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 practitioners 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 aerosols 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%.

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