MOSQUITOES


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Mosquitoes are widely distributed and familiar to us all. Although there are roughly 40 species in Maine, slightly less than half are considered biting pests of humans. In spite of this, one of the most common of all complaints from people trying to enjoy the outdoors during the spring and summer months concerns the annoyance caused by the often enormous populations of these small, slender, long-legged flies and the bites they inflict. Both males and females obtain some nutrition from flower nectar, but it is only the females that feed on blood to acquire the extra protein boost needed to produce and lay eggs. In this process the females can also carry disease organisms and parasites from one host to another and thus may serve as vectors of diseases such as malaria, yellow fever, and various forms of viral encephalitis. In the past these diseases have not been considered a problem in Maine. Recently, however, concern has been expressed regarding the increased incidence in the Northeast of arboviruses (arthropod-borne viruses) such as Eastern Equine Encephalitis (EEE) and more recently the introduction into the eastern U.S. of another form of encephalitis known as the West Nile VirusExternal Link (WNV). As a result of increasing concerns, a monitoring protocol has been developed for Maine mosquito species which may serve as vectors. Heartworm, a disease of canine pets caused by filarial worms, is also transmitted by mosquitoes and is now established throughout most of New England.

In Maine most of the nuisance biting mosquitoes can be broadly placed in three groups on the basis of where they breed or are likely to cause the greatest problem; urban, woodland or salt marsh. A few additional species breed in more remote areas in small stagnant ponds, bogs or swampy areas such as Coquillettidia perturbans  and Culiseta melanura. Most mosquitoes that transmit encephalitis to humans are species that feed on both reservoir bird hosts and humans and so are considered 'bridge' vectors. A list of potential vector species that can be found in Maine would include, but not be limited to: Aedes vexans, A. japonicus, A. triseriatus, A. canadensis, A. cantator, A. sollicitans, Anopheles punctipennis, Coquillettidia perturbans, Culex pipiens, C. restuans, C. salinarius and Culiseta melanura (see Table1).

The introduced species, Aedes japonicus, was first collected in 2001 at several localities in southwestern Maine. This species is a primary arboviral vector and occurs in urban areas. Aedes japonicus breeds in tire dumps and discarded containers. Another introduced species, the Asian Tiger Mosquito (Aedes albopictus), does not occur in Maine and prefers warmer climates to the south.

Biology of Mosquitoes

All mosquitoes pass through four developmental stages: egg, larva, pupa and adult. Eggs are laid either in or near water or in moist depressions which will fill with water during the spring or in flood times. All larvae and pupae require water to develop to adults. Larvae may be predaceous or feed on organic debris and most need to be near the water surface to breathe. Overwintering can occur in any stage depending on the species. One generation per year is the rule for the majority of Maine species except for salt marsh species or others in wet seasons. Adult mosquitoes may persist for many weeks or even several months, but the populations of all except salt marsh species usually start to decrease during hot and dry weather conditions later in the summer. Adults do move around in short flights and may on rare occasions move long distances of up to a few miles. Most mosquitoes are active in the evening or on overcast days. During the day they remain resting on vegetation, taking flight only when a suitable host passes by. June is generally considered the month for woodland mosquitoes except in wet years when the season may be prolonged. Mosquitoes which appear briefly in the fall or spring and occasionally during the winter are females of overwintering species. The distribution and seasonality of Maine species is currently being studied.

Woodland (or Upland) Mosquitoes - The majority of our Maine mosquito species fall in this category and most overwinter as eggs or larvae with only a single brood or hatch each year. In extremely wet seasons more than one brood can occur. Some develop from shallow ponds, swamp pools and ditches. Others develop from temporary pools of water resulting from melting snow. Other breeding sites include rock pools, flood plain pools and even tree holes. Contrary to popular belief, most mosquitoes will not breed in lakes or deep and moving streams. Eggs normally begin to hatch in April or May. The tiny hatching larvae which are also known as "wrigglers" or "wiggle tails" are aquatic and undergo a few molts prior to pupation in May or June. Their rate of development increases as the temperature rises. Emergence of the adults usually occurs in late May or early June in central Maine. Females mate, seek blood, and deposit their eggs from a few days up to a few weeks after emerging depending on the species. Eggs of most species remain dormant until the following spring except in unusually wet years.

Salt-Marsh Mosquitoes

Unlike the woodland mosquitoes, the salt-marsh mosquitoes (Aedes cantator and A. sollicitans) produce many generations per year and fly much longer distances, up to ten to twenty miles or more from the coast, in their search for food. These species breed only in saline pools in or near salt marshes. Eggs occur in and about depressions and hatch upon being submerged as a result of flooding due to heavy rainfall or high tides. As a rule, the frequency of high-run tides determine the frequency of generations, one generation of mosquitoes per high-run tide and with each generation numbers of individuals increase greatly. In some seasons there may be two high-run tides each month with a new generation of mosquitoes with each one. A third more southern salt-marsh mosquito, Aedes taeniorhynchus probably occurs in southwestern Maine as well.

Urban Mosquitoes

Most mosquitoes can be found at one time or another in urban areas but there is a special group of mosquitoes whose habits tend to bring them into urban settings with greater frequency. These are the mosquitoes which breed in water which has collected in an assortment of containers from eave gutters and downspouts, buckets, tin cans and tires to bird baths and dumpsters. These species also frequently feed on birds and a variety of mammals and can transmit arboviruses to humans. This group includes three of the more common WNV vectors, Aedes japonicus, Culex pipiens and C. restuans as well as other possible vectors; C. salinarus and Aedes triseriatus.

Personal Protection

The use of protective clothing and insect repellents are both methods which can provide some personal protection against adult mosquitoes, and are especially suitable for hikers, campers, picnickers, fishermen, and others who are active in mosquito infested areas. Types of protective clothing include veils or mosquito netting worn around the head, high boots, long sleeved shirts, long pants, gloves, etc. Insect netting fashioned into a bed net can also provide excellent protection for those camping or sleeping in the open. As with many biting flies, it is best to avoid the use of colognes and perfumes while in the field as these may enhance biting fly activity!

Insect repellents are chemicals that can be applied to the skin or clothing which will repel mosquitoes and to a lesser extent black flies and ticks. A number of products are available, and come as pressurized sprays, creams, sticks or liquid formulations which are usually spread on exposed parts of the body. Usually a few drops applied to the neck, face, hands, and arms or sprayed onto thin clothing items such as stockings can repel mosquitoes for periods of 2 hours or more. Since repellents can irritate the eyes or the lips, care should be taken in their application. Be sure to read the instructions to make sure the repellent won't harm clothing or especially plastic items. Do not over use repellents especially on young children and particularly those containing DEET.   Clothing treatments with permethrin (a toxicant) products have a long lasting period of effectiveness but cannot be applied directly to the skin; once dried on clothing however, there is little or no transfer of chemical compounds.

Homeowners and campowners can alleviate the mosquito nuisance indoors by installing and maintaining tight fitting window and door screens and keeping outside lighting to a minimum. The screening in of porches and picnic areas may also be a worthwhile consideration. Specific materials for screen treatment containing methoxychlor or permethrin may add to the effectiveness of screens. Space spraying of tents or sleeping areas before use may also help.  Use of burning wicks containing pyrethrum or citronella candles may provide some relief in limited areas provided that there is little or no wind. Novelty approaches to mosquito control include such things as "bug zappers," various sound devices, and scented geraniums ("mosquito plants"). While there may be certain psychological benefits to the use of such things, they are usually expensive and there is little scientific evidence to support the claims of those who market such products. There is no sure fire solution to the problem as some would assert.

Natural Controls

The following are various controls that can be undertaken to reduce the presence of mosquitoes, either by elimination of breeding places or destruction of the adults or larvae.

Eliminate Breeding Sites

Locate prior to mosquito emergence (late April) all stagnant water in unused buckets, pools, old tires, tin cans along with other similar discarded containers, and drain or remove these to destroy mosquito breeding sites. Be sure the eave gutters and downspouts are cleaned. Also, be sure to check and refresh water in small children's wading pools and animal water dishes and tubs to eliminate larvae. Keep dumpsters and trash receptacles covered to prevent water accumulation.

Eliminate Adult Resting Sites

Cut back or remove dense brush and similar vegetation from around houses and camps. Keep grassy areas mowed short. Promote natural breezes to discourage mosquito occurrence.

Encourage Natural Predators

Predators such as dragonflies, bats, birds, frogs and mosquito eating fish provide some natural control of mosquitoes, especially in and around small farm and garden ponds and salt marsh pools. Both the nymphs and adult dragonflies are natural enemies of mosquitoes. The nymphs, which are aquatic, feed on a variety of aquatic insects, including mosquito larvae. Dragonfly adults feed on flying adult mosquitoes. Mosquito eating fish can be used to control mosquito larvae in some situations. However, introduction of fish into any body of water is regulated by the Fisheries Division of the Maine Dept. of Inland Fisheries and Wildlife (207)-287-5261 and requires a permit.

Chemical Controls*

Use of chemical control measures against mosquitoes is a complex and often sensitive issue due to the association of these pests with water. For this reason it is suggested that those wishing to pursue this route secure the services of a licensed pesticide applicator (PCO). A list of licensed PCO's or verification of a pesticide registration status can be secured from the Maine Board of Pesticides Control. The following will give you some idea of the complexity of the issue: as of February 2000, there were over 700 products registered in Maine with mosquitoes on the label: larvicides, insect repellents and adulticides. The larvicides include: Bacillus thuringiensis var. israelensis (Bti), the organophosphate temephos and insect growth regulators such as methoprene and pyriproxyfen. When using a larvicide in standing water, you may need a permit from DEP. Just over 200 of the registered products are adulticides designed to kill adult mosquitoes. The active components of these products include synergized pyrethrins, synthetic pyrethroids (such as: resmethrin, phenothrin, permethrin and sumithrin) and organophosphates (such as: malathion and naled).

*NOTE: These recommendations are not a substitute for pesticide labelling. Read the label before applying any pesticide. Pesticide recommendations are contingent on continued EPA and Maine Board of Pesticides Control [(207) 287-2731] registration and are subject to change.

Caution: For your own protection and that of the environment, apply the pesticide only in strict accordance with label directions and precautions.

NEW Technical Report  Foss, K. A. and R. G. Dearborn. Preliminary  Survey of Mosquito Species (Diptera: Culicidae) with a Focus on Larval Habitats in Androscoggin County, and Additional Larval Data for Portland, Maine. December, 2002. 51 pp. pdf version (8.71 MB)

Foss, K. A. and R. G. Dearborn. Preliminary Faunistic Survey of Mosquito Species (Diptera: Culicidae) with a Focus on Population Densities and Potential Breeding sites in Greater Portland, Maine. November 2001. 32 pp. pdf version (.5 MB)

MAINE DEPARTMENT OF AGRICULTURE, CONSERVATION AND FORESTRY
Maine Forest Service - Forest Health and Monitoring Division
April 2003