Home Mosquito Management Integrated Mosquito Management Mosquito Surveillance
The following is taken wholly from Chapter 3 (INTEGRATED MOSQUITO SURVEILLANCE AND ENVIRONMENTAL MONITORING TO ASSESS CHANGES IN MOSQUITO POPULATIONS) in the Florida Mosquito Control White Paper developed by the Florida Coordinating Council on Mosquito Control. 2009. Florida Mosquito Control: The state of the mission as defined by mosquito controllers, regulators, and environmental managers. University of Florida; Vero Beach, FL, USA
This chapter discusses the importance and legal obligations regarding mosquito and environmental surveillance systems. Included are discussions of how to develop, design, and implement mosquito surveillance systems. Commonly used methods to inventory mosquito habitats, collect immature and adult mosquitoes, and monitor environmental parameters that can be used to predict mosquito emergences are discussed.
Mosquito surveillance is a prerequisite to an effective, efficient, and environmentally sound mosquito control program. Surveillance is used to define the nature and extent of the mosquito problem and to gauge daily mosquito control operations. It provides a basis for evaluating the effectiveness of control operations, the data needed to comply with state rules and regulations regarding the justification for treatments, and a basis for evaluating the potential for transmission of mosquito-borne diseases (see Chapter 8).
Mosquito surveillance is most effective when combined with an ongoing program for monitoring meteorological, astronomical, and environmental factors that may influence mosquito population change. For example, rainfall and ground water levels, temperature, relative humidity, wind direction and velocity, tidal changes, lunar cycles, stormwater and wastewater management, and land use patterns are all factors that may influence mosquito population levels and adult mosquito flight behavior and dispersal./p>
The objectives of this chapter are to characterize mosquito and environmental surveillance systems and to provide a general review of Florida mosquito control programs.
Ideally, the structure and implementation of an integrated mosquito surveillance program should be based on the needs of the local mosquito control agency. Moreover, these needs should define the components of the control program, as well as the budget required to implement them. In fact, this process is often reversed. Mosquito control programs are funded at a specific level, generally without a needs-assessment process. The program director is then required to meet all mosquito control needs within the constraints of a fixed budget. If funding for this process is inadequate, the result is incomplete mosquito surveillance with reliance upon undesirable and less effective control methods.
The steps in developing an integrated mosquito surveillance program as part of an overall mosquito control effort are to:
• Define area-specific mosquito problems
• Define area-specific mosquito control strategies
• Design a mosquito surveillance program to be used as a decision-making aid to help determine when and where mosquito control efforts are needed
There are at least 80 mosquito species in Florida, and every Florida county has several species that are dangerous disease vectors and several more species that create a major nuisance during most months of the year.
The first step in determining which mosquito species must be monitored is to determine which species cause problems. Control efforts can be justified when a mosquito poses a nuisance or is an economic or health-related pest or vector. A nuisance mosquito bothers people, typically in and around homes or in recreational areas. An economically important mosquito reduces property values, slows economic development of an area, reduces tourism, or adversely affects livestock and poultry production. A health-related mosquito problem is the ability of a mosquito species to transmit pathogens that cause disease. In Florida, this definition currently includes only mosquito species that transmit dog heartworm, St. Louis encephalitis virus, West Nile virus, and eastern equine encephalitis virus. However, nearby in the Caribbean and Central America, other mosquito-transmitted diseases are common (e.g., dengue, malaria). Any mosquito that bites or annoys people can be considered a health problem, particularly for individuals who are allergic to mosquito bites or suffer from entomophobia (an unreasonable fear of insects).
A list of important nuisance and vector mosquito species can be compiled from a review of the literature. The geographic and temporal distributions of these species also can be Chapter 3: Integrated Mosquito Surveillance Page 30 found in the published literature. Once target species have been identified, selected areas can be sampled frequently to determine the abundance of adults and larvae of the species of interest. Mosquito surveys should be conducted as needed throughout the mosquito activity season. Data from mosquito surveys can be used to determine the abundance and seasonal distribution of each species.
Control strategies can be developed based on surveillance data. Since mosquito collection methods differ in their effectiveness for sampling different species, more than one collection method may be used to accurately determine the seasonality and abundance of all the important mosquito species in an area. Multiple surveillance techniques for larvae and adult mosquitoes should be used to accurately quantify mosquito abundance.
Once a list of local targeted mosquito species has been compiled, two additional questions must be answered:
• Which mosquito species will be targeted for control efforts?
• What is the geographic and temporal distribution of each targeted mosquito species?
In Florida, temporal and geographical changes in mosquito populations and the problems that mosquitoes cause are measured by monitoring three factors:
• Telephone or website requests for mosquito control services
•Adult mosquito populations as measured by trapping and landing counts
• Immature mosquito populations as measured by larval inspections
• Not all mosquito control programs in Florida monitor all of these variables. Most mosquito surveillance programs rely on the years of experience of district personnel and are usually a compilation of surveillance techniques that have been shown – usually by trial and error – to work for a specific program. Some of the basic mosquito surveillance techniques used in Florida are discussed below.
One method for quantifying local nuisance mosquito problems is through telephone or website service requests. Most Florida mosquito control programs have a telephone number that citizens can call to request mosquito control services. Several programs have developed their own websites where citizens can go to enter a complaint on-line. Service requests are generally related to specific mosquito species. The mosquitoes responsible for service requests vary considerably from region to region and often change during the year. Although service requests are accepted as a way to meet State requirements for monitoring mosquito problems to justify control, these requests always should be verified by other surveillance techniques prior to any treatment.
Service requests can be handled in a variety of ways. Most programs record the information on data sheets, while some programs record complaint data via software programs linked to a Geographic Information System (GIS). The service requests can be displayed on a map using a GIS software program to assist in displaying clusters or patterns of potential mosquito problems. The service request data are ultimately used to determine where to concentrate control efforts once the requests are verified. Typically, an inspector will be sent to verify the service request in areas where high mosquito population densities are not indicated by other surveillance techniques. In some cases, changes in the numbers of requests from one day to the next are used to evaluate the effectiveness of control operations.
Service requests that are generated by the presence of container-inhabiting mosquito species, such as the Asian tiger mosquito (Aedes albopictus), Ae. aegypti, or certain Culex species, may require an inspection to identify potential larval habitats. Inspectors also can assist homeowners with point-source reduction of containers that hold water. If the service request results from floodwater mosquitoes (saltwater or freshwater) or permanent water mosquitoes such as Anopheles, Coquillettidia, or Mansonia, the citizen is usually told by mosquito control personnel the steps that will be attempted to correct the problem. The inspector will assess the adult mosquito population and attempt to
Although service requests are accepted to meet the State requirements to justify control, most mosquito control programs in Florida use one or more methods to measure adult mosquito populations before a control decision is made. The purpose of monitoring adult mosquitoes is:
• To determine where adults are most numerous
• To substantiate service request claims of a mosquito problem
• To determine the effectiveness of source reduction, larviciding, and adulticiding control methods
• To provide data that satisfies Florida Administrative Code 5E-13 to insure that applications of pesticides are made only when necessary
Florida Administrative Code 5E-13.036 dealing with mosquito surveillance is concerned only with the monitoring of adult mosquitoes. According to this rule, before adulticides can be applied, a monitoring program must detect an increase in the mosquito population above a predetermined baseline, collect more than twenty-five mosquitoes in a single trap night, or collect more than five mosquitoes per hour of operation. The rules do not specify the type or number of traps or the species or sex of the mosquitoes captured, but they make the application of adulticides illegal when mosquitoes are not present. This rule was initiated in 1987, and, for the first time, forced many mosquito control programs to use mosquito surveillance to justify spraying.
Landing rates are utilized by more than 90% (pers. comm., FDACS 2006) of Florida mosquito control programs. They are used for measuring adult mosquito activity, augmentation of existing mechanical trap collections, or assessment of customer complaints for making spot treatments with adulticides. The technique consists of counting the number of mosquitoes that land on a person in a given amount of time.
The specific method used to determine a landing rate varies among programs. Important variables are the time of day at which observations are made, the duration of observations, the portion of the subject's body observed for landing mosquitoes, the number and type of habitats, and the number of human subjects used. It is important to choose a landing rate protocol and avoid changing the variables to get meaningful data. Day-to-day changes in the biting population at a given site are best reflected when the same individual performs the landing rate at that site.
Landing rates taken during the day can be effective for monitoring saltmarsh mosquitoes, which bite during the early morning and during the day. Landing rates also are useful for evaluating activity of day-biting, container-inhabiting (including bromeliad) mosquitoes which are common around homes. Although many crepuscular species can be located in well shaded, moist canopied areas during daylight hours, it is best to assess their landing rate at the time of peak activity. The host-seeking females can be collected with a battery powered aspirator for a set time interval and identified later.
The New Jersey Light Trap (NJLT) is traditionally green in color, uses a 25-watt bulb, is placed 5½ feet off of the ground, and is useful in measuring relative abundance of certain mosquito species. The light is the attractant, and many insects other than mosquitoes also are attracted to the trap. The NJLTs were first used in a statewide program in the mid-1950s by the Florida Board of Health mosquito control program. Local programs would operate the traps and send the collections to the Department of Health in Jacksonville for identification. Mosquito identification eventually became the responsibility of the local programs. During this transition, many programs that lacked expertise in mosquito identification stopped trapping.
Because NJLTs require 110V AC power, they have been operating in the same locations for decades, and the historical monitoring data have been valuable for documenting the long-term changes in mosquito populations at those locations. While NJLTs are usually operated overnight, the number of trap sites and the frequency of trapping vary among Chapter 3: Integrated Mosquito Surveillance Page 33 mosquito control programs. Currently, there are no rules of thumb, established standards, or State rules that apply to the operation of NJLTs.
The majority of Florida mosquito control programs (79%; pers. comm., FDACS 2006) use the Center for Disease Control light traps (CDC) to monitor adult mosquitoes. The CDC light trap is a miniature version of the NJLT that operates on six volts DC and can be used anywhere. It costs less to purchase than the NJLT, does not require AC power, and collects primarily mosquitoes. Although there are several manufacturers of CDClike traps, they can be handmade by local mosquito control programs for about onefourth of the retail cost. This cost differential has resulted in a proliferation of different designs for the trap. It is not important that all control programs use the same CDC trap design as long as the same model of trap is used within a program. Some mosquito control programs use carbon dioxide (either dry ice or bottled gas) and/or octenol as a supplement for the CDC trap. Some control programs operate CDC traps for a few hours a night, and other programs operate them overnight. The main reasons for these variations are budget related, rather than entomological. As with the NJLTs, there is no standard protocol for placing or operating CDC traps.
Three programs in Florida use methods other than light traps as their principal adult mosquito surveillance tool. Pasco County Mosquito Control District (MCD) uses permanently located unbaited suction traps, while Lee County MCD uses truck traps. A truck trap is a large screened funnel attached to the top of a pickup truck. Unlike the NJLT and CDC traps, suction and truck traps sample all airborne mosquitoes, which provides a better measure of mosquito density but does not measure the biting mosquito population. Lee County MCD has operated truck traps for more than 30 years. These data have been very useful for making control decisions for saltmarsh mosquitoes in Lee County. In addition, Lee and Pasco County MCDs use a network of CDC traps to monitor mosquito populations to help identify localized mosquito problems. St. Lucie County MCD recently has used the commercially available “Mosquito Magnet” as their primary adult mosquito collection tool.
Other specialized traps are used to trap either specific species or are used to augment collections as part of an arbovirus surveillance program and include the CDC gravid trap, resting boxes, and vacuum aspirators. The CDC gravid (Reiter) traps collect gravid females, including species that transmit arboviruses. Essentially these traps use an “ovilure mixture”, organics in water that attract gravid females that are ready to oviposit. The hypothesis is that since these females have already blood fed at least once, the collected females have a greater probability of having an arbovirus present in their salivary glands, making public risk assessment easier. A number of different “ovilures” are used, and some attract different species, for example, hay infusion for Culex quinquefasciatus, alfalfa infusion for Ae. aegypti, and oak leaf infusion for Ae. triseriatus.
Resting boxes are used for the collection of Culiseta and Anopheles spp. by programs interested in monitoring vector populations. Resting boxes are generally placed on the ground with the open end facing west to minimize the influence of direct sunlight during the early part of the day. A dark, forested habitat with high canopy yields the highest collections (Crans 1989). Mosquitoes utilizing resting boxes as diurnal resting sites enter the boxes during the morning hours, remain inactive during late morning and early afternoon, and then exit the boxes later in the day. The inside of the resting boxes are usually painted black or red, while the outside is painted flat black. The 12” x 12” x 12” plywood cubes have one open end and are usually positioned no closer than 10 feet from one another in either a line or grid design. Collection from these boxes is usually by aspirator and should be conducted in mid morning to late afternoon.
Vacuum aspirators include sweepers, suction traps, and hand-held battery operated flashlight aspirators. These devices will collect a number of resting mosquito species and blood-fed mosquitoes in dark areas and natural cavities. They are especially good for collection in heavy vegetation around homes for assessing the mosquito problems of customer complaints calls if other methods are lacking or problematic.
Collection of Mansonia and Coquillettidia adults is more difficult, since the larvae are associated with the roots of aquatic vegetation. Both species are readily collected as adults in NJLT and CDC traps, but, to assess their population from aquatic plant habitats, a more direct trapping regime may be needed. Most workers use emergence traps to collect Mansonia species associated with water lettuce (Pistia stratiotes) mats. The emergence traps cover a known surface area (typically 4 meter square), and adult collections are made on a weekly basis. Traps are generally spaced between 50-100 meters apart and number from 2-10 per site. Traps are periodically repositioned to compensate for the possible depletion of mosquito fauna at 4-8 week intervals.
If the design of the mosquito control program includes source reduction or application of larvicides, both a mosquito habitat inventory and a larval surveillance system should be in place. The mosquito habitat inventory is a permanent collection of descriptions of all habitats. A larval surveillance system describes the abundance of mosquito larvae at each site. The information can be used to determine optimal times for use of larval control measures, including chemicals, biologicals, draining, or impounding. It also can be used to help forecast the need and timing for adult mosquito control and to help assess the effectiveness of both chemical and biological control measures.
As mosquito control programs evolve, topographical and aerial paper maps are being replaced by geo-referenced high resolution aerial images. The geo-referenced maps can show the location of potential larval habitats and the treatments that occurred within a specific time frame. These maps are used to develop and maintain a program for the surveillance of larvae and the application of larvicides. The maps provide an up-to-date record of the larval habitats within the jurisdiction of the control program.
The map inventory should be dynamic and updated on a routine basis. As new residential or commercial developments are created, the characteristics of mosquito habitats may change. In turn, the species composition of mosquitoes produced at each site also may change. Due to changes in rainfall patterns and intensity of tidal flooding, these habitats can vary greatly.
Deciding which characteristics of the larval habitat should be recorded in an inventory is difficult. Instantaneous measurements of rapidly or frequently changing variables, such as water depth, water temperature, and presence or absence of predators or parasites may be useful to help determine if control treatments are needed and should be included in a larval habitat inventory.
While the field work portion of the initial inventory is time consuming, creating and maintaining maps of larval habitats is even more difficult. It is highly desirable to use a computer-based mapping system using GIS technology to map these larval habitats if possible. A major advantage of a computerized mapping system is the ease with which data can be extracted and compiled. Maps can be displayed on screen or may be printed to highlight areas of concern.
The number of devices and procedures that have been developed to sample mosquito eggs, larvae, and pupae is extensive (Service 1993). Unfortunately, little effort has been made to standardize the most frequently used methods. Each mosquito control program has its own version of the different sampling methods, which makes the comparison of data between programs difficult.
There are many techniques available to sample mosquito eggs (Service 1993), but these methods are seldom used on an ongoing basis or as a primary surveillance system. Sampling mosquito eggs is too labor-intensive for practical purposes, and it is usually easier and simpler to sample mosquito larvae. A few programs have found egg sampling useful to initially describe or find mosquito habitats to be added to the inventory. Once documented, it is usually easier and simpler to sample larvae. One exception to the above is the use of ovitraps, which monitor the distribution of the Asian tiger mosquito, Aedes albopictus, and Ae. aegypti, in Florida. Using a network of highly attractive ovitraps to monitor this species is easier than searching for the small containers in which these species oviposit. Several county programs used ovitraps to detect the initial introduction of Ae. albopictus (Hillsborough, Lee, Leon, Monroe, Sarasota, and the City of Gainesville). Once this species was found and subsequently established, the ovitrap collections were discontinued. Indeed, in certain countries where these species are a major public health risk for dengue or yellow fever, nominal data has been used (i.e., absence/presence) for control determinations (Mogi et al. 1990).
Mosquito larvae and pupae can be collected with dippers, nets, aquatic light traps, suction devices (turkey baster for bromeliad and container collections), and containerevacuation methods. The most commonly used apparatus is the dipper. The term "standard pint dipper" is used in the scientific literature, but, in practice, there is no standard dipper or standardized dipping techniques (Service 1993). The dipper consists of a white plastic cup, 400ml in volume, with a two to five foot handle to allow for an extended reach. The dipper can be used as a survey tool simply to determine the presence or absence of larvae. Such a method usually involves taking several dipper samples from designated areas in the habitat of interest and then counting the larvae captured in each dip. The dipping method will vary with water depth, presence of aquatic vegetation or other debris, and water clarity. Collectors must take into account certain factors of importance, e.g., mosquito species difference in submerging behavior, and stage differences (first and second instar stay under longer). Training, practice, and experience are important when control programs use larval density as a basis for larval control measures: Larvae densities measures = Number of larvae per dip.
The collection of Mansonia and Coquillettidia larvae is difficult because the larvae do not breathe at the water surface but get their oxygen by piercing the stems and roots of aquatic plants. Collection of larvae is problematic since they quickly detach when disturbed and bury themselves in the detritus. To collect Coquillettidia larvae, a pump and wand system has been used (Morris et al. 1985) with good results. Collection of Mansonia larvae also is difficult and labor intensive. Samples are taken of a known surface area using a stainless steel sampling tool with serrated teeth around the perimeter of the bottom to penetrate the water lettuce mat. This trap has a “trap door” to collect the underlying water column to be collected with the sample. In this system, five quadrants are used per site and sampled approximately every 30 days. Another method is to collect plants randomly in the field and to place them in a bucket with water for transport. In the laboratory, the plants are shaken vigorously to dislodge the larvae, and the larvae are then concentrated through a series of sieves and then counted. The number of plants collected varies, but generally from 5-10 plants per site are used.
To maximize the usefulness of mosquito surveillance data, it is important to monitor certain environmental parameters such as rainfall and tidal events. Predicted tide levels in coastal areas are monitored using charts, and tide gauges are useful for measuring the actual tide. Tidal activity and rainfall dictate when high marsh sites will be flooded and when they will need to be inspected for mosquito larvae. The tide gauges also may reflect changes in the water level caused by rainfall and wind that often result in increased mosquito production in salt marshes and mangrove forests. Rain gauges are important in both coastal and inland counties -- in fact, anywhere mosquito production is being monitored. Data from these instruments can be supplemented with data from the National Weather Service and local weather watchers. Because rainfall in Florida is highly localized, it is important to collect rainfall data from many locations.
Knowledge of weather patterns is important during ground and aerial mosquito control applications. High winds, low temperatures, rainfall, and high humidity can deter the product from getting to the target, influence the dispersal of the material applied, and deter it from reaching its target, thereby affecting the efficacy of the application.
Crans, W.J. 1989. Proceedings of the Eighty-Second Annual Meeting of the New Jersey Mosquito Control Association, Inc.
Mogi, M., W. Choochote, C. Khamboonruang and P. Suwanpanit. 1990. Applicability of presence-absence and sequential sampling for ovitrap surveillance of Aedes (Diptera: Culicidae) in Chiang Mai, Northern Thailand. Journal of Medical Entomology 27: 509- 514.
Morris, C.D., J.L. Callahan and R.H. Lewis. 1985. Devices for sampling and sorting immature Coquillettidia. Journal of the American Mosquito Control Association 1: 247- 250.
Service, M.W. 1993. Mosquito ecology: field sampling methods, 2nd ed. London and New York: Elsevier Applied Science.