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New Investigation Shows Spraying Ineffective in Preventing Spread of West Nile

(Beyond Pesticides, August 21, 2006) During the peaking of West Nile
Virus season and an increase in community decisions to spray for
mosquito control, a new study shows that spraying does not reduce the
transmission of West Nile Virus.

Recognizing the widespread use of truck-mounted spraying to control
adult mosquitos, yet the lack of research on the true effectiveness
of this method in reducing the transmission of West Nile Virus (WNv)
disease, a group of scientists and practicioners conducted an
efficacy investigation of truck-mounted spraying in reducing mosquito
populations. The study, "Efficacy of Resmethrin Aerosols Applied from
the Road for Suppressing Culex Vectors of West Nile Virus," is funded
in part by the Centers for Disease Control and the National
Institutes of Health, and led by the Harvard School of Public Health
(Michael R. Reddy, et. al.) appears in the June 2006 issue of
Vector-Borne and Zoonotic Diseases, Volume 6, Number 2.

Three suburban landscapes in eastern Massachusetts with an array of
lot sizes, street, and vegetation patterns provided the study site.
Following U.S. EPA guidelines for flow rates and droplet size, and
heeding important mosquito control considerations for wind speed and
temperature, a typical spray application of the pyrethroid pesticide,
resmethrin, commenced just after dusk and continued for two hours.

Mosquito populations were measured looking at egg-laying rates in
treated and non-treated areas in six different trials during the
months of July and August. Minimum and maximum rates of resmethrin
were applied. Generally, about as many eggs were deposited before the
pesticide application as after in both the treated and untreated
areas, meaning the treatments did not decrease the reproductive
activity of the adult mosquitos. In only one of the trials did the
egg rafts decrease somewhat after spraying, and in another trial the
populations of eggs actually rose after treatment in both the treated
and untreated sites.

The authors conclude "we find that ULV applications of resmethrin had
little or no impact on the Culex vectors of WNV, even at maximum
permitted rates of application, [and] such insecticidal aerosols,
delivered from the road, may not effectively reduce the force of
transmission of WNV."

In further discussion the authors state, "Although numerous field
trials have demonstrated that insecticidal aerosols are lethal to
caged mosquitoes (Mount 1998), few have monitored their impact on
mosquitoes in nature. A previous study (in Memphis, Tennessee by one
of the authors, Paul Reiter,) demonstrated an 80% reduction of Culex
species on the night after a treatment, but concluded that a single
application was probably inadequate for meaningful reduction of human
risk of arboviral infection (Reiter et al. 1990)." The authors
consider the nature of the Memphis neighborhood with its larger plots
(five times larger than New England), extensive lawns and lack of
shrubbery, the major factor in the efficacy numbers found in this
study.

In 1998, "A critical review of ultralow volume aerolsols of
insecticides applied with vehicle-mounted generators for adult
mosquito control." appeared in the Journal of the American Mosquito
Control Association (Mount, G.A.), and concluded that the average
upwind and downwind mosquito kill from truck-mounted spraying to be
between 21% -45%.

Take Action: For responsible, safer and smarter control of mosquitos
and vector diseases in your community see Beyond Pesticides' Tools
for Change at
http://www.beyondpesticides.org/mosquito/activist/index.htm.

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