ASPECTS OF MOSQUITO SURVEILLANCE AND CONTROL PROGRAMS
David N. Gaines, PhD, Public Health Entomologist
Virginia Department of Health - Office of Epidemiology
Mosquito Surveillance
Mosquito control cannot be effectively applied without the use of surveillance as a component of the control program. Mosquito surveillance includes both trapping for adult mosquitoes and dip sampling for larval mosquitoes.
Adult Mosquito Surveillance Through Trapping
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Mosquito surveillance through trapping provides several types of information. This information includes:
a) The identity of the mosquitoes active in the community.
b) The identity or probable location of the mosquito breeding habitats.
c) The relative number of mosquitoes and species active during any particular time of the season.
d) The presence of arboviral activity in a community.�
There are about 60 species of mosquitoes that can be found in Virginia and each species has unique habits and behavior. Some breed in swamps or marshes, others in puddles or temporary pools and others breed in water that accumulates in stump holes or artificial containers. Some species are general feeders and will bite humans, birds and large animals. Others may specialize on one type of animal host such as birds, rodents, amphibians (e.g., frogs), or livestock and humans. Some species typically fly up to 10 miles or more in search of a blood meal whereas other species rarely fly farther than 100 yards from their breeding habitat. Some are known to carry diseases and others are not. Therefore, it is important for one to know the identity of problem mosquitoes in a community to know how to control them.
To identify the mosquitoes that are active or causing problems in an area one must first collect them through some sort of trapping program. Disease carrying mosquitoes cannot be effectively tested for arboviruses until they have been collected and identified. Because the type of breeding habitat and typical flight range of each mosquito species is known, identification of the trapped mosquitoes provides information as to where they might be coming from. The number of mosquitoes collected each week in a trap also gives an indication of the number of mosquitoes active in the area and indicates whether or not pesticide applications are necessary.
Adult mosquitoes may be trapped using a CDC type trap (CDC or ABC traps), a New Jersey light trap or a gravid trap. Each of these trap types has it�s own function and utility. The CDC type traps (CDC and ABC) are portable, battery-operated traps that are useful for investigating newly identified mosquito problem areas. They are also used at regular trapping sites. CDC type traps are ideally suited for collecting mosquitoes for species identification or for pooling for arboviral testing. CDC type traps catch relatively few non-mosquito insects. Thus, the trapped mosquitoes do not get damaged by a lot of larger insects during the trapping process. CDC type traps attract mosquitoes with a small flashlight sized light bulb and are generally baited with carbon dioxide.� New Jersey light traps are stationary and must be plugged into a power source. They use a 25-watt light bulb to attract mosquitoes. They are useful for monitoring mosquito populations in areas where just a few species predominate (e.g., salt marsh mosquitoes), but are not useful for most mosquito/arboviral surveillance. Many other insects get caught in these traps, and mosquito specimens are often too beat up to identify. By regular trapping with a New Jersey trap and counting the trapped mosquitoes, one can obtain information on the relative abundance of mosquitoes in an area from week to week.
Gravid traps are useful for monitoring populations of Culex pipiens and Culex restuans as well as those of some of the container breeding Aedes species. Gravid traps use a container of baited water to attract gravid mosquitoes. Gravid mosquitoes are mosquitoes that have already had a blood meal and are searching for a place to lay eggs. Gravid traps are useful for monitoring arboviral activity because mosquitoes that have had a blood meal are more likely to have picked up and be carrying a virus. Culex species are important vectors of West Nile Virus and the container breeding Aedes species may also be vectors.
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Larval Mosquito Surveillance Through Dip Sampling
Dip Sampling is as important a mosquito surveillance tool as adult trapping. Dip sampling provides information that will indicate:
a) The presence and identity of mosquito species in a particular breeding habitat.
b) The relative size and timing of an emerging mosquito population.
c) The correct time to apply mosquito larvicides to prevent a population of adult mosquitoes from emerging.
� Immature mosquitoes exist in aquatic habitats in a larval state. Personnel conducting a surveillance program should regularly take dip samples of identified mosquito breeding habitats. These breeding habitats may have been identified through identification of adult mosquitoes collected during trapping or through pro-active searching out and dip sampling of potential habitats. Dip samples are obtained using a dipper on the end of a long handle and may contain mosquito larvae in various stages of development.
� Larval mosquitoes collected during dip sampling are placed in Whirl-Pak bags. Larvae can be identified as 4th instar larvae or raised until the adult stage for identification. Some species of mosquito adults do not readily come to traps and dip sampling may be the first and easiest way to detect them in a community. Once the larval habitats in a community are known, one can begin to predict the size and timing of impending adult mosquito populations by counting the larvae in various habitats (number of larvae per dip) and determining their age. Larval counts help determine whether it is a good time to apply mosquito larvicides. Larviciding operations (using pesticides against mosquito larvae) are one of the most effective and environmentally friendly ways to control mosquitoes.
�Typical Surveillance Program Operations
Traps are typically placed in problem areas (based on citizen complaints) or in areas that will allow the capture of a good representative sample of local pest mosquitoes. Traps are typically set once a week at each location. The number of traps needed depends on the size of the area under surveillance or the number of separate areas needing surveillance. If placed correctly, a single trap may be adequate for a town that is � mile across. Surveillance activities in a city the size of Norfolk may require about 15 traps. Spare traps may be needed for investigating new complaint areas.
CDC type traps are baited with dry ice that produces carbon dioxide, or with canisters of compressed CO2 that emit a steady stream of gas (most mosquitoes are attracted to CO2). Dry ice is cheaper to use if there is a ready source nearby. Compressed CO2 gas is the only alternative when sources of dry ice are unavailable. Traps are typically set before 4 PM and collected at 8 AM the next morning. Trap catches are returned to the office, for sorting and identification (trapped mosquitoes may be killed before sorting by placement for in a freezer for 15 minutes). If mosquito pools are to be submitted for arbovirus testing (Eastern Equine Encephalitis, St. Louis Encephalitis or West Nile Virus) mosquitoes are knocked out with triethylamine (TEA), sorted, identified and then placed in Cryo-tubes and frozen until shipment to the testing lab. Mosquito pools are submitted for each species from a location and for each trap date. Each pool must contain from 25 to 50 mosquitoes of a species.
Backpack or hand-held aspirators are useful for collecting types of mosquitoes (e.g., Asian tiger mosquitoes) that are not attracted to traps. Mosquitoes may be collected by disturbing the adult resting habitat (kicking tires or buckets, or shaking vegetation), and vacuuming up the adults as they fly out. One can also collect resting mosquitoes off of walls or ceilings, or collect mosquitoes that attack you during trap setting activities.
Dip sampling is typically performed at identified mosquito breeding habitats on a weekly basis. �Dip sampling is often combined with larviciding operations. Dip sampling may also be performed to find breeding habitats around neighborhoods where citizen complaints have indicated a local mosquito breeding problem. Some mosquito larvae may need to be reared to adult stage to facilitate identification. Larvae may be reared in the Whirl-Pak bags they were collected in.
Personnel Requirements for a Mosquito Surveillance Program
A mosquito surveillance program is best conducted by a person that is educated, dedicated, and self motivated. Good surveillance requires samples at regular intervals (no missed weeks) and good record keeping. Personnel performing surveillance work mostly perform their duties un-supervised and their level of dedication and interest in the work will affect the quality of their surveillance data. Mosquito identification requires some knowledge of microscope use and the patience to learn and follow the mosquito identification keys. It also requires the patience to sit in front of a microscope for several hours at a time. Persons with some college education and a background in field biology or ecology are generally good candidates for this type of work. The biology departments of colleges or universities are a good place to seek personnel (biologists) for mosquito surveillance programs. However, mosquito biology is a specialized field and it may take a season of field work for new personnel to become proficient in this knowledge, even if they have a degree in entomology. Courses and workshops are available through the Virginia Mosquito Control Association (VMCA) or the Virginia Department of Health to train mosquito surveillance and control personnel in various aspects of mosquito identification, surveillance and control.�
Full time personnel working on mosquito surveillance and control programs generally spend the spring, summer and fall doing mosquito surveillance and control. Winters are spent on planning drainage maintenance of mosquito breeding sites (ditches, puddles, etc.), surveillance for potential breeding sites, preparation of educational materials for the public, maintenance of equipment and supplies, budgets, and preparation of surveillance reports.
Some Materials and Supplies Needed for a Mosquito Surveillance Program
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