[Flying Insects] The Basics of Mosquito Control

Mosquitoes are by far the most dangerous animals on earth. Most people don’t realize this fact for a couple of reasons: 1) they don’t know that mosquitoes (and all other insects, for that matter) are "animals," and 2) they don’t comprehend the amount of human disease caused by the little suckers. Some scientists estimate between 500 and 700 million people get malaria worldwide each year. That’s more than twice the entire population of the United States each year! Since mosquitoes are the most medically important insects on earth, it is imperative that we as pest management professionals understand something about their biology and control.

MOSQUITO BIOLOGY. Mosquitoes, like other flies, undergo complete metamorphosis, having egg, larval, pupal and adult stages. Larvae are commonly referred to as "wigglers" and pupae as "tumblers." Larvae and pupae of mosquitoes are always found in water. The breeding source may be anything from water in discarded automobile tires to water in the axils of plants, to pools, puddles, swamps and lakes. Mosquito species differ in their breeding habits, biting behavior, flight range, etc. However, a generalized description of their life cycle is presented here and will serve as a useful basis for understanding mosquito biology and ecology. Most larvae in the subfamily Culicinae hang down just under the water surface by a breathing tube (siphon), whereas anopheline larvae lie horizontally just beneath the water surface supported by small notched organs of the thorax and clusters of float hairs along the abdomen. They have no prominent siphon. Mosquito larvae feed on suspended particles in the water as well as microorganisms. They undergo four molts (each successively larger), the last of which results in the pupal stage. With optimum food and temperature conditions, the time required for larval development can be as short as seven days.

Mosquito pupae do not feed and will "tumble" toward the bottom of their water source upon disturbance. They emerge as adult mosquitoes in two to four days. This process begins with the splitting of the pupal skin along the back. The emerging adult must dry its wings and separate and groom its head appendages before it can fly away. This is a critical stage in the survival of mosquitoes. If there is too much wind or wave action, the emerging adult will fall over while leaving its skin, becoming trapped on the water surface to soon die.

Adult mosquitoes of both sexes obtain nourishment for basic metabolism and flight by feeding on nectar from flowers. In addition, females of most species obtain a blood meal from birds, mammals or amphibians for egg development. Breeding sites selected for egg laying differ by species, but generally mosquitoes can be divided into three major breeding groups: permanent water breeders, floodwater breeders and artificial container/tree hole breeders. Anopheles and many Culex mosquitoes select permanent water bodies, such as swamps, ponds, lakes and ditches that do not usually dry up. Floodwater mosquitoes lay eggs on the ground in low areas subject to flooding. During heavy rains, water collecting in these low areas covers the eggs, which hatch from within minutes up to a few hours. Salt marsh mosquitoes (Ochlerotatus sollicitans), inland floodwater mosquitoes (A. vexans) and dark rice field mosquitoes (Psorophora columbiae) are included in this group. Artificial container/tree hole breeders are represented by yellow fever mosquitoes (A. aegypti), Asian tiger mosquitoes (Aedes albopictus), tree hole mosquitoes (O. triseriatus) and others. However, several species of Anopheles and Culex may also occasionally oviposit in these areas. Some of these container-breeding species lay eggs on the walls of a container just above the water line. The eggs are flooded when rains raise the water level. Other species oviposit directly on the water surface.

Female Anopheles mosquitoes generally lay eggs on the surface of the water at night. Each batch usually contains 100 to 150 eggs. The Anopheles egg is cigar shaped, about 1 mm long and bears a pair of air-filled floats on the sides. Under favorable conditions, hatching occurs within 1 or 2 days. Aedes and Ochlerotatus mosquitoes lay their eggs on moist ground around the edge of the water or, as previously mentioned, on the inside walls of artificial containers just above the water line. Aedes and Ochlerotatus eggs will die if they become too dry when first laid. However, after the embryo in the egg develops, the eggs can withstand dry conditions for long periods of time. This trait has allowed Aedes and Ochlerotatus mosquitoes to use temporary water bodies for breeding. Culex mosquitoes lay batches of eggs that are attached together to form little floating rafts. Upon close inspection of a suitable breeding site, these egg rafts can often be seen floating on the surface of the water.

SHIFT IN THINKING. Mosquito control efforts historically have placed primary emphasis on adulticiding, then larviciding, then breeding source reduction, then public education (including personal protection). Lately there has been increased awareness that the previous model needs to be turned totally around such that public education and source reduction would be top priorities, then larviciding and lastly, adulticiding. We need a paradigm shift in mosquito control.

Breeding source reduction is the elimination of mosquito breeding sites. Reducing the amount of breeding areas in a town can save mosquito control personnel both time and work. A spring clean-up drive involving schools, citizen groups and town maintenance and sanitation departments can help get rid of bottles, cans, tires, stagnant drainage ditches and other sites that produce mosquitoes. Many types of breeding sites, however, cannot or should not be eliminated. Mosquitoes in these areas will have to be controlled by applications of a suitable larvicide. Improper drainage of wetlands and indiscriminate ditching can create more mosquito problems than were there to begin with.

MOSQUITO SURVEILLANCE. Here’s a review of a variety of mosquito surveillance activities:

Treatments Should be Based on Something. First, it must be noted that all mosquito control treatment decisions should be based on something (preferably scientific evidence or empirical testing). Surveillance can help ensure that timing of chemical applications and even chemical choices have a scientific basis. Decisions in mosquito control should be as scientifically based as possible.

Purpose of Surveillance. Mosquito surveillance should be a routine part of any mosquito control program. A good surveillance program will provide two types of information. 1) a list of the local mosquitoes 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 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 (visit www.pctonline.com to learn more about New Jersey light traps) can provide mosquito controllers with valuable information on adult mosquito populations. Many mosquito control agencies routinely station permanent light traps in the backyards of retired people living in mosquito-prone areas throughout the city. These individuals maintain the traps, collecting the mosquitoes after each sampling and mailing the samples to the mosquito control agency. Operational expenses can be reduced by locating light traps at fire stations, city or county barns or other facilities where they can be easily maintained.

Mosquito Egg Surveys. Oviposition jars are black jars containing a paper or wooden strip (paddle) for female mosquitoes to lay eggs on. They are useful tools for collecting information on many container-breeding mosquitoes. 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.

Larval Surveys. Most equipment required to conduct a larval surveillance program can be purchased in a local hardware store. A white plastic or metal dipper is excellent for collecting water from artificial containers and small water bodies that are easy to reach. Larvae can then 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 bought from companies that supply mosquito control equipment. These are useful for sampling hard-to-reach areas.

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.

Adult Surveys. Adult mosquito surveillance is a very important part of any mosquito program. Adult surveillance will provide information on the effectiveness of the larvicide program. However, the presence of some adult mosquitoes does not mean that larviciding efforts are not working. No program will 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 salt marsh mosquitoes, are capable of flying long distances and can move into an area from distant breeding sites.

Daytime Resting Stations. Adult mosquitoes, especially Anopheles, can be found during the daytime resting in both natural and artificial shelters. These areas include houses, barns, sheds, privies, bridges, culverts, hollow trees, overhanging cliffs and foliage. Counts of mosquitoes utilizing daytime resting shelters can give a good indication of population density. Mosquitoes found in these shelters can be easily collected with an aspirator. In areas where no resting shelters are found, an investigator may install an artificial shelter such as a wooden box so that these sites can be routinely sampled. Many mosquitoes that do not usually bite can be collected in this way.

Light Traps. Several types of light traps are commonly used. The CDC light trap, developed by the Centers for Disease Control and Prevention, is a widely used portable model. CDC light traps run 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 are usually set out and turned on at dusk and picked up at dawn. Only selected species of mosquitoes are attracted by light traps and catches tend to be smaller during a full moon. Mosquito catches can be increased by hanging a container of dry ice near the light trap.

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 increasingly being used by mosquito control agencies for WNV surveillance since they predominately catch Culex mosquitoes.

Habitat Mapping and Record Keeping. Habitat maps and records of mosquito populations and application methods used are valuable sources of information to mosquito control personnel. A habitat map should show all known water areas within a town, including artificial containers and floodwater areas. In the beginning of the larvicide program, all known water areas can be recorded upon photocopied quarter-mile quadrants of a town 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 ensures 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 mosquito control 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.

Complaint Calls. The public can provide a valuable service by calling in mosquito problems. Complaint calls can help pinpoint large populations of mosquitoes. Spray efforts can be aimed at these "hot spots" when needed, rather than spraying the entire town. Encourage community members 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.

MOSQUITO CONTROL. There are several types of mosquito control, including biological control, larviciding, natural mosquito control organisms and adulticiding.

Biological Control. Many organisms have been or are being evaluated as potential biological control agents for mosquitoes. A few of these agents have been used to control mosquitoes for years. The World Health Organization has used the mosquito fish (Gambusia affinis) in many parts of the world since the 1940s. A nematode parasite (Romanomermis culicivorax) was at one time commercially available and has been used in many areas for mosquito control with measured success. The bacteria Bti (Bacillus thuringiensis israeliensis) and Bs (Bacillus sphaericus) have been on the market for several years and are some of the most successful biological control agents currently used.

Each biological control agent has merits and restrictions. In order to use a biological control agent successfully, mosquito control personnel must have a basic knowledge of the biology of each agent used. Some biological control agents are limited by salinity, temperature or organic pollution. Some are more effective on certain types of mosquitoes than others. These agents also differ in the ways in which they can be formulated, transported, stored and applied. All of these factors must be considered when choosing the proper biological control agent for a specific habitat or to control a specific mosquito.

Products Used for Larviciding. Here’s a review of various types of products used for laviciding.

· Insect Growth Regulators. These active ingredients disrupt the normal growth patterns that occur in an immature insect (e.g., a mosquito larva) as it molts. It may be used to control second, third and fourth instar larvae. Materials in this group effect only those systems found in insects and closely related arthropods and have little (if any) toxicity to mammals and fish.

· Bacterial Formulations. Two bacterial related formulations are available for larviciding programs: Bacillus thuringiensis var. israelensis (Bti) and Bacillus sphaericus. Bti attacks the cells of the intestinal tract and within a few hours affected larvae are unable to absorb food. The larvae die within 8 to 12 hours. These products must be ingested by actively feeding mosquito larvae hence they have no effect on fourth instar larvae or pupae. Both products are sensitive to ultraviolet light and break down fairly quickly, but they work well in clear to polluted water. Bti probably has a broader spectrum of activity than B. sphaericus. At operational concentrations these products have very little if any toxicity to mammals or fish.

· Monomolecular Films. Products in this class, such as Arosurf and Agnique, reduce the surface tension of the water — making it difficult for larvae, pupae and emerging adults to attach to the surface, causing them to drown. These products may also clog larval and pupal breathing tubes, which also interfere with air exchange. Concentrations of this film do not have any toxicity to mammals or fish.

· Petroleum Hydrocarbons. After application, hydrocarbons, such as Golden Bear Oil, rest of the surface of the water and will enter the breathing tubes of mosquitoes as they approach the surface.

· Abate (Temephos) and Malathion. These materials are organophosphate insecticides. Label directions relative to mixing and application must be followed. Non-target organisms may be affected if these procedures are not followed. If there is interest in these products, consult the label of choice for information relative to site use, mixing and application information.

Natural Mosquito Control Organisms. As mentioned earlier, natural habitats, permanent water bodies, and even artificial containers that have held water for a long time, support populations of mosquito predators. These predators include beetle larvae, certain fly larvae, aquatic bugs, tadpole shrimp, dragonfly and damselfly naiads, and fish. During routine surveillance of mosquito habitats, the larvicide technician should look for and become familiar with these organisms that feed on mosquito larvae in the wild. The occurrence of dead mosquito larvae or pupae in previously untreated sites may indicate the presence of a naturally occurring pathogen or parasite.

Adulticiding (spray trucks). Only selected chemicals are approved for use in a thermofogging unit or ultra low volume unit for mosquito control in the United States. Note: There are numerous formulations and brand names for each of the chemicals listed below. Not all products are labeled in all states. Here’s a list of products used for adulticiding mosquitoes in the United States:

· Organophosphate: Malathion. This product is often used in older "thermal foggers," although there is a formulation for truck-mounted ULV machines.

· Organophosphate: Naled. This product is primarily applied by airplanes, although there is a ground-use label.

· Organophosphate: Chlorpyrifos. This product is not widely used in mosquito control.

· Botanical: Pyrethrins. Pyrethrin-based products — although environmentally friendly — are the most expensive.

· Synthetic Pyrethroid: Permethrin. Various formulations of permethrin (some with a synergist, PBO) are the most widely used mosquito adulticides.

· Synthetic Pyrethroid: Sumithrin. This product is applied both by ground and airplane equipment.

· Synthetic Pyrethroid: Resmethrin. Resemthrin is used both ground and airplane equipment. It is a restricted use pesticide which requires handlers to be certified in pesticide use by their state agriculture department.

Always read the chemical label before buying, mixing, loading, applying and storing insecticides. A few adulticides available are restricted use pesticides and must be purchased and applied under the direct supervision of a certified pesticide applicator. All applicators should review and carry the chemical labels and MSDSs with them while conducting control activities.

Since most mosquito species are night fliers, adulticiding should generally be done after sunset. Chemicals sprayed during the day will be carried upward by thermal currents. Also during the day, mosquitoes generally are resting out of reach of pesticide in protective foliage, while bees are exposed as they gather nectar. Killing honey bees should be strictly avoided. Spray trucks should be driven slowly; usually 10 mph. Great efforts should be made to drive the entire evening at the same speed. This assures application of the chemical at a uniform rate.

Machines must be kept in proper working order and calibrated yearly. Both flow rate and droplet size should be calibrated in ULV machines and flow rate in thermofoggers. Generally, private mosquito contract personnel, Cooperative Extension Service employees and/or equipment manufacturers/distributors are available to aid in calibration. When using corrosive chemicals, machines should be flushed after each use. Pesticide lines should be routed outside of the cab.

Adulticiding only should be done when necessary. Routine adulticiding wastes chemicals, exposes the public to pesticides unnecessarily and promotes buildup of mosquito populations resistant to the chemicals. There should always be a basis for spraying mosquitoes, either 1) mosquito-borne disease in the area, 2) high mosquito numbers collected in traps or 3) increases in complaint calls.

FINAL THOUGHTS. Decisions about mosquito control should be based on something and not be made arbitrarily. Spraying should be based upon established criteria such as disease in the area, a certain number of mosquitoes captured in light traps in a particular area and/or number of complaints received. Acceptable "trigger" numbers vary around the country (even the experts disagree), and should be mutually agreed upon by both the contracting organization and the contractor (mosquito control agency). One program I know of in Louisiana has established triggers of 25 southern house mosquitoes or 25 Asian tiger mosquitoes in a gravid trap per 24 hours or 50 mosquitoes (each species) in a CDC trap. Ideally, mosquito counts should be correlated with complaints. I offer the following caveats as to trigger numbers and complaints:

In evaluating complaints, keep in mind that some people call repeatedly regardless of the presence or absence of significant mosquito numbers.

Mosquito control is a science, but let’s not forget that it is also an art. Experienced mosquito control personnel consider numerous factors in making treatment decisions. For example, factors such as recent rainfall and the "age" of adult mosquitoes should be considered in control decisions. For example, 50 "old" salt marsh mosquitoes caught in a light trap might not elicit as much concern as 20 "newly" emerged ones. I say this to point out that whatever trigger mechanisms are decided upon in a mosquito control program — the "art" of mosquito control should not be restricted.

The author a is medical entomologist for the Mississippi Dept. of Health, Jackson, Miss. He can be reached at jgoddard@giemedia.com.