The Mosquito Control program provides a direct service to approximately 2,100 communities in 16 Maryland counties. The primary goal of this program is to prevent the occurrence of mosquito-borne disease in humans, pets and livestock. 

Mosquito control is an economic necessity in some parts of Maryland dependent on outdoor tourism during the summer. The program relies on surveys and monitoring of the larval and adult mosquito populations to coordinate control activities. Control techniques include breeding source reduction, public education, biological control and insecticide applications from aircraft or ground equipment.

To learn more about the program, check out the Mosquito Control Program AgBrief. ​​

Mosquitoes are members of the family Culicidae in the order Diptera (true flies). Adult mosquitoes are distinguished from other flies by the presence of a long proboscis and scales on the margins and veins of the wing. Males differ from females by having feathery antennae and mouthparts not suited for piercing skin.

Mosquitoes are insects that develop through four distinct life stages - egg, larva, pupa and adult. Only adult females feed on blood which is a required source of protein for egg development. Males and females feed on plant nectar as a source of carbohydrates.

There are three basic types of mosquito eggs - those laid singly on the water surface, each egg being buoyed by floats; eggs laid on the water surface in groups or rafts; and eggs laid singly out of the water on a surface that will subsequently flood. Mosquitoes which deposit eggs on the water surface are commonly known as permanent water breeders and include the genera Anopheles,Coquillettidia, Culex and Culiseta. Floodwater mosquitoes lay their eggs on a moist substrate, out of the water, and represent one of the most successful reproductive strategies in the animal kingdom. Female floodwater mosquitoes are attracted by chemical stimuli to oviposition sites and are not dependent on water for oviposition, hence weather has less impact on the reproductive success of floodwater species than on permanent water breeders. Floodwater mosquito genera include Aedes, Ochlerotatus and Psorophora.

All mosquito larvae are aquatic. The 60 species known to occur in Maryland have adapted to a wide range of larval habitats, including swamps, marshes, tree holes, septic ditches, rock pools, etc. All of the breeding sites have a common characteristic of stagnant pools not subject to significant wind or wave action or water flow. Additionally, the breeding sites generally have a low or non-existent population of parasites or predators that prey on mosquito larvae. Consequently, flowing streams, tidal creeks, large ponds, lakes and other large water bodies are not typical mosquito breeding sites. Larvae feed on microorganisms and particles of organic matter. Mosquito larvae must have access to atmospheric oxygen, which is obtained by means of a siphon tube that penetrates the water surface or, in some species, pierces the roots of aquatic plants. The larval stage lasts from 4 to several days and contains four separate developmental periods termed instars.

The pupal stage is also aquatic but the pupa can complete development on a moist surface. It is during the pupal phase that the transformation from an aquatic larva to a terrestrial adult takes place. The pupal stage lasts only a few days.

Adult female mosquitoes mate once during their lifetime. This occurs shortly after emergence from the pupal case. The blood feeding habits of the female vary between species. Some are general feeders while others feed only on specific groups of vertebrates such as birds or reptiles. The flight habits are also variable, with some species rarely flying more than several hundred feet from their breeding sites and others flying 20 miles or more.

Modern pest management requires surveillance data in order to plan and evaluate control work. Field surveys are the foundation of an effective program. Data on the mosquitoes with the greatest potential to adversely affect public health and comfort, including data on mosquito density and distribution, are essential in order to plan and conduct effective control measures. In Maryland, the species occurring in the greatest abundance, or that have the greatest impact on human comfort, economic growth and public health, are Ochlerotatus sollicitans, Ochlerotatus taeniorhynchus, Aedes albopictus, Aedes vexans, Ochlerotatus canadensis, Anopheles crucians/bradleyi, Anopheles quadrimaculatus, Culex pipiens, Culex salinarius,Coquillettidia perturbans and Psorophora columbiae. Several Aedes and Ochlerotatusspp. and Coq. perturbans are implicated in the transmission of eastern equine encephalitis to humans, equines and ratites. Dog heartworm in canines is the most common mosquito-transmitted disease in Maryland, with several vector species.Culex species are important vectors of West Nile encephalitis and St. Louis encephalitis. Aedes albopictus, popularly known as the Asian tiger mosquito, is a recent introduction to the Maryland mosquito fauna. It was introduced into the United States from Asia by the international used-tire trade and has spread throughout much of the country via interstate tire shipments. This species has become a major pest in several urban areas of Maryland and is a vector of West Nile encephalitis. The newest addition to the mosquito fauna of Maryland is Aedes japonicus, first found in Frederick County in 2000. Ae. japonicus is also a beneficiary of the international tire trade and an excellent vector of disease.

Larval surveillance requires extensive logistical preparation due to the size and remoteness of many breeding areas. The largest expanse of breeding area exists in the southern Eastern Shore region of Maryland. In general, mosquito larval habitat is most widespread in the coastal plain. High-level aerial photography and satellite imagery are used for rural areas to locate potential breeding areas based on soil and plant community associations. In urban environments, Ae. albopictus and Cx. pipiens are found in obscure breeding sites in yard to yard searches.

Potential breeding areas are subject to "ground truthing" to confirm the location of the sites and measure the presence of mosquito larvae. Confirmed breeding areas are mapped on United States Coast and Geodetic topographic maps (scale 1:24,000), or larger scale city maps, and used as a reference for future surveillance. Frequent and regular inspections of breeding areas are carried out from March through September to determine larval density and species composition. 
Larval surveillance data are used to guide the course of the control program and are the most important field data available to the mosquito control manager. This information is used in the decision-making process of whether or not to initiate control efforts, what type of control to employ and in evaluating the effectiveness of the control.

Breeding areas that consistently produce large numbers of important pest or vector species and are located within flight range of a community are considered as potential sites for control. Several Aedes and Ochlerotatus spp. are capable of long distance (up to 20 miles or more) dispersal flights. Other species, such as Ae. albopictus, Oc. canadensis, and Coq. perturbans are weaker fliers (1-5 miles). Culexand Anopheles mosquitoes have a much more restricted flight range than do mostAedes and Ochlerotatus mosquitoes and breeding areas for the former species more than two miles from a community are not considered for control. All breeding sites within a community, or close to the perimeter of a community, are designated as primary control sites for control of mosquito larvae.

Meteorological conditions significantly affect breeding sites. A prolonged drought will evaporate the surface water from a wetland and temporarily halt mosquito larval production. However, a drought will also eliminate the population of mosquito predators and exacerbate mosquito breeding, particularly in areas producing salt marsh Ochlerotatus species, when surface water returns. A summer with frequent rainfall and/or flooding tides will maintain a higher water level, which usually favors larval production of Anopheles and Culex species, but may also boost the population of predators and eliminate mosquito production. Therefore, a good larval surveillance program cannot be static. It must be dynamic and flexible to accommodate fluctuations in weather and land use patterns.

Surveillance for adult mosquitoes is conducted using traps and landing rate counts. Traps are an easy, relatively low cost way to obtain an index of the number of mosquitoes in an area, but, data are subject to wide variation. Trap data provide a good historical record, if trap locations are constant from year to year. Trap collections are time-consuming to process and the information on mosquito abundance is often delayed for several days after the catch is made and of reduced value from the standpoint of operational control. For arboviral surveillance, traps utilize carbon dioxide as an attractant to increase trap capture rates..

In Maryland, a trap designed by the Communicable Disease Center (CDC) is the trap type generally used to monitor pest and vector adult mosquitoes. The CDC trap is battery-powered and easily portable. For pest surveillance, the trap uses no source of attraction other than a small light. A collection of more than 10 anthropophagous (human biting) female mosquitoes per night of trap operation is considered to be the level which causes discomfort and/or complaints from the majority of people. The light trap action threshold for ground spraying of adult mosquitoes is 10 per trap night. The action threshold to suppress pest populations of adult mosquitoes by aerial spraying (application of insecticide by an aircraft) is a light trap collection of 100 female mosquitoes.

Landing rate counts provide immediate indicators of adult mosquito activity. The counts are taken for a short period of time at specific, predetermined locations. Inspectors serve as "bait" to attract mosquitoes which attempt to blood feed. A count is made of the number of mosquitoes landing on the readily visible portions of the inspectors' bodies, below the waist, during a two minute interval. Landing rate counts for salt marsh Ochlerotatus species and Ae. albopictus can be taken during daylight because these species will actively attempt to blood feed at that time. However, most other species, particularly Anopheles spp. and Culex spp., must be sampled during twilight periods, or at night, because of their nocturnal activity. The action threshold for landing rate counts to justify ground spraying for the control of adult mosquitoes is 3 mosquitoes in 2 minutes. The action threshold for aerial spraying is 12 mosquitoes per minute.

The primary goal of the Mosquito Control program is to prevent the occurrence of mosquito-borne disease in humans and domestic animals. When evidence of mosquito-borne disease is detected, measures are taken to reduce vector mosquito populations to as low a level as practical. After the surveillance and demographic data are analyzed and a decision is made that control efforts are justified, several options are available. These range from complex to simple, inexpensive to costly, and short-term to long-term. What option to utilize is dictated by the extent of the mosquito problem and the available resources. 

Source Reduction Using Open Marsh Water Management (OMWM) - OMWM is a water management technique directed toward the control of salt marsh Ochlerotatus larval mosquitoes without using pesticides. OMWM is an example of applied ecology. It accomplishes control by incorporating physical control (digging ditches and ponds in the marsh) and biological control (fish live in the ditches and ponds and eat mosquito larvae). It is a long lasting form of control and a system that, when properly designed, has a life expectancy of 20 years or longer. Due to this longevity, OMWM is the most economical form of control, despite the initial high investment. OMWM not only provides excellent control of mosquitoes at their source, it utilizes wildlife management techniques to enhance the high salt marsh habitat for a variety of game and non-game species of fish and wildlife. The ponds constructed for OMWM projects provide habitat for submerged aquatic grasses, and the ditches enhance the tidal marsh food web while reducing nutrient flow into the Chesapeake Bay.

Modern Maryland OMWM techniques are designed to achieve long-lasting, effective control of mosquito populations while maintaining and often enhancing the ecology of the tidal marsh environment. These management techniques are considered by many environmental scientists to be acceptable alterations to the tidal marsh habitat since they promote the objectives of mosquito control agencies, pesticide reduction advocacy groups, and environmental protection groups. Other environmental scientists argue that no physical change of the tidal marsh environment is acceptable. For example, there is concern that OMWM changes may adversely impact the black rail, a species listed as in need of conservation by the Natural Heritage Program of the Maryland Department of Natural Resources. Since 1991, OMWM has been suspended in Maryland because regulatory agencies have virtually stopped the issue of necessary permits. Most source reduction projects since 1991 have been maintenance of previously managed areas.

Biological Control - Larvivorous fish, invertebrate predators, parasites and diseases to control mosquitoes have been widely used throughout the world. Almost always, biological control agents are used against mosquito eggs, larvae and pupae. Biological control of adult mosquitoes using birds, bats, dragonflies and frogs has been advocated, but supportive data are anecdotal. There is no documented study to show that bats, purple martins, or other predators consume enough adult mosquitoes to be effective control agents. The Mosquito Control Section does not advocate the establishment of bat boxes by the public due to the increased risk of human exposure to rabies.

The use of fish is particularly effective in controlling the aquatic stages of the mosquito. The Mosquito Control Program maintains several rearing ponds throughout Maryland to propagate Gambusia holbrooki (mosquito fish) for distribution to appropriate mosquito breeding sites. The widespread use of larvivorous fish such as Gambusia, a native fish to the Chesapeake Bay and its tributaries, has been discouraged by the Maryland Department of Natural Resources due to concern the fish may prey upon threatened or endangered species of amphibians and fish. As a consequence of this concern, the biological control initiative using fish has been reduced to stocking them only in artificial wetlands, such as stormwater retention ponds. 
Temporary Control- Control of mosquitoes with the use of insecticides is commonly referred to as temporary control because the non-residual insecticides used in the Maryland program provide only short-term reduction of mosquito numbers. The temporary control program is divided into two categories - larviciding and adulticiding. Insecticide applications are made under the supervision of certified pesticide applicators, pest control category VIII, and regulated by State and federal laws. Several certified applicators are employed by the Mosquito Control Section. No restricted use pesticides are applied in Maryland for mosquito control by State or local government agencies.

Larviciding - Insecticide application directed against larval mosquitoes is an important component of an IPM mosquito control program. Larviciding is the most efficient type of temporary control. An important part of the mission of the Maryland Mosquito Control Program is to prevent, or significantly reduce, adult mosquito annoyance to humans, pets and domestic livestock. It is more efficient to eradicate or substantially diminish a brood of mosquitoes while they are concentrated as larvae in the aquatic habitat than to control them as adults. For example, the adult mosquitoes produced on one acre of breeding area can disperse over 50,000 acres, assuming a flight range of five miles.

All larvicide applications are based on a demonstrated presence of mosquito larvae. Larval inspections are conducted by trained personnel capable of identifying instar stages of mosquitoes and distinguishing among various genera. Inspections for Aedes and Ochlerotatus larvae must be conducted quickly after heavy rains or flooding tides because, during the summer, larvae can develop at a rate of one instar per day. Therefore, breeding sites must be located and treated within five days after flooding. In most instances, when widescale flooding has occurred, only a small portion of the breeding area can be inspected and a determination made on the need for treatment.

Larvicides are applied using manually carried or vehicle-mounted spreading equipment or from specially equipped aircraft. Ground equipment application is economical and has the advantage of being able to specifically apply insecticides to larval breeding sites only, as opposed to aerial application where an entire area is treated and much insecticide falls on dry ground. However, aircraft are needed when large areas must be treated within the short time available for treatment. Aircraft are able to apply insecticide evenly over large areas that would be difficult or impossible to traverse on the ground.

All larvicide applications are made under permits issued by the Maryland Department of the Environment. Permit applications are made on a county basis for specific areas within the county and for individual insecticides. The permit allows a limited number of insecticide applications within a specific time frame. The permit review process is time consuming and often prevents the timely application of mosquito larvicides to new or previously undocumented breeding sites.

The insecticides currently used for larviciding in Maryland include Bacillus thuringensis var. israelensis (B.t.i.), a naturally produced bacterial toxin, and methoprene (Altosid), a synthetically produced insect growth regulator. B.t.i. is one of the least toxic materials available for larviciding and, when applied from the ground, it is usually effective. It is the most commonly used larvicide for ground application. B.t.i. must be ingested by the larvae in sufficient concentration to cause death by disruption of the function of the larval midgut. Due to the poor operational results found in our quality control evaluations, B.t.i. is seldom applied by aircraft in Maryland. B.t.i. produces varying levels of control depending on water quality, amount of and type of vegetation and species to be controlled. Under ideal conditions, B.t.i. will control larvae for up to 24 hours.

Bacillus sphaericus is a relatively new bacteria larvicidal product that is very effective against Culex mosquitoes. It is used in Maryland primarily to control Culex salinarius and Culex pipiens. Bacillus sphaericus can be effective against Culexmosquitoes for up to 21 days.

Two other bacterial products have recently been registered as mosquito larvicides. Both provide control over a wider range of mosquito genera and habitat types than B.t.i.. A product that combines B.t.i. and B. shaericus to produce an additive toxic effect controls multiple mosquito genera where they occur in the same breeding habitat. Sacchropolyspora spinose, the most recently registered bacterial larvicide product, appears to provide consistent control over most mosquito genera and is available in extended release formulations that control larvae over long periods. Both products are more expensive than B.t.i. (approximately 5 times) and have limited, special use in Maryland.

Methoprene is the most commonly used larvicide for aerial application and is also commonly applied by ground equipment. This product provides 90-100% control of emergence of adult floodwater mosquitoes. There has been no observed negative environmental impact as a result of the use of methoprene for mosquito control in Maryland to our knowledge. 
Larviciding is not allowed on certain State parks, Federal refuges or Assateague Island National Seashore. This is an important factor impacting mosquito control near these State and federal lands, which serve as a source of adult mosquitoes to the nearby residential areas. 

Criteria for Application of Larvicides
Populations of mosquito larvae are sampled using a standard dipper which is immersed and quickly withdrawn from water. An assessment of the number of larvae, instar stages and number of pupae, is made. Field identification of mosquito larvae is made to the genus level. Wetlands are sampled to determine the presence, spatial distribution and density of a larval population. A decision to use a larvicide to control a larval population takes into consideration the type of mosquito and the distance to a residential area.

The physical nature of some larval habitats makes standard larval dipping difficult; samples may not be representative of the actual larval populations. In sites such as, but not limited to, dredged spoils containments, wetlands with heavy phragmites encroachment, cattail ponds, stormwater management ponds and containers, the use of larvicides is warranted based on the observed presence of larvae, historical treatment records and/or adult mosquito surveillance data. Wetlands and containers that lie in or near residential sites will be treated on the demonstrated presence of larvae.

A larvicide may be applied to a wetland for the control of mosquito larvae when larvae are present at a minimum average of one larva per dip. Breeding habitats with a minimum larval density of one per dip and that lie within a two mile radius of an area to be protected may be treated. The breeding sites of Ae. vexans and Coq. perturbans will be treated if they lie within a five mile radius of an area to be protected. For long distance flying salt marsh species, Oc. sollicitans and Oc. taeniorhynchus, breeding sites may be treated without regard to distances to protection areas.

Adulticiding - Despite all efforts to prevent adult mosquito populations from reaching annoyance levels, it is inevitable that outbreaks will occur. When this happens, it is the mosquito control manager's responsibility to reduce mosquito numbers to a point below the action threshold to protect public health and comfort. Adulticiding is most effective when the adult mosquito population is localized or when spraying is carried out uniformly over a large area to prevent reinfestation of treated areas. Multiple spraying, spaced 2 to 3 days apart, may be necessary to reduce the population of adult mosquitoes to a low level, particularly if mosquito-borne disease transmission is possible.

Localized adult mosquito populations which exceed the action threshold can be managed with an application of insecticide dispersed from truck-mounted, ultra low volume (ULV), aerosol generators. The principle insecticide applied for adult mosquito control in Maryland is permethrin, synergized with piperonyl butoxide (PBO). ULV units disperse the synergized pyrethroid insecticides (0.003 lb. active ingredient per acre) over an effective swath width of 300 feet. Applications ideally are made when mosquito activity is high, wind velocity is 2 to 10 mph, air temperature is between 60 to 85 degrees F, relative humidity is high and a temperature inversion exists.

Aerial spraying for adult mosquito control can be conducted when a large number of mosquitoes, exceeding the aerial spray action threshold, infest a community or populated area of 500 acres, or more. Spraying is conducted with a Department-owned Beechcraft King-Air, twin-engine, fixed-wing aircraft equipped with an Ag-Nav global positioning system. The principal insecticide used for aerial adulticiding is naled (Trumpet EC) applied at the rate of 0.8 to 1.2 fluid ounce per acre (0.08 to 0.10 lb. a.i./acre). Aerial spraying is conducted when weather conditions favor high mosquito activity and maximum retention of the spray particles within the treatment area. Night vision technology has enabled the aircraft to be operated at night since 2005, resulting in a higher degree of mosquito control. Due to the high mosquito population needed for justification (12 mosquitoes per minute landing rate count and/or 100 mosquitoes per light trap), most aerial spraying is conducted in the southern Eastern Shore region of Maryland.

Public Education - Mosquito control staff members meet with the press, attend community meetings and communicate one to one with residents to emphasize what steps can be taken by individuals to reduce mosquito problems. Cleanup of old tires, buckets, cans and any other water holding containers can significantly reduce mosquito breeding sites in a community, particularly in parts of the State with few natural wetlands. Window screening will keep mosquitoes outdoors. Pet owners are urged to have their dogs protected against dog heartworm disease, which is transmitted by mosquitoes. Owners of horses and other equine are urged to have their animals vaccinated twice a year to prevent occurrence of eastern equine encephalitis. 
An initiative in 2000 and 2001 conducted large scale mailing of information about Ae. albopictus and its control. This information was distributed in communities known to be infested with Ae. albopictus and included parts of Anne Arundel, Baltimore, Calvert and Prince George's counties and Baltimore City. Residents were urged to conduct neighborhood cleanups to remove containers used as mosquito breeding sites. The evaluation of this effort has not been able to demonstrate any appreciable effect on reducing the mosquito breeding containers in residential areas.

Traditionally, homes and commercial developments were sited away from wetlands and located in breezy, open areas. However, during the past four decades, there has been a shift in development areas. Homes frequently are built adjacent to, or in, wetland areas. This occurs because of several possible factors, including: (1) lack of, or greater expense for, upland sites; (2) a desire to live on or near waterfront property; (3) diminished public concern about mosquito bites or mosquito-transmitted disease; and (4) reliance on government, private contractors or self to alleviate problems associated with wetlands. In addition to mosquitoes, residences near wetlands frequently are impacted negatively by the presence of deer flies, horse flies, stable flies, biting midges ("no-see-ums"), black flies and ticks.

The Mosquito Control Section receives numerous service requests from residents of communities in or near wetlands who were unaware that their community was prone to infestation by biting arthropods prior to moving there. Planning and zoning agencies could restrict development in areas known to be potential sites of arthropod-borne disease and nuisance. At the very least, it should be an ethical obligation of realtors to disclose to buyers that living in or near some types of wetlands will subject residents to greater than normal exposure to insect and tick bites. The Mosquito Control Section is available to advise planning and zoning agencies and realtors on known mosquito producing wetlands. This public education would reduce insecticide use by government agencies, contractors and homeowners, and have other ecological benefits. 
Products Advertised for Mosquito Control - There are numerous products being advertised today alleging that they are effective for mosquito control. Unfortunately, these products have limited or no value in reducing mosquito annoyance. 
A mechanical trap (the Mosquito Magnet(R)) is promoted as being capable of "controlling" adult mosquitoes in an area of up to 1 acre. This trap was evaluated by MDA in 2001 and, while it was found to be a good tool for collecting mosquito specimens for surveillance purposes, the claim of control could not be corroborated. 
Electronic devices that emit a high frequency sound are advertised as being effective to repel mosquitoes, as well as other pests. These claims are false. The devices do not deter female mosquitoes from attempting to bite, nor do they cause mosquitoes to flee from the sound.

Electrocuting devices, popularly known as "bug zappers" do not control mosquitoes. Studies have shown that mosquitoes make up less than one percent of the insects killed by the zappers. Beneficial insects such as beetles and moths make up the bulk of the catch.

Plants such as the Citrosa plant are claimed to have a mosquito-repelling quality. The Citrosa plant is a genetically engineered houseplant that incorporates tissue cultures of the grass that produces citronella oil into hybrid varieties of geranium to produce a cultivar that emits a citronella-like odor. Citronella oil does repel mosquitoes, and it is a logical assumption that the Citrosa plant's aroma would produce similar results. However, the citronella-like aroma of the Citrosa plant does not repel mosquitoes. Mosquitoes have been observed resting on the Citrosa leaves. Crushing the Citrosa leaf and rubbing it on the skin does not repel mosquitoes. The idea of the Citrosa plant was sound, but the results do not prove the hypothesis that it is effective in repelling mosquitoes.