David L. Keith
IANR/University of Nebraska
Wayne L. Kramer
Nebraska Dept. of Health & Human Services
THE CURRENT SITUATION
While it is dry at this time, whenever significant heavy rainfall arrives, greater mosquito activity is 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).
INTRODUCTION TO MOSQUITOES
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
("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.
SURVEYS FOR MOSQUITOES
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.
|Adult Resting Position
A key element in mosquito control is drainage of standing water. On occasion this conflicts with wildlife management objectives since many wildlife species rely on mosquitoes for food and many more depend on wetlands for habitat. It is possible to control mosquitoes with minimal impact on wetlands and wildlife.
For Towns and Municipalities
Conduct some areawide surveys to locate problem areas -- standing water -- the sources of mosquito breeding. Find out if anything can be done to accelerate drainage of temporary ponds if this does not conflict with wildlife management issues. Appoint a team of interested individuals and city employees to help stay on top of developing mosquito problems. Keep them in contact with local officials, including the Extension Educator who can provide advice.
Bear in mind that perhaps no issue is as politically sensitive as a decision by the City Council to fog or spray a community for mosquitoes. Such sensitivities must be explored before a decision is reached. Questions that must be answered are the following: How serious is the mosquito situation? Where are the mosquitoes coming from? Where are the worst areas? Is it a genuine threat to public health or is it merely a nuisance? Should a public hearing be held to discuss the need for mosquito control? What is the level of feeling for or against a control effort? Are funds available for this purpose? If control procedures are undertaken, a monitoring program should be established to estimate mosquito populations both before and after control procedures are used and to follow up and
monitor any post treatment effects.
- Foggers produce an insecticide smoke by vaporizing an insecticide/oil mixture dripped onto a hot metal plate. Particles are extremely fine and do not readily stick to surfaces. They can burn vegetation, however. Fogs are used only when the air is still in the evening hours. Since there is no appreciable residue, they must be repeated often as new mosquitoes emerge and move into town. Malathion (Fyfanon ULV) and chlorpyrifos (Mosquitomist 1.5 ULV) are registered for use as thermal fogs for mosquitoes. Only a few communities are still using foggers, having switched in most cases to Truck-Mounted ULV sprayers.
Truck-Mounted ULV Spraying
- Such sprayers apply specially formulated insecticide formulations as very fine droplets, using no water to deliver the material. This seems to be the most efficient method of ground application for most small communities. Formulations which may be used include: malathion (Cythion ULV), chlorpyrifos (Mosquitomist 1.5 ULV), permethrin (Biomist 3 + 15 OR Aqua-resilin/water-based), sumithrin (Anvil 2 + 2 OR 10 + 10) and resmethrin (Scourge 4 + 12 OR 18 +54).
Conventional Aerial Spraying
- Conventional aerial sprays deliver an insecticide in a small volume of water. They provide quick knockdown and can cover relatively large areas in a short period. Aerial spraying is often the preferred approach when controlling large outbreaks of mosquitoes. Given an extended mosquito emergence period, multiple sprays might be needed. Aerial sprays can be applied over municipalities (if provided for on the insecticide label), outside or around town, even over some breeding sites. Control option is: naled (Dibrom).
ULV Aerial Spraying
- With ULV aerial application, pure undiluted formulation is applied directly as extremely small droplets at very low total volumes, usually less than 0.5 gallon per acre. Options for ULV application include: permethrin (Biomist 3 + 15 ULV), resmethrin (Scourge 18 + 54 OR Scourge 4 + 12), chlorpyrifos (Mosquitomist 1.5 ULV) and malathion (Fyfanon ULV).
Treatment of Stagnant Water to Kill Larvae
- Temephos (Abate) is a conventional insecticide that is available for larval mosquito control. Some refined petroleum distillates -- Arosurf MSF, GB-1356 and BVA-2 larviciding oil are also registered. Do not use diesel fuel, kerosene or motor oil. In areas where honeybees, fish and wildlife are of great concern, consider the following products for larval control: methoprene (Altosid), Bactimos (BTI) and Vectobac (BTI).
The Bacillus thuringiensis israelensis (BTI) products
- Vectobac and Bactimos are composed of BTI spores and/or toxin, and are completely safe to birds, fish, mammals and other organisms. They are effective only on mosquito larvae and those of certain other aquatic flies. The use of methoprene (Altosid), an insect growth regulator, for mosquito larval control, is not permitted where fish are present. Bactimos and Vectobac are available either as briquets or as granules which are easy to apply. Once wetted, the active chemicals are gradually released from the formulated product. BTI formulations may be effective for as long as 30 days. A new formulation of Bacillus sphaericus
(Vectolex CG) can be used in water with a high level of organic matter such as stagnant sewage lagoons. Methoprene (Altosid) may control mosquitoes for 30 to 150 days (depending on which of the two available formulations of Altosid is used). Altosid and Vectobac are also available as liquid formulations.
Check product labels for specific instructions, application sites, restrictions and warnings. Choose product based on cost, effectiveness, environmental safety and label restrictions. Be aware that some products may damage wildlife or spot automobile paint. Applications to control adult mosquitoes should not include open water, but should include temporary shallow water with dense vegetation, trees and shrubbery in which mosquitoes rest during the day.
For the Homeowner
Eliminate all mosquito breeding areas on your property. Examine leaf-clogged gutters, rain pools, bird baths, sewage lagoons, old tires, cans, bottles, children's wading pools and construction debris. Look for anything that might catch and hold rain. Drain water from these containers. Rinse the bird bath out weekly. Walk around your neighborhood. Try to spot possible breeding places near your home. Point out possible problems to your neighbors and suggest corrective action.
- Wear long-sleeved shirts and full length trousers. Two layers of clothing are more difficult to penetrate by biting mosquitoes. Wearing light colored clothes will reduce your attractiveness. If you do a lot of garden work, consider buying a mosquito net at a sporting goods store -- wear it over your hat or cap. Use DEET (diethyl toluamide) repellents, applying them to clothing. Skin applications may cause problems for sensitive people, children and the elderly. Work outdoors when it is cooler, or there is brisk air movement or strong sunlight. Mosquitoes are most active in early evening hours.
Keeping the Home Mosquito-Free
- Check all doors, windows and window screens, making sure these are in good repair and are tight. Replacement screens should be 16 mesh per inch. Mosquitoes in the home may be sprayed with aerosol sprays containing pyrethrins. Reduce outdoor lighting to reduce attraction of the home to mosquitoes. Keeping porch lights off and replacing traditional white light bulbs with yellow ones will also help to reduce attraction.
- Treat flower borders, smaller trees and shrubs around the patio with either malathion, permethrin or diazinon about 3 hours before the cookout. Check the labels to verify uses on plants to avoid possible plant sensitivity. Burn citronella candles or oil in lanterns during the barbecue. Remember that the barbecue smoke itself will repel mosquitoes. If you don't want to spray, consider issuing your guests some repellent or holding your event indoors. A new light trap (Mosquito Magnet) which generates carbon dioxide as a mosquito attractant is now on the market, will help to reduce mosquitoes, but is quite expensive. Several years ago some homeowners purchased thermal foggers, which produce an insecticidal smoke. The foggers usually came packaged with a small amount of oil-based insecticide. Several have asked, since they are out of material, what they can use as a substitute. Thermal foggers, in our opinion, are not very effective in reducing adult mosquitoes and are not recommended. However, for those who have the equipment and wish to try, they may contact local chemical supply stores and ask for malathion (Fyfanon) ULV. This product is not generally available and usually cannot be purchased in small quantity, i.e. less than one gallon of formulation. Follow label instructions carefully.
Dogs and Heartworm Disease
- Heartworms, a filarial worm or nematode, are transmitted to dogs and occasionally to cats, by mosquitoes. Adult female worms, which may reach a length of 12 inches, live in the dog heart, producing microfilariae which are picked up by feeding mosquitoes. Infective stages develop within the insect and are eventually transmitted to other, healthy dogs. Once inside, they develop into adult worms which lodge in the heart. Dogs exposed to mosquitoes should first be tested by a veterinarian. Animals which have already contracted the worms must be treated under close veterinary supervision. Non-infested dogs should be put on a heartworm preventive program. All pharmaceutical remedies must be obtained from a licensed vet after examination and testing of the pet.
We have drawn information from the following references in developing this material:
- Anonymous, 1967. CDC Manual. Pictorial Keys. Arthropods, Reptiles, Birds and Mammals of Public Health Significance. US/HEW, Public Health Service. 192 pp.
- Anonymous, 1986. Modern Mosquito Control. The Problem/The Solution. Sixth Ed., American Cyanamid Company. 35 pp.
- Kramer, W. 1993. Environmental Health Circular. Bureau of Environmental Health. Nebraska Department of Health.
- Lewis, Don, 1993. Horticulture and Home Pest Newsletter. July 21, 1993. Mosquito Control Following the Floods of '93.
- Mock, D.E. and L. Brooks. Kansas Insect Newsletter. May 21, 1993.