Prevention and Control: Integrated Vector Management

Prevention and control of arboviral diseases is accomplished most effectively through a comprehensive, Integrated Vector Management (IVM) program applying the principles of Integrated Pest Management. IVM is based on an understanding of the underlying biology of the arbovirus transmission system and utilizes regular monitoring of vector mosquito populations and arboviral activity levels to determine if, when, and where interventions are needed to keep mosquito numbers below levels which produce risk of human disease, and to respond appropriately to reduce risk when it exceeds acceptable levels.

Operationally, IVM is anchored by a monitoring program providing data that describe

• Conditions and habitats that produce vector mosquitoes
• Abundance of those mosquitoes over the course of a season
• Arboviral transmission activity levels expressed as infection rate in mosquito vectors
• Parameters that influence local mosquito populations and virus transmission

These data inform decisions about implementing mosquito control activities appropriate to the situation, such as

• Source reduction through habitat modification
• Larval mosquito control using the appropriate methods for the habitat
• Adult mosquito control using pesticides applied from trucks or aircraft when established thresholds have been exceeded
• Community education efforts related to risk levels and intervention activities

Monitoring also provides quality control for the program, allowing evaluation of the effectiveness of larval and adult control efforts, and causes of control failures (e.g., undetected larval sources, pesticide resistance, equipment failure).

Mosquito control tools target mosquitoes at the adult or immature stage depending on program objectives. Multiple species are involved in eastern equine encephalitis (EEE) virus transmission, and different populations of a single species may vary their activity based on environmental conditions. The decision to conduct mosquito control activities is based on mosquito and meteorological surveillance, established local thresholds and triggers (mosquito, human, and non-human animal), and insecticide resistance status of the target species (Table 1). Mosquito control professionals should have detailed knowledge of the local mosquitoes involved in EEE virus transmission to prevent and control disease. Programs should use pesticides and other control tools registered by the U.S. Environmental Protection Agency (EPA) in compliance with label instructions and any local, state, and federal laws regulating their use.

Table 1. Summary of Mosquito Control by Life-stage, Method, and Objective

http://www.epa.gov/mosquitocontrol

The objective of larval mosquito control is to reduce immature mosquito populations before they emerge as adults. This can be an efficient method of managing mosquitoes where larval sites are accessible, but habitats of EEE virus vectors are often hard to find and labor-intensive to treat. Few studies have shown efficacy of larval control methods against EEE virus vectors.

Culiseta melanura larvae develop in crypts filled with water in swamp and bog habitats. A single study showed aerial application of methoprene penetrated larval crypts and had 81% efficacy (emergence inhibition) over 5-weeks post-treatment (Woodrow et al. 1995). Temephos was also evaluated and not detected in the larval habitats (crypts) of Cs. melanura. Although not evaluated yet for EEE, aerial or ULV Bacillus thuringiensis israelensis (Bti) water-dispersible granules can penetrate foliage and water in covered areas to control other mosquitoes that occur in cryptic larval habitats (e.g., Aedes aegypti, Culex quinquefasciatus) (Pruszinski et al. 2017). These delivery techniques may be also useful against Cs. melanura and the larval habitats of epizootic bridge vectors. Although further studies are needed on the efficacy and implementation of larval control of EEE virus, applying a larvicide at the same time as an adulticide application to reduce adult mosquito populations may help prevent population rebound due to newly emerged adults and mosquitoes not active at the time of application.

Larvicides (and pupacides) are applied directly to water sources or placed in areas where flooding is expected to target the aquatic habitats of vector species. Larvicide can be applied by ground or aerial dispersal methods. For small aquatic larval sites or areas that cannot be reached by vehicles, backpack sprayers and dusters are used to apply liquid, granules, or pellets. Formulations can be short-acting (up to 2 weeks) or extended-release products (lasting more than 1 month). Larvicides may kill on contact through ingestion, or act as stomach poisons or growth regulators. Information on pesticides for larval mosquito control is available from the U.S. EPA (http://www.epa.gov/mosquitocontrol/controlling-mosquitoes-larval-stage).

Adult mosquito control aims to reduce the abundance of biting, infected adult mosquitoes to prevent them from transmitting arboviruses to humans and to break the mosquito-host transmission cycle. Where populations are increasing above acceptable levels, adulticides are used to reduce vectors. Vector mitigation strategies should be applied quickly once arboviral activity is detected and be targeted to the local EEE virus epizootic and enzootic vectors. Programs should use pesticides registered by EPA for this purpose (http://www.epa.gov/mosquitocontrol/controlling-adult-mosquitoes).

Adulticides can reduce the numbers of adult mosquito vectors for EEE virus, but not enough cases occur annually to demonstrate clear impact on EEE virus transmission to humans. Indicators of high transmission risk are used to decide when to apply adulticides and often by the time aerial applications occur, transmission to humans has already occurred. Also, due to the epidemic nature of this disease, untreated areas relevant for comparison might not be available, which limits the ability to make conclusions about the efficacy of using adulticides to reduce disease (Grady et al. 1978).

Adulticiding can be conducted from the ground with backpack spray equipment, truck-mounted equipment, or by air with fixed-wing or rotary-wing applications. Types of treatment include space-spray (e.g., ULV) adulticides and residual treatments.

• Space-spray and ULV treatments rely on mosquitoes and insecticide droplets coming into direct contact in the air column. These are temporary measures to reduce the mosquito population active at the time of treatment (Lloyd et al. 2018). ULV formulations applied in small volumes prevent deposition and enhance degradation of the active ingredients in the environment (Bonds 2012). Mosquitoes not active at the time of application are not exposed. Because there is little to no deposition of insecticide, no residual control of mosquitoes occurs. As a result, multiple applications may be needed for sustained control (Andis et al. 1987).
• Long-lasting adulticides, also called residual or barrier treatments, can be applied to surfaces and to vegetation. To be effective, the mosquito must land on the treated surface and directly contact the insecticide. This type of application targets the resting mosquito population and is typically used in urban pest management and residential properties (Lloyd et al. 2018).
• Other methods of control: Traps and baits have been proposed to control mosquitoes (e.g., Ae. aegypti, Ae. albopictus) but few studies have been conducted for vectors of EEE virus. A single study on attractive targeted sugar baits (ATSB), which attract and kill sugar feeding mosquitoes, found reductions in adult Cs. melanura populations in the 2-weeks post-treatment; however, the study design, number of mosquitoes trapped, and background insecticide used in the study limit the conclusions. At present, more evidence is needed before for broad scale use can be recommended. (Qualls et al. 2014).

References

Andis MD, et al. Strategies for the emergency control of arboviral epidemics in New Orleans. J Am Mosq Control Assoc 1987;3:125–130.

Bonds JA. Ultra-low-volume space sprays in mosquito control: a critical review. Med Vet Entomol 2012;26:121–130.

Grady GF, et al. Eastern equine encephalitis in Massachusetts, 1957-1976: A prospective study centered upon analyses of mosquitoes. Am J Epidemiol 1978;107:170–178.

Lloyd AM, Connelly CR, and Carlson DB (Eds.). Florida Coordinating Council on Mosquito Control. Florida Mosquito Control: The state of the mission as defined by mosquito controllers, regulators, and environmental managers. Vero Beach (FL): University of Florida, Institute of Food and Agricultural Sciences, Florida Medical Entomology Laboratory; 2018. https://fmel.ifas.ufl.edu/media/fmelifasufledu/7-15-2018-white-paper.pdf

Pruszynski CA, Hribar LJ, Mickle R, Leal AL. A large scale biorational approach using Bacillus thuringiensis israelensis (Strain AM65-52) for managing Aedes aegypti populations to prevent dengue, chikungunya and Zika transmission. PLoS One 2017;12:e0170079.

Qualls WA, et al. Evaluation of attractive toxic sugar bait (ATSB)—Barrier for control of vector and nuisance mosquitoes and its effect on non-target organisms in sub-tropical environments in Florida. Acta Tropica 2014;131:104–110.

Woodrow RJ, et al. Field trials with methoprene, temephos, and Bacillus thuringiensis serovar israelensis for the control of larval Culiseta melanura. J Am Mosq Control Assoc 1995;11:424–427.

Insecticides to control larval and adult mosquitoes are registered specifically for that use by the EPA. Instructions provided on the product labels prescribe the required application and use parameters and must be carefully followed. Properly applied, these products do not negatively affect human health or the environment. In persons living in treated areas, ULV application of mosquito control adulticides does not produce any detectable biological changes indicating exposure or increase asthma or other adverse health events (Currier et al. 2005; Duprey et al. 2008; Karpati et al. 2004). The morbidity and mortality from arboviruses demonstrably exceed the risks from mosquito control practices (Davis and Peterson 2008; Macedo et al. 2010; Peterson et al. 2006).

Individually owned private properties may be major sources of mosquito production. Examples include accumulations of discarded tires or other trash, neglected swimming pools, and similar water features that become stagnant and produce mosquitoes. Local public health statutes or public nuisance regulations may be employed to gain access for surveillance and control or to require the property owner to mitigate the problem. Executing such legal actions may be a prolonged process during which adult mosquitoes are continuously produced. Proactive communication with residents and public education programs may alleviate the need to use legal actions. However, legal efforts may be required to eliminate persistent mosquito production sites.

Pesticide products and application procedures (for both larval and adult control) must periodically be evaluated to ensure an effective rate of application is being used and that the desired degree of control is obtained. Application procedures should be evaluated regularly (minimally once each season) to assure equipment is functioning properly to deliver the correct dosages and droplet parameters and to determine appropriate label rates to use locally. Finally, mosquito populations should routinely be evaluated to ensure insecticide resistance is not emerging.

Surveillance data describing vector sources, abundance and infection rates, records of control efforts (e.g., source reduction, larvicide applications, adulticide applications), and quality control data must be maintained and used to evaluate IVM needs and performance. Long-term data are essential to track trends and to evaluate levels of risk.

For vector control to be effective, mosquitoes must be susceptible to the insecticide selected for use. In order to delay or prevent the development of insecticide resistance in vector populations, IVM programs should include a resistance management component (Lloyd et al. 2018). This should include routine monitoring of the status of resistance in the target populations to

• Provide baseline data for program planning and pesticide selection before the start of control operations
• Detect resistance at an early stage so that timely management can be implemented
• Continuously monitor the effect of control strategies on insecticide resistance, and determine potential causes for control failures, should they occur

Insecticide resistance may be monitored using bioassays in larvae or adult mosquitoes (Brogden and McAllister 1998). The CDC bottle bioassay is a simple, rapid, and economical tool to detect insecticide resistance by determining the time taken for a pesticide active ingredient to kill mosquito vectors. The results can help guide the choice of insecticide used for spraying. The CDC bottle bioassay can be used as part of a broader insecticide resistance monitoring program, which may include field cage tests and biochemical and molecular methods. A practical laboratory manual for the CDC bottle bioassay is available online https://www.cdc.gov/mosquitoes/mosquito-control/professionals/cdc-bottle-bioassay.html. For additional information, contact CDC at USBottleAssayKit@cdc.gov.

The IVM program should include options for managing resistance that are appropriate for local conditions. The techniques regularly used include the following:

• Management by moderation. Prevent onset of insecticide resistance by reducing overall chemical use or persistence by
  • Using doses no lower than the lowest label rate to avoid genetic selection
  • Using chemicals of short environmental persistence and avoiding slow-release formulations that increase selection for resistance
  • Avoiding use of the same class of insecticide to control adult and immature stages
  • Applying locally; many districts treat only hot spots and use area-wide treatments only during public health alerts or outbreaks
  • Using less frequent applications; leaving generations, population segments, or areas untreated (when appropriate)
  • Establishing higher thresholds for mosquito mitigation with insecticides, except during public health alerts or outbreaks.
• Management by continued suppression. This strategy is used in regions of high value or persistent high risk (e.g., heavily populated regions or locations with recurring outbreaks) where mosquitoes must be kept at very low densities. It involves the application of dosages within label rates but sufficiently high to be lethal to heterozygous individuals that are partially resistant. If the heterozygous individuals are killed, resistance will be slow to emerge. This method should not be used if any significant portion of the population in question is fully resistant. Another approach more commonly used is the addition of synergists that inhibit existing detoxification enzymes and thus eliminate the competitive advantage of these individuals. Commonly, the synergist of choice in mosquito control is piperonyl butoxide (PBO).
• Management by multiple methodology. This strategy involves the use of insecticides with different modes of action in mixtures or in rotations. There are economic limitations associated with this approach (e.g., costs and logistics of switching or storing chemicals), and critical variables in addition to the pesticide mode of action that must be taken into consideration (i.e., mode of resistance inheritance, frequency of mutations, population dynamics of the target species, availability of refuges, and migration). Programs should evaluate resistance patterns routinely and the need for rotating insecticides at annual or longer intervals.

Continuing education for operational vector control workers is required to instill or refresh knowledge related to practical mosquito control. Training focusses on safety, applied technology, and requirements for the regulated certification program mandated by most states. Training should also include information on the identification of mosquito species, their behavior, ecology, and appropriate methods of control.

References

Brogdon WG, McAllister JC. Insecticide resistance and vector control. Emerg Infect Dis 1998;4:605–613.

Currier M, et al. Human exposure to mosquito-control pesticides—Mississippi, North Carolina, and Virginia, 2002 and 2003. MMWR 2005;54:529–532.

Davis RS, Peterson RK. 2008. Effects of single and multiple applications of mosquito insecticides on nontarget arthropods. J Am Mosq Control Assoc 2008;24:270–280.

Duprey Z, et al. Community aerial mosquito control and naled exposure. J Am Mosq Control Assoc 2008;24:42–46.

Karpati AM, et al. 2004. Pesticide spraying for West Nile virus control and emergency department asthma visits in New York City, 2000. Environ Health Perspect 2004;112:1183–1187.

Lloyd AM, Connelly CR, and Carlson DB (Eds.). Florida Coordinating Council on Mosquito Control. Florida Mosquito Control: The state of the mission as defined by mosquito controllers, regulators, and environmental managers. Vero Beach (FL): University of Florida, Institute of Food and Agricultural Sciences, Florida Medical Entomology Laboratory; 2018.

Macedo, PA, et al. Evaluation 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. J Am Mosq Control Assoc 2010;26:57–66.

Peterson RKD, et al. A human-health risk assessment for West Nile virus and insecticides used in mosquito management. Environ Health Perspect 2006;114:366–372.

The objective of a phased response to EEE surveillance data is to implement public health interventions appropriate to the level of risk in a community (Table 2). A surveillance program adequate to monitor EEE virus activity levels associated with human risk must be in place to detect epizootic transmission in advance of human disease outbreaks. Human case reports lag behind human infection events and are poor indicators of current risk levels. Effective public health action depends on interpreting the best available surveillance data and initiating prompt and aggressive intervention when necessary.

Table 2. Recommendations for a Phased Response to EEE Surveillance Data