[Special Report: Mosquito Control] Under Watch

Many PCOs these days are choosing to offer mosquito control as an add-on service for their customers. While this effort is commendable as a welcomed expansion of pest control into the public health arena, it is not to be entered into lightly. Mosquito control is a specialty service and involves much more than just spraying for mosquitoes. The best mosquito control program is an integrated one that includes point source reduction of breeding areas, routine larviciding in those breeding areas that cannot be eliminated and adulticiding when necessary.

In this day of environmental consciousness, pest control personnel must use integrated methods and not just routinely spray. A good adulticiding operation should be a backup system, used when mosquito populations have gotten out of hand. An unseasonable emergence of mosquitoes or an encephalitis outbreak may require the use of adulticides to contain the problem. If the fog truck has been parked, being used only when necessary, then the program manager can feel confident that the chemical will be fast and effective, encountering no problems with resistance among mosquito populations.

PURPOSE OF MOSQUITO SURVEILLANCE. Mosquito surveillance should be a routine part of any mosquito control program. Good surveillance will provide two types of information: 1) a list of the local mosquitoes (including distribution and population size estimates) and, 2) the effectiveness of the control strategies being used. Routine surveillance can keep control personnel informed about locations of major breeding areas, helping to identify problem sites where control should be concentrated. Carefully interpreted survey data can provide vital information. For instance, large numbers of Culex egg rafts around the edge of ditches or Aedes eggs on oviposition strips are indicators that these breeding sites should be watched closely during the next few days. Treatment should be timed to catch the heavy crop of resulting larvae during the period of their life cycle when they are active feeders. Heavy adult catches in light traps stationed near treated areas may indicate that an important breeding site has been overlooked in the survey or that mosquitoes are migrating in from other areas, depending upon the species captured.

Establishment of fixed light traps (such as New Jersey Light Traps, discussed later in this article) can provide mosquito controllers with valuable information on adult mosquito populations. Mosquito control agencies routinely station permanent light traps in the backyards of retired people living in mosquito prone areas throughout the neighborhood or city. Sometimes, these individuals maintain the traps, collecting the mosquitoes after each sampling and mailing the samples to the mosquito control agency. In other cases, all these tasks are performed by pest management personnel. For public agencies, operational expenses can be reduced by locating light traps at fire stations, city or county barns or other facilities where they can be maintained easily.

COMPLAINT CALLS. The population served by mosquito control (a town, neighborhood or gated community) can provide a valuable service in mosquito surveillance by calling in mosquito problems. Complaint calls can help pinpoint large populations of mosquitoes. Spray efforts can then be aimed at these “hot spots” when needed, rather than spraying the entire area.

Community members should be encouraged to notify their local mosquito control agency when mosquitoes get out of hand. Those complaint calls can then be plotted on a large map to provide information on probable areas to target. If manpower allows, complaint calls can be turned into service requests. Customer service representatives or office managers should gather as much information as possible over the phone, including the name, address and phone number of the caller, time of day or night the mosquitoes are biting and areas of standing water that the resident may know about. When possible, an inspection should be arranged when the resident is home. This is a good opportunity to discuss with the homeowner various aspects of mosquito biology and control, source reduction and personal protective measures. The inspector should conduct a thorough inspection of the homeowner’s and adjacent yards.

Taking time to educate residents will help reduce the number of callback inspections. The PCO should try to empower the home-owner by leaving literature on mosquito control and offering sound suggestions and advice. Getting residents to change their behavior may help eliminate neighborhood mosquito problems. Pest controllers should record the date, location, density, stage of larval development and habitat. A sample can be kept for identification and to create a breeding site card for locations that will need regular monitoring and treatment. Information on adult mosquito landing rates encountered and the decided treatment should be recorded. Information should be kept in a file system for rechecks and control activities, historical information and for legal purposes.

MOSQUITO EGG SURVEYS. Oviposition jars can be used for collecting information on container breeding mosquitoes, such as the Asian tiger mosquito (Aedes albopictus), the yellow fever mosquito (Aedes aegypti) and the tree hole mosquito (Ochlerotatus [Aedes] triseriatus). Counting eggs collected from an ovitrap will give a good indication of the number of Aedes larvae that will hatch in an area following the next rain. Some eggs can be quickly identified to species under a microscope, but at a minimum, eggs should be counted. The oviposition jar should be a black plastic or glass jar or even an aluminum can that has had the top cut out and painted black. The oviposition jar is fitted with a strip of felt-covered paper or masonite hardboard clipped to the side. The jar is filled about halfway with water.

Female mosquitoes are attracted to both the black jar and the water and they will lay eggs on the rough surface of the strip just above the water line rather than the smooth surface of the jar. A hole punched in the side of the jar about two inches from the rim will prevent water from flooding the eggs during heavy rains, thereby causing many of them to hatch. Some mosquito control districts attach an oviposition jar to both sides of a piece of white painted board. This arrangement reduces the chance of oviposition jars being turned over and, also, the contrast of black jars against the white boards seem to attract mosquitoes more readily.

LARVAL SURVEYS. For larval surveillance, all that is needed is a white plastic dipper or a metal dipper for collecting water from artificial containers and small bodies of water. Often, larvae are simply counted and released. If necessary, they can be gathered from the dipper with a medicine dropper and placed in a small jar containing a little water, to be preserved later.

Long-handled white graduated dippers can be purchased from companies that supply mosquito control equipment. These are useful for sampling ditches, margins of lakes and streams and hard-to-reach areas. Descriptive information should be written down for each larval sample collected. Accurate descriptions of habitats sampled, including those places where no mosquitoes are found, are equally important. By having a good background on the type of local areas that breed mosquitoes, future routine surveys can be conducted efficiently, concentrating on those areas that are known breeding sites. Occasionally a thorough survey of all water areas should be conducted to ensure that previously unproductive areas have not become mosquito breeding sites.

For a really top-notch mosquito program, estimates of population densities of larvae can be obtained by counting the number of larvae per dip, using a standard size dipper. Three to five dips should be taken and counted at each site. The number of dips counted and number of larvae in each dip should be recorded. Information on the life stage of larvae and pupae can also be recorded.

By noting numbers of larvae in each instar or size category (small, 0-5; medium, 5-15; large, 15+), number of pupae per dip and water temperature, the investigator will be able to make an educated guess as to when mosquitoes will emerge and what control efforts should be used. Generally, larvae develop faster at higher temperatures. Large numbers of pupae indicate that a large number of adults will emerge within hours or days. Since pupae do not feed, use of bacterial larvicides such as Bacillus thuringiensis israelensis (Bti) or other products that must be eaten by mosquitoes will not control them.

On the other hand, if most larvae are small, it may be five to 14 days before adults emerge, depending upon the species and the temperature. The investigator may decide that in this case an application of Bti is suitable. Large numbers of pupal skins floating on the surface is a sign that adult mosquitoes have recently emerged. The experienced investigator also will be able to determine the genus of many larvae based upon a few key characteristics. This knowledge will be useful in selecting the right larval control agent.

For example, the larvicide Bacillus sphaericus is highly effective on Culex mosquitoes but not Anopheles mosquitoes. If identification is necessary, mosquito larvae should be handled carefully. When handled roughly, distinguishing hairs and other structures may fall off or become damaged, making identification difficult or impossible. Larvae that are to be preserved should be removed from the pan or dipper with a large-tipped medicine dropper. These larvae should be placed in a small jar containing alcohol, labeled and carried back to the office.

ADULT SURVEYS. Adult mosquito surveillance is a very important part of any integrated mosquito program. Adult surveillance will provide information on the effectiveness of the control program. However, the presence of some adult mosquitoes does not mean that larviciding or source reduction efforts are not working. No program can be successful in totally eradicating mosquitoes. The objective is to control mosquito populations, keeping their numbers at an acceptable level. Also, several species, such as the salt marsh mosquito (Ochlerotatus sollicitans) and the dark rice field mosquito (Psorophora columbiae), are capable of flying long distances and can move into an area from distant breeding sites.

Information that can be gained from a routine adult mosquito surveillance includes: 1) checklist of adult mosquito species in the local area, 2) estimate of adult mosquito population density and distribution, 3) indication of the presence of breeding sites that were overlooked, 4) identification of sites where larviciding efforts need to be stepped up and 5) source of adult female mosquitoes that can be used in West Nile surveys.

The equipment needed to collect adult mosquitoes is more complicated and expensive than that required for collecting larvae. Adult mosquitoes are very fragile. They readily lose legs, scales and wings when handled roughly, making identification difficult or impossible. The special collection equipment described below is designed to capture adult mosquitoes with minimum damage.

LIGHT TRAPS. Several types of light traps are available. The CDC light trap, developed by the Centers for Disease Control and Prevention, is a portable model that is widely used. This light trap runs on a six-volt lantern battery; a smaller version uses two D-cell batteries. Mosquitoes are attracted to a small light at the top of the trap and are then sucked into a net at the bottom of the trap by a fan. The traps usually are set out and turned on at dusk and picked up at dawn. Timing devices can be installed on the traps so that they will only run during those hours of peak mosquito activity, conserving batteries. Only selected species of mosquitoes are attracted by light traps and catches tend to be smaller during a full moon. Mosquito catches are increased by hanging a container of dry ice or an octenol lure near the light trap.

The New Jersey light trap is a larger metal device, usually located at a permanent sampling station. This trap often is equipped with a timing device that turns it on during selected hours on certain days of the week. It works on the same general principal as the CDC light trap, except that it uses 110-volt AC power and mosquitoes are sucked into a paper cup inside a jar containing a killing agent, such as a piece of pest strip. The paper cup prevents mosquitoes from coming into direct contact with the pest strip. Generally, New Jersey traps require little maintenance.

OVIPOSITION OR GRAVID TRAPS. Oviposition traps or gravid traps are available through some supply companies. These devices are similar to oviposition jars in that they provide a black plastic container partially filled with water as an attractant. Female mosquitoes visiting the trap to lay eggs are sucked into a net by a small fan motor like those used on many light traps. Oviposition traps are very selective for female Culex mosquitoes. The catch data is not comparable to light trap data. These traps are not used by all mosquito control agencies, but are extremely valuable for West Nile surveys since they specifically target WNV mosquito vectors.

PRESERVING ADULT MOSQUITOES. If it becomes necessary to retain or permanently store specimens, adult mosquitoes should be handled very carefully to prevent them from losing scales, legs or wings. Collections taken from a New Jersey light trap using a pesticide strip in a killing jar usually are dead within the perforated paper cup. These mosquitoes should be gently shaken from the cup into a small tissue-lined cardboard jewelry box or equivalent container. Mosquitoes should be arranged evenly over the tissue and one layer deep. A piece of tissue should be placed on top of the mosquitoes to prevent them from being shaken about. A label containing the necessary sample information should be placed on top of the last layer of tissue and the lid secured on the box with a rubber band. Only one sample should be placed in each box. If many mosquitoes are captured in a single trap, extra boxes can be used to hold the sample and these boxes bound together with a rubber band. Mosquitoes can be removed from net bags of light traps with an aspirator. Ice or frozen reusable ice packets can be used to “freeze” adult mosquitoes in the field within an ice chest.

A better method is to place the entire mosquito trap net in a freezer for about 30 minutes. Killed mosquitoes can be shaken from the net bags onto a piece of paper or into a small pan and then transferred to a small collection box. Adult mosquitoes should not be handled, if possible. When it is necessary to pick them up, use a pair of forceps, grabbing each mosquito gently by a group of legs. For identification by a specialist, mosquitoes can be mailed by placing the collection boxes in a larger, sturdy container and filling loose spaces around the collection boxes with paper or Styrofoam® peanuts. The lid should be carefully secured and the container marked “fragile.” As an extra precaution, a wisp of cotton can be placed on top of the upper tissue layer of each collection box and replace the lid. This will help prevent the mosquitoes from being shaken around inside the collection boxes.

These steps may seem to be a little extreme. However, it is necessary to provide the identifying entomologist (such as a technical director or someone at the cooperative extension service) with good quality samples in order that correct identifications can be made. This is especially important when adult collections are first being made from an area and a reference collection and species list is being compiled.

HABITAT MAPPING AND RECORD KEEPING. Habitat maps and records of mosquito populations and application methods used are valuable sources of information both for the pest controller and regulators. A habitat map should show all known water areas within a neighborhood or town, including artificial containers and floodwater areas. In the beginning of the larvicide program, all known water areas can be recorded on photocopied quarter-mile quadrants of a town or neighborhood street map or aerial photograph. The best way to conduct a habitat survey is by foot, inspecting each site for evidence of mosquito breeding. This insures a thorough inspection and allows the inspector to become familiar with the area. Both mosquito positive and mosquito negative sites should be recorded.

Positive sites should be distinguished from negative sites by placing a small star (*) next to those sites where mosquitoes were found. Water sites can be recorded by type, using a numerical code. Later, when making routine larviciding rounds, the technician can then quickly determine locations and types of water habitats in an area at a glance. He or she will know which of these sites were positive during the initial survey.

Habitat maps can be verified during the first couple of larvicide applications. Newly discovered sites should be added. Locations of ditches and storm drains and tree lines can be checked. When the technician feels comfortable with the accuracy of the maps, photocopies can be made and the master copy kept on file. During each larviciding trip, a set of these copies can be used as field maps. Notations concerning the day’s activities can be recorded on each quadrant as the larvicide technician visits each site. Maps showing each week’s activities should be kept on file for future reference on mosquito breeding trends.

Some mosquito control personnel choose to laminate quadrant maps. During routine visits to breeding sites, the technician can make notations on these maps in grease pencil. These maps should be kept in a metal clipboard with a cover in order to prevent smudging the grease pencil. Information should be transferred from the laminated maps to regular copies and filed for future reference. Keeping records on each site can be useful. Knowing information such as previous larvicide treatments, past estimates of mosquito numbers, life stages found and when a site was wet or dry will allow the technician to predict when a particular site will become a problem.

FINAL THOUGHTS. All of this mosquito surveillance information can be useful in other ways. Many vector-borne diseases occur in cycles. Knowing something about breeding trends of local mosquito species over the past few years may indicate the likelihood of a disease outbreak. Any advance warning of a potential epidemic would allow mosquito control technicians to take precautions such as larviciding an area more frequently or, if necessary, fogging adult populations. Remember, surveillance is important. It lets pest management professionals know what mosquitoes are in the area, when and where to spray and whether or not our control efforts are working. n

The author is a medical entomologist for the Mississippi Dept. of Health, Jackson, Miss.