Stormwater | Meeting the Challenges of Stormwater Management and Vector Control

Meeting the challenges of Stormwater Management and Vector Control

Integrating stormwater management and vector control in a Phase II city

By Kenneth E. Banks

Managing stormwater in Denton, TX, took a new direction on August 6, 2002, when a mosquito—captured as part of a local University of North Texas research project—tested positive for West Nile virus (WNV). Based on the cases of WNV being found in surrounding states, as well as in other areas of Texas, city officials suspected that it would be only a matter of time before the disease reached Denton. After August 2002, however, Denton’s mosquito control philosophy shifted from being associated mainly with nuisance controls to potentially having major public health implications. The city needed a unified, coordinated approach to mosquito management that ensured the protection of both public and environmental health. This article outlines the rationale behind Denton’s mosquito control program and describes how the city has attempted to meet the multiple—and sometimes conflicting—challenges of stormwater management, integrated pest management, and public health protection.

One of the immediate issues for the mosquito control program involved determining which control methods were most appropriate. Before WNV arrived in Texas, several groups of citizens had worked with the City of Denton to decrease the number of pesticides and herbicides applied in parks and other public places. The city has worked with these groups to adopt an integrated pest management approach to controlling both animal and plant pests within city properties. The approach for mosquito control was no different, and city staff members desired to put together a program based on preventative—rather than reactive—controls. The overriding goal of the program was to instigate scientific and appropriate responses for mosquito control, based on data about mosquito habitats, breeding seasons, and control measures and on the overall risk to public health. Staff also realized, however, that this type of program could not be administered solely by the City of Denton. Citizen involvement and guidance were needed, as was technical support.

Public health is an overriding goal in any municipal environment. In addition, relatively recent National Pollutant Discharge Elimination System (NPDES) regulations have challenged small to medium-sized (Phase II) cities to adopt measures to improve stormwater runoff quality by using myriad best management practices (BMPs). Although improving stormwater quality is the goal of the Phase II program, the public health implications of some stormwater improvements are often ill understood and in some instances might not be considered at all. This situation might become particularly pronounced in smaller cities that are struggling to meet the regulatory compliance deadlines of the Phase II program without the benefits of additional staff and resources. As resources become increasingly taxed and compliance deadlines approach, meeting the immediate stormwater-quantity and -quality requirements might become the only programmatic goal of stormwater management.

Stormwater Best Management Practices and Mosquitoes
Managing stormwater has traditionally involved activities related to the quantity and quality of stormwater runoff; recent concerns about disease vectors that might be associated with structural stormwater BMPs, however, have caused many stormwater professionals to begin to consider the potential public health impacts of certain structural BMPs. Those involved in BMP design, implementation, operation, and regulations are finding that the responsibilities of stormwater management go beyond simple compliance with urban runoff regulations. The challenge now has become an issue of maintaining compliance with state and federal stormwater regulations while simultaneously minimizing the potential impacts to public health. Many Phase II municipalities just becoming involved with a new and somewhat bewildering array of stormwater regulations, requirements, and BMPs might be unprepared to deal with the additional complicating factor of public health.

Most of the recent public health concerns about disease vectors have centered around mosquito control. On a global basis, there is no doubt diseases transmitted by mosquitoes are among the most significant causes of human illness and death, with millions of people being affected by mosquito-borne illnesses every year. These problems might be exacerbated in urban areas, which tend to have numerous human-made and natural mosquito-producing habitats. Habitats might include, but are certainly not restricted to, certain stormwater BMPs, particularly those that retain water for a designated period of time. Such systems might be designed to have relatively still water and/or densely vegetated areas to settle contaminants and provide areas where biological activity can mitigate pollutants. Unfortunately the same conditions can also provide optimum conditions for some mosquito life cycle stages.

A mosquito’s life cycle consists of four stages—egg, larvae, pupa, and adult—and eggs must be laid in stagnant water or on damp soils likely to be flooded with water. Eggs typically hatch in 24–48 hours, and the resulting larval and pupal stages will typically last five to 18 days before producing an adult mosquito (Floore, 2002). The amount of time spent in any given life stage depends on environmental conditions, particularly temperature. Anything done to disrupt the cycle from the egg to the adult phase, however, can effectively prevent mosquitoes from being able to transmit diseases.

In some instances, the intensive media coverage concerning mosquitoes and their relation to human health produces the perception that any type of standing water is a breeding ground for large numbers of disease-infected mosquitoes. This perception might lead citizens or public officials to suggest that all sites having standing water should be filled, drained, sprayed, or otherwise managed to completely eliminate all mosquitoes. These concerns are not unfounded; some recent studies conducted in California have demonstrated that many common BMP designs are capable of providing breeding habitats for mosquitoes, some of which are capable of transmitting human diseases (Metzger et al., 2002). Studies have also shown, however, that proper design, construction, and maintenance of BMPs can dramatically reduce the suitability of these structures for producing mosquitoes (Kluh et al., 2002).

The relatively recent and rapid proliferation of WNV and the resulting impacts on public health have many municipal separate storm sewer system permittees concerned about the potential impact of stormwater BMPs on disease transmission. Although the newness of this illness has garnered a lot of media attention and created a general concern among many people, according to the national Centers for Disease Control and Prevention (CDC), the risk to any one person is extremely low.

WNV first appeared in the eastern United States in 1999 and rapidly spread westward, becoming isolated in birds, humans, and mosquitoes in 44 states and Washington, DC, by the end of 2002. The number of humans affected by the virus increased dramatically during 2002 and 2003 (see Table 1). Typical symptoms of WNV in humans are flulike, characterized by mild fever, headache, body ache, swollen lymph glands, and occasional development of a rash. According to the CDC, about 13% of those infected develop West Nile fever, with headaches and flulike symptoms from which they eventually recover. Some individuals who become infected with WNV (thought to be less than 1% of the total number of people infected) will develop meningitis, an infection of the membranes of the brain or spinal cord, or encephalitis, an infection of the brain. Both conditions can cause death or permanent injury, and advanced age seems to be an important risk factor for developing a life-threatening form of the disease. The fatality rate is approximately 3–15% among those who contract meningitis or encephalitis.

Based on mosquito biology, any standing water has the potential to promote mosquito growth. Because some stormwater management practices use standing water to promote water-quality improvements—or because they have a possibility of creating standing water if not properly maintained—there is a chance stormwater management practices will contribute to mosquito problems. Stormwater BMPs, however, are essential to minimizing the adverse water-quality impacts caused by development and thus are a vital component of urban surface-water conveyance systems. The challenge facing the City of Denton was to provide water-quality improvements through stormwater management without jeopardizing public health.

Denton Setting
The city of Denton occupies an approximately 64-mi.² area in north-central Texas, about 30 mi. north of Dallas. Three major watersheds drain from Denton, and there are many ponds, wetland areas, streams, and stormwater conveyances within these watersheds. The city also has numerous parks and a large number of protected greenbelts and environmentally sensitive areas along riparian corridors. Denton is located in close proximity to Lake Lewisville, a large (45-mi.²) multipurpose reservoir. Many of these areas have the potential to provide a mosquito habitat.

Soon after the discovery of the first WNV-positive mosquito sample, the City of Denton developed and implemented the Mosquito Surveillance and Response Plan. The plan is designed to establish the programmatic assessment of mosquito populations within the city and to guide the use of various control measures. City staff recognized that the magnitude of the mosquito control program required the cooperation and coordination of many different departments within the city, county, and state public health agencies and of the citizens of Denton. The city also decided that employing a proactive, targeted approach to mosquito management was much more desirable than merely responding to an outbreak once it occurred.

Mosquito Response Plan
City staff recognized early in the process of developing the mosquito response plan that there was a potential for overlap between mosquito control and stormwater management. Because there is a real potential for vector production in stormwater BMPs, the City of Denton’s Watershed Protection Department, which administers both the city’s watershed monitoring program and its stormwater program, was chosen as the lead department for mosquito control issues. The Watershed Protection Department, acting as a general coordinator, works closely with the city’s Drainage, Engineering, Animal Control, and Code Enforcement Departments and the Public Information Office concerning various aspects of control. The city staff also maintains close contacts with the Denton County Health Department. Cooperation among the various city and county departments has been crucial to providing the city with the best possible means of achieving clean-water goals without sacrificing public health.

Mosquito Surveillance
During the fall of 2002, the City of Denton entered into an agreement with the University of North Texas to begin an active adult mosquito monitoring program. Beginning in April 2003, several adult mosquito traps were deployed throughout the city. Two main types of traps are used: a CDC light trap baited with carbon dioxide and a gravid trap designed to collect ovipositing female mosquitoes. Light traps rely on a carbon dioxide attractant to lure mosquitoes into an area where a fan conveys the mosquitoes into a capture net. Gravid traps contain a pungent liquid mixture of water, hay, and other organic material that lures female mosquitoes ready to lay eggs. Currently almost all of the mosquitoes that have tested positive for WNV within Denton have been captured using gravid traps.

After capture, adult mosquitoes are transferred from the traps to specially designed transport boxes and then are sent to the Texas Department of Health for identification of species level and testing for the presence of viruses. If viral infections are detected, further testing is conducted to determine if the virus is West Nile or some other mosquito-borne virus, such as Saint Louis encephalitis or eastern equine encephalitis. Trapping is conducted approximately once a week using four fixed locations, four locations randomly chosen within four large geographic areas in the city, and two completely random locations. Some limited trapping has also been based on additional information, such as the number of complaint calls or observations of dead birds.

Mosquito Trap Information
To date, 28 different mosquito species have been identified within Denton, and more than 7,000 mosquitoes have been submitted for testing. As of September 9, 2003, 12 samples tested positive for WNV. All of the information collected by the trapping network is used to create maps showing sampling locations where WNV-positive samples were collected (see Figure 1). Sites where multiple samples were collected are color-coded so citizens can quickly tell where the disease appears to be more prevalent. Maps are posted on the City of Denton Web page as soon as the information is obtained.

Green labels indicate a single positive sample. Yellow labels indicate two samples have tested positive from the same location.

Information concerning the dynamics of mosquito densities (see Figure 2) and the overall incidence of WNV (see Figure 3) can be produced quickly from trap results. This information gives city officials and citizens a better understanding of mosquito population densities within the City of Denton and a sense of the relative risk of WNV transmission to humans. Citizens are quickly informed about the appropriate city risk level and are provided with information concerning the most appropriate protection activities. The information is also used to direct city staff to areas that have consistently high mosquito population densities, to determine if target mosquito species appear to be associated with stormwater infrastructures or homeowner management practices, and to help determine the appropriate larviciding approach.

Trapping information, along with information on animal or human cases of WNV, is used to establish risk levels for the city. Each risk level is triggered by data concerning the likely public health risk and coincides with a particular series of activities. Control activities become more pronounced as the threat to public health increases. An outline of triggers, associated risk levels, and resulting activities of the city’s Mosquito Surveillance and Response Plan can be viewed by going to the Water Utilities Department page at www.cityofdenton.com.

Adult Mosquito Control: To Spray or Not To Spray
The most important maintenance cycle from a public health standpoint involves humans and mosquitoes in urban and suburban environments. The principal vector of WNV in Denton, Culex quinquefasciatus, is a highly domesticated species that uses the transient waters common to the urban environment as larval habitat. Other mosquito species might be involved in suburban or rural maintenance cycles, but in Denton they do not seem to be a major contributor to the transmission of WNV.

Traditional mosquito control often uses insecticides as space sprays to kill adult mosquitoes. One of the most popular approaches to the application of these sprays in urban and suburban environments has been ultralow-volume (ULV) applications of either organophosphates—for example, Malathion—or synthetic pyrethroid products. Some research, however, suggests ULV applications—either by ground or by aerial equipment—have had little lasting effect on certain mosquito species (Hudson, 1986; Gubler, 1989).

One of the greatest concerns raised during the development of the mosquito response plan was that adulticiding operations—if used—would create a false sense of security among residents. Although minimal data exist concerning the efficacy of ULV applications in an urban environment under real field conditions, control that is anywhere near 100% is unlikely because of several factors: (1) the nature of the insecticide (the droplets produced by the ULV machinery must contact the mosquito to kill), (2) the large number of obstructions to ULV applications in an urban setting, and (3) the number of places a mosquito can hide in a typical urban environment. Citizens who are used to the relatively high level of insect control obtained by insecticide applications in home or yard settings, however, might not understand that the entire mosquito population will not be killed by broad-scale ULV spraying. The fast breeding cycle of mosquitoes during certain times of the year also makes it likely that a new generation of biting adults will appear soon after ULV applications.

Because of the many concerns about chemically based adult mosquito control, emphasis was placed on community-focused, integrated approaches. The rationale was that the best control could be achieved through source reduction and, in some cases, larviciding activities conducted by both community members and city staff. Through this approach, community members have a role in and a responsibility for program implementation and maintenance. It is important to realize, however, that the Mosquito Surveillance and Response Plan is not a no-spray program. At higher risk levels, limited spraying is considered a management option based on the flight range and densities of target mosquitoes, the prevalence of virus-positive mosquito pools, the perceived risk to the human population, and the time of the current weather patterns. If spraying is conducted, ULV applications with synthetic pyrethroid products are performed within areas of highest risk.

Although certainly the sustainability of disease control programs of this magnitude can benefit from a sense of community ownership, community members must find neighboring mosquito habitats unacceptable and must be convinced their best interest is to control mosquitoes both in their own backyards and in their neighborhoods. This requires education and continued reinforcement, and it might take some time for a substantial number of community members to become involved in actions that have traditionally been perceived to be a government responsibility. Denton’s approach is somewhat top-down; citizens are provided with information on how to best accomplish source reduction and larval control. To date, this approach appears to be successful.

Larviciding: Stop Mosquitoes Before They Can Bite
Over the past few years, major advances have been made in the area of biological mosquito control. Biological control strategies involve natural predators, including Gambusia affinis (mosquito fish), fungi, protozoans, round worms, flat worms, and such bacterial agents as Bacillus thuringiensis israelensis (Bti). Each biological control agent has certain benefits and restrictions. To use a biological control agent successfully, the applicator must have a basic knowledge of its biology. Some biological control mechanisms, for example, are limited by salinity, temperature, or organic pollution, and some mosquito species are much more susceptible to specific types of biological control agents. All of these factors must be considered when choosing and applying biological control agents.

The perfect pesticide is easily applied, reasonably inexpensive, and nontoxic to nontarget organisms and eliminates the pest quickly before it becomes a threat. Although no single pesticide combines all of these factors, certain types of Bacillus bacteria have been developed into pesticides that are very close to the perfect model. Bti, for example, is a naturally occurring soil bacterium that produces a poison capable of killing mosquito larvae. Bti is considered ideal for mosquito management because of its specificity to mosquito larvae and because of its lack of toxicity to nontarget organisms.

Under adverse conditions, Bti bacteria form asexual reproductive cells, called endospores, which enable the cells to survive. The endospores of Bti also contain crystals of delta endotoxin, an insecticidal protein toxin. When eaten by a mosquito larva, the crystals dissolve in the larva’s intestine and perforate the cells of its gut, disrupting its normal digestion and preventing it from feeding. When the larva stops feeding, its gut pH is lowered by equilibration with its blood pH. This lowered pH enables the bacterial spores to germinate, and the bacteria then invade the host, causing a lethal infection. Death typically occurs a few hours after digestion. Based on laboratory studies, the United States Environmental Protection Agency concluded that the toxicity and infectivity risks to nontarget animals are minimal to nonexistent (USEPA, 1998). Currently Bti is commercially available in powder, liquid, granular, capsule, and briquette formulations.

The City of Denton decided that biological control agents would be the main tools for mosquito control. Bti and the closely related bacterium Bacillus sphaericus (Bs) were considered the most environmentally acceptable and commercially available biological control agents because of their relative specificity for mosquitoes and their negligible toxicity to vertebrates (Rishikesh et al., 1983). Limited applications of larvivorous fish (Gambusia affinis) also were considered to be a valuable component of an integrated control program, either alone or together with other control agents (Walton et al., 1990; Walton and Mulla, 1991; Reed et al., 1995). It is very important, however, to consider the ecological implications of releasing mosquito fish into certain areas. Some states also might require permits for the release of these organisms.

Mosquito Control and the Construction Review Process
Cooperation and collaboration among various city and county departments are crucial in minimizing vector production from BMPs. By reviewing BMPs before implementation and by monitoring them after installation, the risk to public health from vectors can be decreased. Because the Watershed Protection Department staff is familiar with issues of vector control, members can request stormwater design modifications that will minimize the potential for producing vector breeding habitats.

The Watershed Protection Department staff also is involved in the acceptance of stormwater pollution prevention plans (SWPPPs) for projects within the city. Although the general permit to discharge stormwater within the state of Texas does not address the issue of mosquito control, the Texas Health and Safety Code specifically states that a collection of water in which mosquitoes are breeding in the limits of a municipality is not allowed (§341.011). Other states have similar codes. For Denton, the Texas Health and Safety Code is reinforced at the local level through ordinances and is used as justification for requiring mosquito control within the SWPPP review process. If a city believes mosquito control is an issue on a particular project, it can request the permittee to add mosquito control measures to the SWPPP. In these situations, most of the requested SWPPP activities involve source reduction.

Public Education and Citizen Involvement Efforts
A common misperception among citizens is that distant water bodies are contributing to local mosquito densities. Although some species of mosquitoes are strong fliers capable of many-mile flights, most mosquito species collected in Denton’s trapping operations are considered weak fliers. For Denton, as for many areas of the southern US, the mosquito species that represents the greatest risk for WNV transmission is the Southern house mosquito, Culex quinquefasciatus. Because this mosquito has an average flight range of approximately 0.5 mi. from its breeding habitat, it is crucial to inform the public of the importance of eliminating local mosquito habitats and treating local, private sources of water for larval control.

The City of Denton encourages active citizen participation in localized control and provides Bti materials free of charge to all citizens aiding in local private-property larviciding activities. This program has been a great success and has fostered a sense of partnership among citizens and city staff. Citizen phone calls have also proven to be a very useful source of information for Denton staff. Although the number of calls can become overwhelming during the peak of the mosquito season, they provide useful information regarding the density of localized mosquito populations, the presence of dead birds, and the notification of potential mosquito breeding habitats.

Public information distributed by the city directs citizens to the Watershed Protection Program staff for mosquito-related issues. Because the program staff is familiar with the city’s stormwater infrastructure and mosquito surveillance plan, this process has been very efficient in determining the relationship between the mosquito problem and stormwater systems. If the problem seems to be related to a component of the stormwater infrastructure, city staff will conduct a site visit and perform source reduction, vegetation maintenance, or larviciding activities. If the problem does not seem to be related to the stormwater system, the staff will provide information on how citizens can minimize mosquito populations around the home. If many complaints are received from a single area, workshops on mosquito control may be conducted through neighborhood organizations. The city’s Public Information Office distributes additional information, including fliers, newspaper ads, notices in public buildings, and public service announcements on local cable channels. This approach has proven to be an efficient way to convey information to a large number of citizens.

Poorly designed culvert and eroded area with standing water likely to produce large amounts of mosquitos

Poorly dsigned underground stormwater junction where hundreds of mosquito larvae were found after a large storm

Maintenance of City Stormwater Facilities
For the most part, wet facilities within the city do not appear to create significant mosquito habitats, as long as the quantity and quality of water is sufficient to maintain sufficient mosquito predators. If inspections reveal the presence of mosquito larvae or a large number of adults, limited treatments of these facilities may be warranted, particularly when excessive plant growth creates isolated areas where mosquitoes can proliferate. In these situations, treatment with granular forms of Bti might be more effective than using briquettes.

For permanent bodies of water, the city takes an active role in vegetation management and might introduce mosquito fish. The goal is to manage emergent vegetation so it does not reach density levels that prevent predators from being able to effectively locate mosquito larvae. In situations where mosquito larvae are present and vegetation removal is impractical or undesirable, granular Bti might be used.

Drainage swales and structures that maintain standing water in sumps or basins have proven to be some of the more problematic areas for mosquito control. Often swales will have some undercutting associated with poorly designed or eroded culverts, which can retain water long after the rest of the swale is dry. Underground stormwater conveyances that retain water can also produce similar habitats. The resulting pockets of water usually are still and highly organic and have minimal mosquito predators. The city’s Drainage Department monitors and treats these areas as a part of normal stormwater infrastructure maintenance. Drainage staff are trained to recognize potential breeding sites and mosquito larvae and to appropriately treat these areas. Treatment usually involves either repair of a system malfunction or treatment with Bti or Bs. Maps and records concerning which locations were treated are maintained to make the process more efficient.

Click here for larger image

Table 2 summarizes some of the common mosquito species in Denton, their respective stormwater habitats, and control measures used for particular types of stormwater controls.

Regulatory Requirements of Larvicide Applications
Stormwater managers not only must face the regulatory requirements of NPDES, but if they become involved with mosquito control they also need to understand the regulatory requirements for vector control agents. Although Bti and Bs are both bacterial agents, they are listed by EPA as registered pesticides. From a regulatory standpoint, therefore, most states treat them no differently than chemical agents. Typically, municipal pesticide applicators have to obtain a noncommercial applicator’s license in vector control from the appropriate state agency to legally apply mosquito pesticides. The licensed applicator then can provide a relatively extensive training program for nonlicensed individuals to work under the applicator’s license.

Some states do not require extensive training for the application of certain types of pesticides. The Illinois State Department of Agriculture, for example, enacted an emergency rulemaking provision on August 16, 2002, to allow individuals who have been trained for at least one hour by a licensed mosquito applicator to apply Bti and Altosid products (products containing the insect-growth regulator methoprene) during certain times of the year. The training must cover pesticide labels, use restrictions, application rates, application methods, and any other information the trainer feels is appropriate for the safe and effective use of insecticides.

Critical Review of the Program
Mosquito control programs realistically cannot eliminate mosquitoes entirely; rather, they serve to reduce numbers and thus the risk of disease transmission. For situations that warrant control measures, it is more efficient to control mosquitoes where they are produced as larva than to attempt to control adult populations through spraying programs. It also is important to realize that public perception is a critical part of both stormwater control and mosquito management. Public education efforts should strive to demonstrate how a properly maintained and treated stormwater BMP can actually work to reduce mosquito populations. Areas that appear to be good larval habitats will attract laying adults, which will deposit their eggs in the area. If proper maintenance and/or treatment activities are performed, however, the chance of the eggs developing into biting adults is greatly reduced. This is not the case in habitats where there are no predators and where no treatments or other management activities are performed.

West Nile virus usually becomes a more pronounced human health issue in late summer or early fall, when mosquito populations are larger and the number of infected mosquitoes increases (see Figures 2 and 3). The best time to prevent problems, however, is earlier in the season, when source reduction and larviciding can be effectively employed. If your community is at risk for WNV, start these programs early and maintain a consistent effort throughout the mosquito season. Coordinated source reduction and larviciding efforts involving cooperation among stormwater managers, municipal staff, county agents, citizens, and businesses are likely the best approach in an urban setting. As with many municipal issues, vector control is a problem shared by many different entities and will likely require a concerted effort to reach an acceptable resolution. Through cooperation among multiple city departments and public health agencies, the City of Denton has made substantial progress toward accomplishing the dual goal of protecting water quality and human health.

Addressing the threat of an emerging infectious disease like WNV depends on sustained and coordinated efforts of many parties. Undoubtedly the control of WNV depends on establishing and maintaining effective integrated pest management control systems; however, collaboration among state and local health departments, academic centers, city staff, and citizens is also crucial for program success. A strong and flexible plan involving all of these partners is the best defense against WNV outbreak or other vector-borne illnesses that might result from stormwater BMPs.

References
Floore, T. “Mosquito Information,” www.mosquito.org/info.php. The American Mosquito Control Association. 2002.
Gubler, D.J. “Aedes aegypti and Aedes aegypti-borne disease control in the 1990s: top down or bottom up.” American Journal of Tropical Medicine and Hygiene, 40:571–578. American Society of Tropical Medicine and Hygiene, Northbrook, IL. 1989.
Hudson, J.E. “The 1982 emergency ultralow volume spray campaign against Aedes aegypti adults in Paramaribo, Suriname.” Epidemiological Bulletin, 20:294–303. Pan American Health Organization, Washington, DC. 1986.
Kluh, S., M.E. Metzger, D.F. Messer, J.E. Hazelrigg, and M.B. Madon. “Stormwater, BMPs, and Vectors: The Impact of New BMP Construction on Local Public Health Agencies.” Stormwater, March/April 2002.
Metzger, M.E., D.F. Messer, C.L. Beitia, C.M. Myers, and V.L. Kramer. “The Dark Side of Stormwater Runoff Management: Disease Vectors Associated With Structural BMPs.” Stormwater, March/April 2002.
Reed, S.C., R.W. Cites, and E.J. Middlebrooks. Natural Systems for Waste Management and Treatment. McGraw-Hill, New York. 1995.
Rishikesh, N., H.D. Burgess, and M. Valdekar. Operational use of Bacillus thuringiensis serotype H-14 and environmental safety, WHO/VBC/83.371. World Health Organization, Geneva. 1983.
USEPA, Prevention, Pesticides and Toxic Substances. Reregistration Eligibility Decisions: Bacillus thuringiensis, p 16. United States Environmental Protection Agency, Washington, DC. 1998.
Walton, W.E., M.S. Mulla, M.J. Wargo, and S.L. Durso. “Efficacy of a microbial insecticide and larvivorous fish against Culex tarsalis in duck club ponds in southern California.” Proceedings of the California Mosquito Vector Control Association, 58:148–156. Mosquito and Vector Control Association of California, Sacramento. 1990.
Walton, W.E. and M.S. Mulla. “Integrated Control of Culex tarsalis Larvae Using Bacillus sphaericus and Gambusia affinis: Effects on Mosquitoes and Nontarget Organisms in Field Mesocosms.” Bull.Soc. Vector Ecol., 16(1):203–221. Society for Vector Ecology, Corona, CA. 1991.

Kenneth E. Banks, Ph.D., CPSWQ, is the water resources manager for the City of Denton, TX, specializing in watershed science, stormwater management, and vector control, and is an adjunct faculty member of the University of North Texas.

SW March/April 2004

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