show up in waters draining
Roseville sites free of residential development.

In toxicity, bifenthrin dominated the suburban sediments. Indeed, Lydy
told Science News, "80 percent of our samples had enough toxicity due
to bifenthrin alone to cause at least half of our [amphipods] to die."
The team recorded pesticide concentrations as high as 437 ppb -- that's
about 100 times as great as its LC50 for H. azteca and 15 times the
highest bifenthrin concentration seen in sediments of creeks running
through Central Valley croplands.

This indicates, Weston says, that the highest concentrations of
pyrethroids in creek sediments trace to "classic suburbia -- we're
talking Mom, Dad, two kids, and a dog."

Although pesticides applied by professional exterminators around the
perimeters of homes are a possible source of the creek contamination,
the research group strongly suspects that much of the bifenthrin comes
from lawn-care products. Some fertilizers even include bifenthrin, so
that homeowners can feed their grass and kill bugs in one pass.

In the Roseville study, the pesticides didn't appear to travel far
once they reached a creek, with the high concentrations appearing only
within 100 yards or so of storm-drain outfalls.

What's not clear, Weston and others observe, is whether the California
data reflect what's occurring nationally or might instead represent a
worst-case scenario. For instance, Amweg presented data at the SETAC
meeting indicating that creeks near Sacramento and San Francisco
showed substantial sediment contamination but streams in Nashville

The California sites, unlike Nashville, get little summer rainfall to
dilute stream pollutants. Moreover, many of California's urban areas
rely on concrete storm drains to channel lawn runoff directly into
streams, whereas the Nashville sites were separated from waterways by
a corridor of greenery.

Too excited Joel R. Coats of Iowa State University in Ames and his
colleagues have been probing why pyrethroids "are as nasty as DDT [is]
to a lot of aquatic life -- including fish."

HOW NEAT? Aquatic caddis fly nymphs build protective cases from plant
debris. Ordinarily, a nymph cuts and stacks materials, log-cabin
style, into an orderly, well-aerated covering (top inset). Pyrethroid-
exposed nymphs, however, make chaotically structured dwellings from
uncut parts (bottom inset) or forgo such protection altogether.

Pyrethroids poison pests by wreaking havoc on their nervous systems,
as most insecticides do. When nerves transmit an impulse, Coats
explains, "there's an electrical ripple that's triggered by sodium
gates in [each cell] opening in sequence." Pyrethroids perturb the
nerve cells' sodium gates, however, so that once open, they never
fully close, Coats says. The resulting sodium leaks maintain nerve
cells in a state of overexcitation that kills the insects.

Because the nervous systems of crustaceans and many other soft-bodied
aquatic animals resemble those of insects, these nontargeted animals
are also vulnerable to pyrethroids.

Coats observes that mammals and birds gain some protection from
pyrethroid poisoning by two mechanisms: production of esterase enzymes
that inactivate the poisons by splitting them in half, and another
metabolic process that employs oxidation. He reported at the SETAC
meeting that although rainbow trout, bluegill, and fathead minnows can
all oxidize pyrethroids, their esterase enzyme activity doesn't break
apart the pesticides.

Although these pesticides may induce ill effects that fall short of
lethality, toxicologists have generally been forced to focus on their
deadliness, Weston says, because fatal concentrations tend to be at or
near the minimum value at which current technology can detect the
pesticides. If the pesticides cause sickness, therefore, it's likely
to happen at concentrations too low to measure, he says. To get around
this difficulty, some scientists have added minute amounts of the
compounds to tanks of water containing aquatic animals.

At Oregon State University (OSU) in Corvallis, Katherine R. Johnson
and her colleagues administered esfenvalerate to aquatic nymphs of the
caddis fly (Brachycentrus americanus) -- an insect eaten by many fish.

For protection from predators, these nymphs enshroud themselves in
hard cases. As the OSU researchers increased pyrethroid concentrations
above 0.05 ppb, formerly resting animals began fleeing their cases in
increasing numbers, notes coauthor Jeffrey J. Jenkins. Among nymphs
that fled, three-quarters of those exposed to as little as 0.2 ppb
esfenvalerate didn't rebuild their cases. Rebuilt cases were
disordered and much weaker than the originals, the scientists reported
at the SETAC meeting.

Conditional toxicity Environmental stressors can sabotage pesticide-
detoxification systems, even in animals that would otherwise withstand
the chemicals, notes Larry G. Talent. At Oklahoma State University in
Stillwater, he studied adult green anole lizards (Anolis
carolinensis), 6 to 8 inches long, exposed to a pyrethroid product
used to treat birds for mites and lice.

When he doused the lizards with a solution of the pesticide and then
maintained the reptiles at a comfortable 95 deg. F, none died. However, 70
percent of treated lizards died within 2 days when they were instead
housed at a cool 68 deg. F. Without pesticide exposure, the lizards showed
no mortality at the lower temperature, Talent reports in the December
2005 ET&C.

Low temperatures, which might mimic night or winter environments, pose
a double whammy for pyrethroid effects: Not only is the lizard's
nervous system more vulnerable to poisoning but its metabolic
breakdown of pollutants also slows.

Mark A. Clifford last year reported a similar synergy between two
environmental stressors -- pyrethroid exposure and a viral infection --
in young salmon. The University of California, Davis fish pathologist
exposed 2-month-old chinook salmon for 4 days to either esfenvalerate
or chlorpyrifos, an organophosphate pesticide. He then seeded some of
the aquariums holding the fish with infectious hematopoietic necrosis
virus, which can kill juveniles.

Fish exposed to low doses of the virus survived, as did those exposed
to either pesticide alone, Clifford's team reported in the July 2005
ET&C. Deaths occurred only in fish exposed to high concentrations of
the virus or to both the pyrethroid and virus. Within 3 days of being
exposed to either dose of virus, roughly 70 percent of the pesticide-
exposed salmon fry were dead.

The pyrethroid's impact "was totally unexpected," Clifford says. Two
follow-up trials confirmed that the initial observation was not a

Winds of change? EPA considers new data when it periodically reviews
its approvals of pesticides registered before 1984. Reevaluations for
permethrin, resmethrin, and cypermethrin are slated for completion
this year, and three other pyrethroids are to be reviewed by 2008.

Because bifenthrin was registered in late 1985, it's not scheduled for
such a reevaluation. In a statement to Science News, however, EPA's
Office of Pesticide Programs (OPP) notes that this pesticide's
manifestation of "certain toxic properties at the level of detection
[makes it] challenging for the agency to determine whether risks from
the use of this pesticide are acceptable."

In fact, the statement says, to better understand pyrethroids'
toxicity and bioavailability to nontarget organisms, OPP is "reviewing
the sediment toxicity studies on bifenthrin, cypermethrin, cyfluthrin,
and esfenvalerate that were recently submitted [by Weston's group and
others]." These pesticides were chosen as "surrogates," the statement
says, for assessing the exposures and toxicity of other pyrethroids.

Indeed, OPP notes, despite their use on some 50 agricultural crops,
some pyrethroids have only "conditional" approval from EPA, pending
future evaluation of their sediment toxicity and of the value of
buffer zones in keeping treated areas from tainting streams.

OPP says that it anticipates completing a "comparative assessment for
pyrethroids" by December.

Pyrethroid manufacturers are already bracing for change.

Jim Fitzwater, a spokesman for bifenthrin-maker FMC Corp. of
Philadelphia, says that homeowners need to be educated about how and
when to apply lawn-care products containing pyrethroids. He notes that
his company sells to consumer-products companies rather than consumers
and says, "We're looking at working with [these] end-use manufacturers
to do a better stewardship job."


2005. Pyrethroid pesticides found at toxic levels in California urban
streams. University of California, Berkeley press release. Oct. 25.
Available here.

2004. Sediments in many Central Valley streams contain toxic levels of
pyrethroid pesticides. University of California, Berkeley press
release. May 6. Available here.

Amweg., E.L., D.P. Weston, J. You, and M.J. Lydy. In press. Pyrethroid
insecticides and sediment toxicity in urban creeks from California and
Tennessee. Environmental Science & Technology.
Abstract available here.

Amweg, E.L., and J. You. 2005. Pyrethroid pesticide distribution and
toxicity in urban creeks. SETAC North America 26th Annual Meeting.
Nov. 13-17. Baltimore. Abstract.

Amweg, E.L., D.P. Weston, and N.M. Ureda. 2005. Use and toxicity of
pyrethroid pesticides in the Central Valley, California, USA.
Environmental Toxicology and Chemistry 24(April):966-972. Abstract
available here.

Clifford, M.A., et al. 2005. Synergistic effects of esfenvalerate and
infectious hematopoietic necrosis virus on juvenile chinook salmon
mortality. Environmental Toxicology and Chemistry 24(July):1766-1772.
Abstract available here.

Coats, J.R. 2005. Toxicology of synthetic pyrethroids to fish. SETAC
North America 26th Annual Meeting. Nov. 13-17. Baltimore. Abstract.

DeLorenzo, M.E., et al. 2005. Toxicity of the pyrethroid insecticide
permethrin to adult and larval grass shrimp (Palaemonetes pugio).
SETAC North America 26th Annual Meeting. Nov. 13-17. Baltimore.

Johnson, K.R., J.J. Jenkins, and P.C. Jepson. 2005. Exposure to
esfenvalerate induces case-leaving in the caddisfly Brachycentrus
americanus. SETAC North America 26th Annual Meeting. Nov. 13-17.
Baltimore. Abstract.

Lydy, M., D. Weston, and J. You. 2005. Relative contributions of
agricultural or urban pyrethroid usage to toxicity in California
streams. SETAC North America 26th Annual Meeting. Nov. 13-17.
Baltimore. Abstract.

Talent, L.G. 2005. Effect of temperature on toxicity of a natural
pyrethrin pesticide to green anole lizards (Anolis carolinensis).
Environmental Toxicology and Chemistry 24(December):3113-3116.
Abstract available here.

Weston, D.P.... and M.J. Lydy. 2005. Aquatic toxicity due to
residential use of pyrethroid insecticides. Environmental Science &
Technology 39(Dec. 15):9778-9784. Abstract available here.

Weston, D.P., R.W. Holmes, and T. English. 2005. A tale of two creeks:
an intensive study of pyrethroids and related toxicity in urban
environments. SETAC North America 26th Annual Meeting. Nov. 13-17.
Baltimore. Abstract.

Weston, D.P., J. You, and M.J. Lydy. 2004. Distribution and toxicity
of sediment-associated pesticides in agriculture-dominated water
bodies of California's Central Valley. Environmental Science &
Technology 38(May 15):2752-2759. Abstract available here.

Further Readings:

Belden, J.B., and M.J. Lydy. 2006. Joint toxicity of chlorpyrifos and
esfenvalerate to fathead minnows and midge larvae. Environmental
Toxicology and Chemistry 25(February):623-629. Abstract available

Cheplick, J.M., et al. 2005. National exposure analysis of pyrethroids
(Part 2): Erosion assessment using PRZM 3.12 at the watershed level.
SETAC North America 26th Annual Meeting. Nov. 13-17. Baltimore.

Holmes, C.M., et al. 2005. National exposure analysis of pyrethroids
(Part 1): Spatial proximity of agriculture to surface water. SETAC
North America 26th Annual Meeting. Nov. 13-17. Baltimore. Abstract.

Lydy, M.J., and K.R. Austin. 2004. Toxicity assessment of pesticide
mixtures typical of the Sacramento- San Joaquin delta using Chironomus
tentans. Archives of Environmental Contamination and Toxicology
48(December):49-55. Abstract available here.

Raloff, J. 2003. POPs treaty enacted. Science News 164(Nov. 8):301.
Available to subscribers here.

______. 2000. The case for DDT. Science News 158(July 1):12-13.
Available here.

______. 1999. Thyroid linked to some frog defects. Science News
156(Oct. 2):212. Available here.

Ritter, A.M., et al. 2005. National exposure analysis of pyrethroids
(Part 3): Sensitivity analysis of exposure to drift and erosion. SETAC
North America 26th Annual Meeting. Nov. 13-17. Baltimore. Abstract.


Erin L. Amweg University of California, Berkeley Building 102-RFS
Berkeley, CA 94720-3140

Mark Clifford Fish Health Laboratory Medicine and Epidemiology
University of California, Davis Davis, CA 95616

Joel R. Coats Iowa State University Department of Entomology Ames, IA

Jim Fitzwater FMC Corporation 1735 Market Street Philadelphia, PA

Jeffrey J. Jenkins Department of Molecular Toxicology Oregon State
University 1007 Ag and Life Science Building Corvallis, OR 97331-7301

Katherine R. Johnson Department of Environmental and Molecular
Toxicology 1007 ALS Building Corvallis, OR 97331-7301

Michael J. Lydy Department of Zoology Southern Illinois University
Carbondale, IL 62901-6501

Mah Shamin Environmental Risk Branch 5 Environmental Fate & Effects
Division 1200 Pennsylvania Avenue, N.W. Washington, DC 20460

Society of Environmental Toxicology and Chemistry 1010 North 12th
Avenue Pensacola, FL 32501-3368

Donald P. Weston University of California, Berkeley Building 102-RFS
Berkeley, CA 94720-3140

From Science News, Vol. 169, No. 5, Feb. 4, 2006, p. 74.

Copyright 2006 Science Service.