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.