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Type: Pasture Mosquito
entific Name: Ochlerotatus nigromaculis

General Information
Life Cycle
Habits (Adult Behavior)
Economic and Medical Importance
Control Methods

Ochlerotatus nigromaculis is commonly referred to as the “pasture mosquito” in California because of its prevalence in irrigated pastures. It is a medium sized mosquito having blackish to brown coloration. Individual mosquitoes usually have a white band near the middle of the proboscis (beak). A pale white stripe occurs lengthwise on the top of the pointed abdomen. Brilliant white bands occur around the bases of each (tarsal) segment composing the end portions of the legs. Males resemble the females but may be recognized by the bushy antennae on their heads and “claspers” on the tip of their abdomen.

This mosquito is distributed from Mexico to southern Canada throughout the Western and Central States. Although widespread in California, this species is most abundant within the great Central Valley in association with poor irrigation practices on pasture lands. It is also found in alfalfa, rice fields, row crops, irrigation seepage and associated drainage ditches with changing waterlevels. Breeding has been found from sea level to an elevation of 6,000 feet.

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Mosquitoes have four distinct life stages: egg, larval, pupal, and adult as seen in the illustration at the bottom. The female Oc. nigromaculis deposits its eggs singly (up to 150) on grass stems at or near the ground in moist places. In permanent pastures, eggs are concentrated in great numbers at the base of clumped grasses. The eggs remain unhatched until flooded by the following irrigation. If water does not flood the eggs, many may remain dormant and viable for a number of years. This species is able to produce a brood following each flooding.

The eggs hatch into larvae (wigglers) within a short period of time after coming into contact with water. Small organic particles and microorganisms suspended in the water are fed upon either at the bottom or near the water surface. Breathing takes place at the water surface and is accomplished by means of orienting upside-down with the tip of the tail (siphon) pointing up. In the hottest climates, Oc. nigromaculis require about 3 days minimum for larval development. At the end of the larval stage, the mosquito molts and becomes a pupa (tumbler). Although aquatic, the pupa can survive on damp soil. The pupa is active only if disturbed, for this is the “resting stage” when the transformation from the larval stage to winged adult takes place. This usually takes about two days, but may occur in less than one. After this transformation has been completed, the adult splits the pupal skin and emerges. It can take as little as four days for this mosquito to develop from egg to adult or much longer in cooler periods and localities. This depends upon the environmental conditions such as temperature and food quality. This species overwinters in the egg stage.

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Female Oc. nigromaculis are vicious biters, attacking human beings and other mammals such as horses, cattle, dogs, and rabbits during the day. They are most persistent near twilight.

Newly emerged adults may remain near their larval habitat for the first 24-48 hours before flying elsewhere. Flights of many miles to nearby communities are common. Females are capable of flights up to 20 miles when seeking a blood meal. Males do not bite, instead they feed on nectar and plant juices. Females may also feed on plant juices, but must obtain a blood meal in order to develop their eggs.

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Pasture mosquitoes are considered pests and can interfere with agricultural operations as well as the use of recreational areas. Large numbers of pasture mosquitoes around livestock can result in weight loss and in some areas may affect milk production. This species is not known to be a natural carrier of disease. It may be capable of serving as a vector (carrier) of Venezuelan Equine Encephalitis (VEE) if this disease of man and horses should ever become established in California.

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Important methods of controlling Oc. nigromaculis that occur in irrigated pastures and alfalfa fields include physical control methods (also known as source reduction or environmental modification) and improvements in water management practices. Physical control methods include proper grading of fields, and adequate drainage capacity to prevent irrigation water from standing long enough to allow mosquitoes to complete their development. Water management techniques include efficient water usage (quantity and length of time applied). These methods and techniques provide long-term control and serve to eliminate or greatly reduce the need for chemical application.

The most commonly used biological control agent in mosquito control is the mosquitofish (Gambusia affinis). Due to the factors of intermittent irrigation and temporary presence of water, the use of mosquito fish is not practical. Other biological control methods are being studied.

Problems with drainage or prevention may develop which can make it necessary to use chemical control. Only trained mosquito and vector control personnel should apply chemical control agents. Control agencies have knowledge of the proper compounds and application techniques to assure minimal environmental side effects.

It is important to remember that chemical control provides only temporary relief and is used by mosquito control agencies until other measures can be implemented.

Insect repellents may be useful if it is necessary to be in an area where large numbers of these adults are present. Always read and follow the directions on the label carefully when using a repellent.

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