How to Manage Pests

Mosquitoes

Managing Mosquitoes on the Farm Section 12: Common Mosquitoes and Their Life Cycles Published 2005 Sections of this publication:

Why You Should Care About Mosquito Control West Nile Virus You May Be Raising the Mosquito That Is Biting You! Mosquito Prevention Three Basic Principles of Mosquito Prevention Natural Waters Associated With Farms Managing Stagnant Waters Created by Agricultural Activities Irrigated Fields for Upland Crops Mosquito-Free Irrigated Pastures Rice Dairy Operations Biological and Chemical Mosquito Control Common Mosquitoes and Their Life Cycles Acknowledgments For More Information

You must understand the life cycle of mosquitoes to control them. The life cycle of all mosquito species consists of four parts: egg, larva, pupa, and adult. Nearly all species have adult females that bite animals to supply nutrients for one or more batches of eggs. However, each species has its own peculiarities as to where eggs are laid, how long it takes for larvae to develop, and what happens to adults in winter.

This section provides brief descriptions of some major mosquitoes that occur in California. Because 53 species of mosquitoes are found in California, this section includes only some of the major disease carriers and pest species that are particularly abundant in agricultural areas. Local MVCDs can provide more detailed information about species in your area. Note that common names are sometimes different from place to place. The Latin name in italics is the official scientific name.

Culex spp. Mosquitoes

Several Culex spp. mosquitoes can carry viruses that cause encephalitis, a dangerous brain disease. Mosquitoes in this group lay eggs on top of the water, in floating rafts. Females can lay up to 300 eggs at a time, sometimes as many as 7 batches over a 2-month period, although most adult females do not survive this long. Adults have blunt abdomens and short palps, feelers that lie next to the long proboscis with which the female mosquito bites.

Western encephalitis mosquito (Culex tarsalis)

The western encephalitis mosquito is probably California’s most important species from the perspective of disease and mosquito control. It can carry viruses that cause encephalitis (sleeping sickness), which can be transmitted to humans and horses. It is very effective in transmitting West Nile virus, St. Louis encephalitis virus, and western equine encephalitis virus. This species is widespread, occurring in every county in California.

Larvae take from 8 days to several weeks to reach the adult stage, with faster development in warmer waters. However, they do not survive well below 70°F (21°C) or above 95°F (35°C). In the Central Valley of California, populations peak in June or July, but larvae may be found into early winter. Great numbers of larvae are found in rice fields, ditches, and in practically any freshwater pool, especially around weedy edges. They tolerate salinity up to 1 percent and can inhabit lightly polluted waters, but not very polluted waters. In some areas, C. tarsalis populations have become resistant to certain pesticides.

The adult encephalitis mosquito can fly for up to 10 to 15 miles (16 to 24 km), but most stay close to the area in which they were bred. They prefer feeding at dusk and night, but may bite during the day in deep shade. Females obtain blood meals from birds or mammals and can transmit diseases between these groups. They are active in spring through fall. They survive through the winter as adults in barns, culverts, caves, and similar dark, protected places.

Northern house mosquito (Culex pipiens) and Southern house mosquito (Culex quinquefasciatus)

These two closely related mosquitoes have a life cycle and biting habits similar to C. tarsalis but they favor water with organic pollution. Thus they are common in areas subject to manure and dairy runoff, fertilized areas, dirty bird baths, and improperly constructed septic equipment. They are very similar in appearance. C. pipiens is more common in the north and cooler coastal regions of California, while C. quinquefasciatus (sometimes called “quinqs”) are more common in the south. Both species are capable of transmitting encephalitis viruses. These mosquitoes are particularly good at getting into houses.

Anopheles spp. Mosquitoes

Anopheles spp. mosquitoes are capable of carrying malaria. Mosquito control and medicine have eliminated sources of malaria in the United States, but these mosquitoes can transmit malaria from persons that have been infected abroad. These mosquitoes lay hundreds of separate eggs on standing water. Unlike most other mosquito larvae, Anopheles spp. larvae lie parallel against the water’s surface, instead of hanging down vertically. They have a small breathing valve at the end of the abdomen, rather than a dark shiny tube, or siphon, like most other species. Adult females have palps that are as long as the proboscis. Females also “stand on their heads” when feeding.

Western malaria mosquito (Anopheles freeborni)

The western malaria mosquito was once feared as a carrier of malaria; now it is mostly feared for its numbers and appetite. It is widespread and can occur in practically any standing fresh water. It becomes abundant in rice fields or other wetlands in late summer, or earlier in cooler areas.

A female can lay successive batches of up to 300 eggs during the breeding season. Larval periods range from 12 to 20 days (longer in cooler weather). Adult females bite at dusk and dawn and prefer to feed on mammals. Most adults stay within a few miles of their source, but they may migrate further when seeking hibernation sites in fall. Early broods are found in pools, ditches, and wetlands. Later they move into the rice fields if these are available. By September, if no control efforts have been made, people and livestock for miles around can be assailed by swarms of these mosquitoes.

Adults hibernate in protected areas in winter. Many adults survive hibernation and some come out during warm periods in the winter. The number coming out of hibernation on the first warm days of early spring is frequently so great that people mistakenly think that a new batch has hatched from the fields.

Ochlerotatus (Aedes) spp. Mosquitoes

These mosquitoes, known as floodwater mosquitoes, lay eggs on damp soils or in dry treeholes and containers rather than on standing water. The larvae and adults are similar to Culex spp. in overall appearance, but the adults have pointed abdomens. Summer-breeding species often develop very rapidly; these species are the main reason for the rule that no wastewater should be allowed to stand for more than 4 days in warm weather. The eggs of these mosquitoes develop on dry land but do not hatch until the area is flooded. By virtue of their rapid larval development, they are sometimes out of the water before most predators have even had a chance to colonize, so natural biological control gets little chance to work. Eggs can build up in the soil because females lay successive batches of eggs, so great numbers can hatch after flooding. Worse yet, these mosquitoes bite during the day as well as at dusk and prefer to feed on mammals. Many Ochlerotatus spp. were formerly placed in the genus Aedes; older publications use Aedes, followed by the same species names given below.

Irrigated-pasture mosquito (Ochlerotatus nigromaculis)

The irrigated-pasture mosquito swarms in hordes to attack livestock and ranchers. The eggs are laid on damp soil, and a new brood hatches with each irrigation. In hot weather, adults can emerge within 4 days after the eggs are wetted. This species is widely distributed in California but is predominant in the San Joaquin Valley. With such a short life cycle and a brood emergence with each irrigation, there may be 10 or more broods during the summer. It develops slowly in the spring, faster in summer, and tapers off in the fall, and it passes the winter in the egg stage in the soil of pastures. The irrigated-pasture mosquito is a vicious day-biter and in general does not migrate far. It is not considered to be an important carrier of disease.

Wetlands mosquito (Ochlerotatus melanimon)

The wetlands mosquito has a similar life cycle and habitat to O. nigromaculis, but in addition to irrigated fields it breeds in deeper waters such as seasonal wetlands and rice fields and is more cold tolerant. Laboratory tests show that it can carry West Nile virus.

Field mosquito, or pale marsh mosquito (Ochlerotatus dorsalis)

The field mosquito is a big problem for growers and communities near coastal marshes. Emerging adults often fly 10 miles (16 km) or more inland to feed. It can become quite abundant in brackish areas and salt marshes, including those in eastern parts of the state. Its habits and life cycle are similar to those of the irrigated-pasture mosquito, but it develops somewhat more slowly, taking over 1 week to grow from egg to adult even in warm weather. Larvae can be found from January through October. Eggs do not develop in cold weather. The eggs are the overwintering stage; adults die off in early winter.

California salt marsh mosquito (Ochlerotatus squaminger)

This mosquito also breeds in farms with coastal marshes. It is a winter breeder, and larvae develop slowly in the cold water. It has only 1 generation per year. Biting females can appear in enormous numbers in springtime.

Western treehole mosquito (Ochlerotatus sierrensis)

The western treehole mosquito breeds in treeholes and other containers, especially in the foothills of the Coastal Range and Sierra Nevada. Females lay batches of 200 to 300 eggs. The eggs hatch when rain fills the containers with water. Development is slow and adults emerge between late February and June. Usually there is only 1 brood per year. This day-biting mosquito is annoying to humans but can be fatal to dogs because it carries dog heartworm.

Managing Mosquitoes on the Farm, UC ANR Publication 8158 Sharon P. Lawler and Gregory C. Lanzaro, Department of Entomology, University of California, Davis

Copyright © 2005 The Regents of the University of California, Division of Agriculture and Natural Resources. All rights reserved.