UC Mosquito Research Program: Aquatic Pest Control: About Mosquitoes

University of California Division of Agriculture and Natural Resources
Publication 3337
By Patrick J. O'Connor-Marer and Kathy Keatley Garvey

Pesticide Application Compendium, Volume 5
Published 2001 - $30 - 168 pages - ISBN 1-879906-53-8

Habitats for Mosquitoes

Waterlettuce (Photo by Jack Kelly Clark) Hydrilla (Photo by Jack Kelly Clark) Waterhyacinth (Photo by Jack Kelly Clark)

Cattails (Photo by Kathy Keatley Garvey) Bulrush (Photo by Jack Kelly Clark) Waterlily (Photo by Kathy Keatley Garvey

Other Pests in the Aquatic Environment (Mosquito information from Chapter 4)

Besides aquatic weeds, you may also need to control animals that become pests in aquatic areas. These include some invertebrates, such as insects, crabs, and mussels, and a few vertebrates—certain fish, birds, and mammals. As pests, these invertebrate and vertebrate organisms compete with or destroy native plants and animals, interfere with water flow, weaken levees and embankments, or cause health hazards to people and livestock.

Invertebrate Pests

Invertebrates are usually considered aquatic pests if they

create health or safety hazards
impede water flow and hinder navigation
clog hydraulic systems such as pipes and water intakes
interfere with human activities
are annoying or nuisances

For example, some species of mosquitoes transmit organisms that cause malaria, encephalitic, other human diseases, and canine heartworm. Update: West Vile virus. Pest clams and crabs interfere with water transmission systems in California. Certain snails harbor parasitic larvae that burrow into human skin. And mammals such as squirrels and rats damage levees and other structures with their burrowing activities.

Insects

The insects that are of most concern in aquatic environments are mosquitoes, black flies, and midges that feed on the blood of people or other animals. Some of these are capable of transmitting diseases organisms during blood feeding. Other insects become nuisances when they occur in recreational area in large numbers or occasionally inflict painful bites on swimmers. The larval stages of some aquatic insects attach to pipes or other structures in sufficient numbers that reduce water flow.

Mosquitoes

Mosquitoes are the most serious aquatic insect pests, because some species vector human and animal disease organisms (see sidebar below). The most significant pest genera are Anopheles, Culex, and Aedes. Anopheles species are the principal malaria-carrying mosquitoes.

Mosquitoes belong to the order Diptera, which also includes flies, midges, and gnats. Adults in this order have sucking mouthparts and only one pair of wings. Mosquitoes are distinguished from other Diptera by the structure of their mouthparts, which include a slender proboscis. Adult females use this specially adapted proboscis to penetrate skin and feed on blood. Experts identify mosquito species by the scales along the wing veins and body. Scales of individual species vary in color and arrangement.

Of the more than 2,600 species of mosquitoes worldwide, approximately 50 live in California. Only adult females feed on blood, but not all females are bloodfeeders. Adult male mosquitoes generally feed on nectar and other plant juices.

Mosquitoes typically breed and lay their eggs in water clogged with such vegetation as waterlettuce, hydrilla, waterhyacinth, and cattail. They also deposit their eggs in drainage ditches, on river and stream flood plains, and in other standing water.

Mud pumps are one means used to clean out drainage ditches to destroy mosquito breeding areas. (CDFA Photo)

Mosquito-Transmitted Organisms. Mosquitoes in the United States transmit several blood-borne disease organisms. In California, mosquitoes vector the organisms that cause two important human diseases, malaria and encephalitis. (Update: West Vile virus.) In addition, mosquitoes vector the canine heartworm organism, a parasite that causes a serious and often fatal disease in dogs and cats. Controlling Mosquitoes. The most effective mosquito control method involves draining or eliminating breeding areas. When this is not possible, control efforts include spraying insecticides and using natural enemies such as small larvae-eating fish.

Mosquito abatement districts in California are responsible for controlling mosquito breeding areas. Before beginning any mosquito management efforts, contact your local mosquito abatement district for information and coordination of efforts.

Draining. The task of draining or eliminating breeding areas includes cleaning drainage ditches of debris and weeds, packing tree holes with sand or cement, emptying or overturning water-holding containers, and covering structures and vessels that could hold water and serve as mosquito-breeding areas.

Mosquitofish. (Photo by Jack Kelly Clark)

Biological Control. Mosquitofish are effective biological control agents for mosquito larvae. The term "mosquitofish" usually refers to Gambusia affinis, a species that resembles guppies. Used in mosquito control in California since 1921, the mosquitofish is an extremely important biological control agent. Mosquito abatement districts stock them in marshlands, canals, roadside ditches, creeks, ponds, garden pools, rice fields, livestock-watering ponds, and water storage reservoirs. Another biological control agent is a strain of the bacterium Bacillus thuringiensis (Bt), which occurs naturally in the soil and on plants. Different varieties of this bacterium produce a crystalline protein that is toxic to specific groups of insects. The variety israelensis is used to control mosquitoes, black flies, and fungus gnat larvae.

Diseases Transmitted by Mosquitoes

Mosquitoes belong to the insect order Diptera. They are the most serious aquatic insect pests because of their potential to vector disease organisms.

Adult female mosquitoes are blood-feeding pests of people and animals. Mosquito bites usually result in red, swollen areas that itch severely and may persist for several days. Some species of mosquitoes also vector microorganisms to people or animals. These microorganisms include those that cause malaria, yellow fever, encephalitis, dengue, canine heartworm, and filariasis.

Malaria

Malaria is a serious disease cause by protozoan parasites passed on in the saliva of an infective mosquito each time it takes a new blood meal. The parasites penetrate red blood cells, multiply, and progressively break down the cells. This induces bouts of severe fever and anemia. Left untreated, malaria can cause kidney failure, coma, and death. The occurrence of malaria in California, however, is extremely low.

Encephalitis

Several species of mosquitoes transmit encephalitis viruses to people and animals. In the majority of human cases, these viral infections manifest themselves only as general flu-like symptoms, ending with full recovery. Infection, may, however, lead to encephalitic (inflammation of the brain, which can be fatal or leave permanent neurological damage. Culex spp. mosquitoes transmit the virus that causes St. Louis encephalitic and Western equine encephalitis. Severe cases of St. Louis encephalitis can cause seizures, double vision, paralysis, and death. Aedes spp. mosquitoes transmit the viruses that cause LaCrosse and Eastern equine encephalitis.

Canine Heartworm

Canine heartworm infects dogs and related canines (foxes, coyotes, and wolves, and cats through mosquito-feeding activities. Tiny microfilarial worms, a lifestage of the filarial nematode Dirofilaria immitis, enter the animal's blood through the mosquito bite. Once inside the animal, they grow quite large, measuring up to 10 inches long, and they typically live in the animal's pulmonary artery and "right" heart. The resulting thickening and inflammation of the heart cause symptoms such as difficulty in breathing, chronic cough, and vomiting, and the disease can be fatal. Canine heartworm occurs worldwide and is transmitted by several species of mosquitoes.

West Nile Virus (Update for Web)

West Nile Virus is spread by the bite of an infected mosquito, primarily from the genus Culex. Mosquitoes become infected when they feed on infected birds, including crows and magpies. The Centers for Disease Control and Prevention has identified some 284 species that can be infected by WNV. Infected mosquitoes can then spread WNV to humans and other animals when they bite.

West Nile virus (WVN) was first identified in the West Nile district of Uganda in 1937. The first recognized epidemic took place in Egypt in 1950. In the United States, it was first reported in New York in 1999, and in California in 2002. The virus has been isolated from 43 mosquito species, predominantly from the genus Culex, according to the Encyclopedia of Arthropod-Transmitted Infections, published by CABI Publishing, N.Y. in 2001. WVN accounted for 18 deaths in California in 2005 and infected more than 900 others throughout the state. Fifty-four of the 58 counties reported WNV activity in 2005.

Entomologists believe that WNV is established as a seasonal epidemic in North America that flares up in the summer and continues into fall. People over the age of 50 are more likely to develop serious symptoms of WNV and should take special care to avoid mosquito bites. In a very small number of cases, WNV also has been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby, according to the Centers for Disease Control and Prevention.

About the Authors:
Entomologist Patrick O'Connor Marer served as the pesticide training coordinator, Pesticide Safety Education Program, UC Statewide Integrated Pest Management Program, from 1987-2004 when he retired from the university. Veteran journalist Kathy Keatley Garvey, a UC Davis writer and photographer since 1986, joined the Pesticide Safety Education Program in 1996, and the UC Mosquito Research Program in 2005. The photographs are the work of principal photographer Jack Kelly Clark, UC Agricultural and Natural Resources. Garvey also captured some of the images published in the manual.

Aquatic Pest Control: About Mosquitoes (from Chapter 4)