No Evidence of Effectiveness
    It seems that almost all discussions about using adulticides over populated areas reflect the implicit assumption that they are effective. Perhaps officials make this assumption because of past success in controlling mosquitoes in other contexts, but the paucity of evidence that it is effective for reducing the transmission of the West Nile virus to humans is glaring. Indeed, a Vanderbilt University molecular biologist who teaches an advanced course on environmental toxins, Dr. Wallace LeStourgeon captures this succinctly, "I think we need to turn the argument around and begin insisting to see convincing evidence that spraying reduces west Nile disease. It simply does not exist." Furthermore, more and more scientific evidence is emerging that adulticiding is in fact not effective.

    Locally, we have repeatedly requested evidence that spraying with adulticides is safe and effective. As to effectiveness, in the forum at the Davis City Hall on August 23, 2005, Dr. Vicky Kramer, Chief, Vector-Borne Disease Section, California Department of Health Services, cited a presentation by Dr. Robert Nasci as evidence. This report is also cited in the answers to questions submitted for that forum, posted on the City of Davis website.  (See our responses to those answers).  Also, several times in those answers "Louisiana papers" are cited, but no specific reference is given.  There is also a general reference to the CDC West Nile web page. No studies are immediately apparent on the CDC website. The only specific citation given in the answers is of the Nasci report.

    There are two sides to this question, of course.  The first is if vector control and public health officials are acting from a background of strong evidence that what they are doing is effective.  The answer is "no."  The second is if there is evidence that the spraying of adulticides is not effective.  This answer is "yes."  We first review what was originally offered as evidence of effectiveness, we then discuss some of the evidence against the effectiveness of adulticiding, and we finally make a few comments about two more recent papers being touted as evidence by the district.

The Nasci Report.

    In his report, which we have not seen published in a scientific journal, of work conducted in the Ft. Collins area of Colorado in 2003, Dr. Nasci draws the two conclusions that 1) "Adulticide caused a measurable decline in vector mosquito density," and 2) "The number of new human cases in Ft. Collins with onsets after adulticide applications declined relative to other communities in the region."

    Some scholars have challenged Nasci's conclusions, however, on the grounds that there were serious deficiencies in the data that fail to support the conclusion that adulticiding resulted in a dramatic reduction of adult mosquito populations. Martin Walter, Professor of Mathematics at the University of Colorado, who studied the presentation and discussed the issues with colleagues, has written that:

"The Nasci presentation does not in any way show that the number of cases of West Nile were reduced by spraying. (While there definitely was a risk to the environment posed by the pesticide.) The City of Boulder did not spray and had better results (as measured by number of West Nile cases) than any of the communities around it that did spray. I would have to agree . . . that spraying has a large component of politics mixed in with it. If you spray you are visibly 'doing something.' Unfortunately, the only thing we know we are doing for sure when we spray is that we are adding poison to our environment." [See Walter Note ]

The "Louisiana Papers."

    We wanted to read the actual papers and contacted Dr. Kramer, who faxed us a copy of one paper with the references omitted.  The title of the paper is “Impact of West Nile Virus Outbreak upon St. Tammany Parish Mosquito Abatement District,” and it appeared in the Journal of the American Mosquito Control Association, 21(1):33-38, 2005.

    The important question is whether adulticide reduces transmission of WNV as compared to doing nothing or to treating with larvicides and using other methods only.  What is the decrease in effective WNV transmission when adulticide is used?  This paper cannot possibly answer those questions, as it was not a comparative study.  It was not a study in any way, shape or form; there were no alternative approaches examined; there were no controls.  Rather this is a documentation of what one district did and the impacts upon that district.  The goal of the district was to prevent new WNV cases, yet it had no marker against which to gauge success or failure, there was no comparison of treatment methods, and there were no control areas for comparison.

    Our district officials did not answer the question if they had done models to show effectiveness, and this paper they rely on does not even discuss spraying vs. not spraying.  While the numbers went down, this might have been from the increased larvaciding, and it might have happened in spite of any control measures as the numbers went down naturally anyway.  No conclusions about the efficacy of adulticiding can be drawn from this administrative report, yet it was cited as one of the two best pieces of evidence in August of 2005.

The DHS "Report."

    In 2006 Vector Control officials cite a report by DHS officials as evidence.  This report has significant problems, however.  The first one to note is that the arithmetic in the chart is flawed.  We suspect that DHS officials can do simple arithmetic, so we suspect that this report was not checked before it was published.  It has other flaws, but no conclusions whatsoever can be drawn with the incorrect arithmetic.  Please see a full critique here.

    We made several Public Record Act requests to get all information associated with that report, but full information was never supplied.  In May of 2008 authors from the CDPH published the final version of their report.  It is fatally flawed, however.

    These papers and the report were the best that the head of the California Department of Health Services, Vector Borne Diseases Section, and the SYMVCD could come up with to justify the use of aerial ULV spray of populated areas.  They have no evidence, no models, no scientific studies. 

Actual Scientific Evidence -- as to Inefficacy -- the Reddy, Bowman, and Pimentel papers.

    In a 2006 paper resulting from a study led by the Harvard School of Public Health, Michael Reddy and the other 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. A model simulating the major outcomes of such treatments indicates that they are unlikely to reduce the force of transmission of such an arbovirus."  The model to which they refer appears in another paper, and its abstract states that "The model indicates that ULV has little impact on disease incidence, even when multiple applications are made, although the peak of the epidemic may be delayed. Decreasing the carrying capacity of the environment for mosquitoes, and thus the basic reproduction rate of the disease, by source reduction or other means, is more effective in reducing transmission." If the peak of the epidemic is delayed, this could necessitate more "treatments," which is why some experts insist that the best thing to do is to eschew adulticiding altogether and allow the disease to run its course and move into chronic endemicity.

    A very careful deterministic model of WNv transmission is presented in "A mathematical model for assessing control strategies against West Nile virus," by C. Bowman, A.B. Gumel, P. van den Driessche, J. Wu, and H. Zhu, Bulletin of Mathematical Biology 67 (2005) 1107–1133. The model involves a system of nine nonlinear differential equations that models birds, humans, and mosquitoes.  It is built along the lines of the classic SIR model for infectious diseases (see Infectious diseases of humans, Anderson, R.M. and May, R.M. 1991, Oxford University Press, and "The mathematics of infectious diseases," Hethcote, H.W., 2000, SIAM Rev. 42 (4), 599–653).  The authors ran numerous simulations and concluded that "a mosquito control strategy that allows the daily birth of 104 mosquitoes and mosquito lifespan of at least ten days will fail to eradicate the disease in the mosquito–bird populations," whereas "if larvaciding can ensure that less than 4 × 103 mosquitoes are born daily, then the disease will be eradicated regardless of the presence or absence of other control measures."  While the exact numbers might change in any given setting, and while probabilistic components will cause some oscillations in the actual numbers, two conclusions are inescapable from this work: 1) early, and sufficient, larvaciding is the key to control, and 2) adulticiding late in the season cannot possibly eradicate the virus, particularly with the protocol that SYMVCD is using.  In particular, biological controls that establish ongoing populations would be particularly effective, as they would begin to work at the very beginning of the season.

    In a 2004 paper, West Nile Virus and Mosquito Control, PhD entomologist David Pimentel of Cornell University writes that "Widespread ULV 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."  As to proper control, Pimentel writes "The prime method of control is the elimination of the breeding habitats for larval mosquitoes, such as water accumulating in bird baths, flowerpots, old tires, and other containers."  This discussion is continued here

    Indeed, in their paper “A Model of the Transmission of Dengue Fever with an Evaluation of the Impact of Ultra-Low Volume (ULV) Insecticide Applications on Dengue Epidemics,” Elizabeth A. C. Newton and Paul Reiter, American Journal of Tropical Medicine and Hygiene, 47(6), 1992, pp. 709-720, the authors write about ULV treatments that "Even when the percent reduction of the adult mosquitoes is high, the impact on incidence is very low. Thus, the benefit in medical terms appears low, and may be almost worthless unless other factors, such as the need to reassure the human population with high visibility action, are taken into account" (emphasis ours).  While there are important differences in dengue fever and West Nile disease, two of which are that in dengue the hosts are humans and the disease is not seasonal, the aims of reducing infection rates by limiting mosquito populations are the same, and these models call into serious question the impact of even high rates of killing adult mosquitoes.  And, we have suspected from the very beginning that public health and vector control officials are adulticiding for political reasons with "high-visibility actions" instead ones to protect the public health, so it is very interesting to see the existence of this apparently well-known dynamic acknowledged in a research paper.

Recent Papers Cited

    In the minutes of their Board meetings vector control officials cite two newer papers as evidence of the success of their activities, Macedo, et al1 and Elnaiem, et al2.  The first is about the 2007 spray in north area of Sacramento County only.  There is a better evaluation of impact on mosquitoes than in the Carney paper and there are no greatly overstated claims.  It also includes monitoring of exposure to the spray.  The claim is that the spray reduced the risk of human infection without the risk of hazardous exposure.  However, the link between a small and temporary reduction in the virus in the mosquitoes and reduced risk of human infection is assumed but not demonstrated.  As has been shown in careful studies temporary kills at the rate local officials claim have been ineffective.  Note the 90% kill rate needed for effective control, and local methods do not get in the ballpark of that rate. 

    The Elnaiem paper uses a different data set than the Carney paper claimed, collected by the state researchers.  The overall claim is much more modest about the impact on the virus load in the mosquitoes, but there is no human data.  Again, the link between a small and temporary reduction in the virus in the mosquitoes and reduced risk of human infection is assumed but not demonstrated.  Careful scientific studies use blood serum anaysis to monitor the transmission of the virus to humans. 

    A number of scientists have complained about the recent denigration of science, notably during the last administration.  The Union of Concerned Scientists speaks forcefully and eloquently about the dangers of a continuation of this trend.  In its minutes the local Board touts its "peer-reviewed studies," but these studies do not come close to meeting appropriate scientific standards.  For example, the Carney paper was posted on the CDC website for its "publication," the list of "referees" contained names of vector control officials who are not scientists, the design and implementation of the experiment were fatally flawed, the paper was apparently based on no formal data, and no formal referreeing was apparently done.  Upon seeing this, one scientist remarked that "their idea of peer review seems to be to pass it around to everybody who is already in agreement."  As to the lack of data or referees, we requested all materials related to the paper in a PRA request, and neither data nor referee reports was supplied, after repeated follow-ups.  It is not clear if the scientists on the Board and SYMVCD are asleep at the switch or if people working in these capacities simply do not understand proper scientific process, but the term peer-review must be used only in the appropriate context, to refer to rigorous review by independent scientists.  SYMVCD's use of the term is certainly very different than proper scientific use, and therefore its "peer-reviewed studies" must be taken with a large grain of salt. 


    We spoke about the Bowman paper at length during PBR's "Insight" on July 31, 2007, but neither David Brown nor Glennah Trochet asked for a reference or to see it.  We can only conclude that they do not wish to see any information that might show that spraying adulticides is not effective.

    The platform of the Alliance for Informed Mosquito Management (AIMM) states succinctly that "In too many municipalities across the country, there are inadequate mosquito management policies in place.  In some cases, a coherent management plan does not even exist.  As a result, there is often a heavy reliance on mass spraying of pesticides to kill adult mosquitoes.  This method of mosquito management is widely considered by experts to be the least effective and most risky response to this important public health concern.  There is no credible evidence that spraying pesticides used to kill adult mosquitoes, also known as adulticides, reduce or prevent WNV incidents or illnesses In fact, communities that do not generally use adulticides as part of their mosquito control often have lower cases of WNV than their neighbors that do.  Pesticides used in the battle against mosquitoes have been linked to numerous adverse health effects including asthma and respiratory problems, dermatological reactions, endocrine disruption, chemical sensitivities, and cancer.  Adulticides can also be harmful or fatal to nontarget wildlife.  There are much safer and more effective ways to manage mosquitoes and protect the public from mosquito-borne illnesses like WNV than the spraying of adulticides." (emphasis ours)

    It is also important to note that Shawnee Hoover of Beyond Pesticides has indicated to us that she advised Dr. Kramer of the serious problems with the Nasci study well before August 23, 2005.

    Given that the current evidence is strongly against the effectiveness of spraying, it is important to note that effective alternatives exist, revolving around the concept of integrated pest management. Such alternatives can involve larviciding, use of mosquito fish, implementing more safe and effective biological controls, eliminating standing water, discovering hot spots and treating them aggressively, and extensive public education. The success of the Boulder, Colorado, program is a good example.  Without good evidence of effectiveness, there is simply no reason to spray adulticides no matter what level any of the District's triggers reach.

    We address safety concerns elsewhere on these pages. Actually, at the time of the debut of this webpage the U.S. EPA was roughly halfway through the process to reevaluate the registration of pyrethrins and PBO for use in pesticides. The docket (OPP-2005-0042 and OPP-2005-0043) revealed that the preliminary risk assessment indicated new areas of concern and that there was significant evidence that this assessment had to be revised since it underestimated the risk of harm.

1.  Paula Macedo, Jerome Schlier III, Marcia Reed, Kara Kelley, Gary Goodman, David Brown and Robert Peterson, "Evaulation of Efficacy and Human Health Risk of Aerial Ultra-low Volume Applications of Pyrethrins and Piperonyl Butoxide for Adult Mosquito Management in Response to West Nile Virus Activity in Sacramento County, California," Journal of the American Mosquito Control Association, 26(1):57–66, 2010.  

2.  Dia-Eldin Elnaiem, K. Kelley, Stan Wright, Rhonda Laffey, Glenn Yoshimura, Maria Reed, Gary Goodman, Tara Thiemann, Lisa Reimer, William Reisen, and David Brown, "Impact of aerial spraying of pyrethrin insecticide on Culex pipiens and Culex tarsalis (Diptera: Culicidae) abundance and West Nile virus infection rates in an urban/suburban area of Sacramento County, California." Journal of Medical Entomology 45: 751-757, 2008.

Updated 9-7-11.