Urban Entomology - Mosquito Update

Whenever significant heavy rainfall arrives, greater mosquito activity can be anticipated. Nebraska communities along rivers and streams often experience moderate to severe flooding, particularly in May, June, July and sometimes late in the summer and fall. Flooding or heavy rainfall produce temporary quiet pools of water which can remain for weeks, available for mosquito breeding. Mosquito reproduction is rapid and a generation can be completed in as few as 5 -7 days. The weather over the next several weeks will play a major role in mosquito development. Hot, dry periods would quickly dry out water pools and strand many mosquito larvae while warm weather with continuous rainfall would encourage rapid buildup. Some species, however, are able to increase in mid-to-late summer, especially in irrigated areas of our state. These often include the Culex mosquitoes which can transmit the West Nile Virus, recently arrived in our country and now present in many Nebraska counties (including the city of Lincoln).

Mosquitoes belong to the Insect Order Diptera, or flies. There are about 140 species in North America. They are found in many diverse aquatic habitats, from salt to fresh water. Mosquito-borne diseases have changed the course of history, helping bring to an end the ancient Roman and Greek civilizations and halting the advances of the Crusades and the conquests of Alexander the Great. In the Americas, they prevented early colonization of many areas and contributed to great losses of life during the construction of the Panama Canal. In the United States, there were still 900,000 confirmed cases of malaria as of 1935. Today the relation of mosquitoes to disease transmission is well known. Both West Nile Virus and encephalitis are virus diseases which circulate in a bird-mosquito cycle that can be picked up and carried by mosquitoes to humans. Horses are also affected. Canine heartworms are carried to dogs by mosquitoes. Also, some individuals experience an allergic response to mosquito feeding. Bites and subsequent scratching may open the way for secondary infectious organisms which invade the wound. Further, mosquitoes are a tremendous nuisance, interfering with outdoor work and recreational activities. Female mosquitoes possess piercing-sucking mouthparts and are blood feeders. Male mosquitoes have "feathery-looking" antennas and mouthparts and feed on floral nectars. When the female mosquito bites, she injects salivary enzymes and anticoagulants. The saliva may also contain disease organisms picked up from an earlier feeding on an infected host. Through successive feedings, these disease-causing organisms can readily be transmitted among birds, other animals and humans.

Mosquitoes pass through four distinct stages -- egg, larva ("wiggler"), pupa ("tumbler") and adult. Eggs may be laid singly or grouped into a "raft". They may be deposited on soil, vegetation, in tree holes, or on the surface or along the edges of still water. Eggs may hatch immediately or may persist for years, waiting for the proper conditions for hatching -- still water with depleted oxygen. Low oxygen tension triggers egg hatching. Once hatched, larval development proceeds rapidly. Typically, larvae hang with their heads downward, suspended from the water surface, diving when disturbed or to feed on tiny aquatic organisms. The larvae are called "wigglers" due to their unusual "S-shaped" swimming habit. Each has an air tube which pierces the surface film. The tube is provided with a breathing hole (spiracle). Each larva grows a step as it molts its skin -- usually there are 4 size classes, one for each step, or larval instar. After the larva is mature, it transforms into the pupa or "tumbler" after the fourth molt. The larval stage lasts about 5-7 days for most species. Tumblers (pupae) do not feed, but are mobile, tumbling into deeper water if disturbed. After 2-3 days in the pupal stage, the adult mosquito emerges. A female mosquito probably lives about 2 weeks given favorable conditions. They are good fliers, and can move several miles from an emergence site if necessary. In larger cities, where river flooding is not involved, most mosquitoes emerge locally. Eggs are developed and laid in batches between blood meals. One female may deposit several hundred eggs in her lifetime.

Community leaders in recently flooded areas might consider appointing mosquito survey teams to keep track of local mosquito development. These volunteers could work with city employees responsible for mosquito control, helping them to concentrate their efforts for greatest efficiency. Identification of mosquito breeding sites is accomplished through regular examination of potential breeding locations and selection of water samples. Rapidly moving water (streams or rivers) and lakes with no vegetation and good wave action are not likely places. Stagnant, still water is essential for mosquito breeding. Around large lakes, look for areas of emergent vegetation (cattails, Sagittaria, bull rushes), accumulations of algae and duckweed or trapped pools. Near rivers, examine oxbows and overflow areas for stagnant ponds. Roadside ditches and seepage areas at bases of hills are excellent habitats for mosquitoes. Water sampling is accomplished with an old fashioned water dipper attached to a 2-3 foot extension (doweling) handle. Move quietly in and out of breeding sites, taking samples by skimming the water surface gently with the dipper until full, then examining it for larvae and pupae. Counts of 8 or 10 per dipper indicate cause for concern. Map sampled areas and keep records. Note date and time, areas of standing water, aquatic vegetation and note counts and stages found. Evidence of many male mosquitoes (normally active in the early evening hours) indicates a nearby breeding site. The presence of many large larvae and tumblers indicates an impending heavy emergence. Mosquito breeding sites in urban and residential areas may include children's wading pools, flower pot bases, clogged eaves, bird baths, pails, discarded tin cans and used tires -- anything that can trap and hold water.

The more commonly encountered mosquito genera are Anopheles (day/night feeders which breed in both permanent and flood waters), Aedes (daytime biters which breed in flood waters) and Culex (evening/dusk feeders which breed in both flood and permanent waters). The figure below from CDC/HEW, compares the features of the three common mosquito genera.

ANOPHELES

AEDES

CULEX

Egg Stage

Larval Stage

Pupal Stage

Adult Stage

Adult Resting Position

 

We have drawn information from the following references in developing this material:

1. Anonymous, 1967. CDC Manual. Pictorial Keys. Arthropods, Reptiles, Birds and Mammals of Public Health Significance. US/HEW, Public Health Service. 192 pp.
2. Anonymous, 1986. Modern Mosquito Control. The Problem/The Solution. Sixth Ed., American Cyanamid Company. 35 pp.
3. Kramer, W. 1993. Environmental Health Circular. Bureau of Environmental Health. Nebraska Department of Health.
4. Lewis, Don, 1993. Horticulture and Home Pest Newsletter. July 21, 1993. Mosquito Control Following the Floods of '93.
5. Mock, D.E. and L. Brooks. Kansas Insect Newsletter. May 21, 1993.

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