Mosquito Aerial Spray Programs Endanger Human Health, Don’t Work


Aerial mosquito spraying over populated areas this year by the local mosquito control district used a more hazardous pesticide than in previous years.  While there is no scientific evidence that the spray is effective in stopping the spread of West Nile virus (WNv), there is evidence that the spraying endangers health.


The more dangerous pesticide used this year is an organophosphate.  Similar to chemical warfare agents produced during World War II, this chemical adversely affects the human nervous system even at low exposure levels, and ingredients are on California’s Proposition 65 list of chemicals known to cause cancer.


The district justifies its spray protocol by such factors as the number of WNv-infected mosquitoes and birds, but this is a rare human disease.  According to the Centers for Disease Control (CDC) only a tiny fraction of the human population becomes infected, and only 1 in 150 of those infected develop serious symptoms The total serious human case count for Sacramento and Yolo counties for 2011 and 2012 is 1.  

It is well established that it is safer and much more effective to target mosquito larvae before they become biting adults.  Although the district uses mosquito fish to some extent, there are other effective larvae predators, such as nematodes (small roundworms) that establish populations for ongoing control and are effective from the very beginning of the mosquito season.  These nematodes infect only mosquito larvae so they pose no risk to human health or the environment.  Even more effective against the supposed vectors of WNv is a mosquito fungus that has exhibited 95% mosquito mortality and has been shown effective in areas where mosquito fish cannot be used.


Both peer-reviewed scientific research and mathematical modeling demonstrate that aerial spraying is ineffective for WNv control.  Cornell University entomologist David Pimentel writes: “Widespread ULV [ultra-low volume] spraying from ground equipment or aircraft for control of mosquitoes and West Nile virus is relatively ineffective, costly, and has been associated with environmental and public health risks.”  Also, a Harvard School of Public Health study concludes: “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.”


A model widely used for infectious diseases produced two important conclusions when applied to WNv transmission: 1) early, sufficient, treatment for mosquito larvae is the key to control; 2) treatment aimed at adults later in the season cannot possibly eradicate the virus, particularly with the protocol the district uses.


Justifications the district has used to support its aggressive aerial spray program have been very weak at best.  In 2005 the district cited a slide show about treatments in Fort Collins, Colorado and a report from a Louisiana parish, but they revealed nothing about the efficacy of spraying.  In fact, the Fort Collins spraying, when viewed in context, demonstrates the ineffectiveness of spraying: nearby Boulder did not spray and had better results than surrounding communities that sprayed.


The district cites a purportedly peer-reviewed paper from the CDC website on the efficacy of its 2005 Sacramento spraying; however, the authors did not provide a single review in response to a Public Records Act request.  Furthermore, that spray event was not set up as a study and parameters were adjusted after the fact.  Additional flaws include: human infection locations were based on a false assumption and the spray was halted for 8 consecutive days by wind, which inhibits mosquito flight and biting behavior.  The wind is not even mentioned in the report.  No valid conclusions can be drawn, yet officials continue to cite this report as evidence of efficacy.

The district’s claim that the spray “breaks the transmission cycle” is contradicted by research and the facts – within only 17 days after the June 11-12, 2012 spray the District’s own risk assessment triggered spraying once again, confirming that even the more dangerous pesticide did not work.  And, while they have more potent vectors and a humid climate that promotes greater transmission of the virus Washington, D.C. officials do not spray precisely because of spray inefficacy and health risks to residents.


The price tag for this ineffective program is staggering.  For example, the district reports that the cost of the 2005 aerial spraying was  $701,790.


The district must base its policies on sound science and recognize that its current treatments pose a greater risk to public health than the virus against which the spraying allegedly protects.


For references for these points and further discussion, see

Kim Glazzard, Organic Sacramento
Samantha McCarthy, Better Urban Green Strategies
Jack Milton, Stop West Nile Spraying Now
Asael Sala, Pesticide Watch