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Mosquitoes and the Diseases They Transmit

Mosquitoes are among the most important insect pests affecting the health of people and animals. Biting female mosquitoes not only irritate people and animals, but they can also transmit many disease-causing organisms.

Annoying populations of mosquitoes can occur anywhere in Texas because there are habitats favorable for mosquito species almost everywhere in the state. To control mosquitoes effectively, it helps to understand their life cycle, to be able to identify the various kinds of mosquitoes, and to know what steps work best for the different species and specific locations.

Life History

Mosquitoes have four distinct stages during their life cycle: egg, larva, pupa and adult. The adult stage is free flying; the other stages are aquatic. The length of time that a mosquito takes to complete its life cycle varies according to food availability, weather conditions and the species of mosquito. Under favorable conditions, some mosquitoes can complete their entire life cycle in only 8 to 10 days.

Egg: One way to identify the breeding sites of mosquitoes is to find the eggs. Mosquito eggs may be laid in clusters called rafts on the water surface. They may also be laid singly on the water surface or in dry areas that are flooded periodically. When first laid, mosquito eggs are white, but within a few hours they become dark brown to black. The shape and size of mosquito eggs vary, with most being football or boat shaped and 0.02 to 0.04 inch long. In warm water, the eggs may hatch in 2 to 3 days. Some mosquito eggs can remain dormant in dry conditions for many months.

Larva: Mosquito eggs hatch into long larvae called wigglers, which are seldom over 1/2 inch long. Wigglers have three body sections: a small head, an enlarged thorax (the middle section) and a long, cylinder-shaped abdomen. Wigglers live only in water. Most of them feed on microscopic plants, animals and organic debris suspended in the water. They filter the food particles from the water with their brush-like mouth parts. The larvae of some mosquito species are predators that feed on other mosquito larvae. Most mosquito larvae mature in 4 to 10 days, passing through four instars (growth stages) and then transforming into the pupal stage. The length of a mosquito’s larval development period depends on food, temperature and species.

While feeding or breathing, mosquito larvae assume distinctive positions in the water. For most species, the larva breathes through an air tube located near the end of the abdomen. It projects the air tube through the water surface and hangs head down at an angle to the water surface, with only the tip of the breathing tube coming into contact with the surface of the water.

An exception is the larvae of Anopheles mosquitoes. They lack air tubes and tend to lie flat against the water surface.

One way to control mosquitoes in the larval stage is to apply a surface film of certain petroleum products in standing water. This film disrupts their breathing and kills the larvae.

Pupa: The pupal stage is the transitional stage between the mosquito larvae, which live in the water, and the adults, which live on land. Mosquito pupae do not eat. They spend most of their time at the water surface and tend to move only when disturbed. Mosquito pupae are sometimes called tumblers because of their tumbling motion in water when they are disturbed. Mosquito pupae are comma-shaped and, like the larvae, breathe through air tubes at the surface of standing water. The front region of the pupa’s body is greatly enlarged, consisting of a fused head and thorax. A pair of respiratory tubes, or trumpets, extends from the back of the thorax and are used at the water surface to breathe air. The pupal abdomen or tail consists of several segments that move freely.

The pupal stage may last from 1 to 10 days, or even more, depending on the temperature and the mosquito species involved. Pupae may also be killed with surface film treatments.

Adult: Adult mosquitoes have wings. Male mosquitoes feed only on nectar, plant juices and other sources of liquid carbohydrates. Male mosquitoes usually emerge a few hours up to a few days before the females emerge. The males rest in the vegetation surrounding the emergence site, waiting for females to emerge.

Female mosquitoes also feed periodically on nectar, plant sap and other sources of plant carbohydrates for energy. However, the females of most species must have a blood meal as a source of protein before they can produce eggs.

Mating usually occurs near the emergence site and coupling occurs quickly in the air. Female mosquitoes can fertilize all of their eggs after a single mating because they can store the sperm internally. Male mosquitoes usually die shortly after the mating period.

Adult female mosquitoes typically live for about a week to a month, but this can vary depending on several environmental factors. Some species overwinter (spend the winter) as engorged mated females that may live up to 6 months or more.

When female mosquitoes are inactive, they rest in protected areas that are typically dark or shaded, humid, and cool in the summer or warm in the winter.

The mouthparts of female mosquitoes are complex and form a prominent beak or proboscis. When a mosquito takes a blood meal, it uses its mouthparts to puncture the skin of the host and feed directly from the capillaries (small blood vessels).

As it feeds, the mosquito injects a small amount of saliva into the wound before drawing blood. The saliva makes penetration easier and prevents the blood from clotting during feeding. In most cases, the itching and swelling caused by the saliva subside within a few hours.

The adult mosquitoes around your home may have come from a breeding site near or far away, depending on the species, wind patterns and the flight habits of the females:

• Aedes aegypti breed primarily in and around human habitations and fly short distances, usually only about 200 yards.
• Most Anopheles mosquitoes have a flight range of about 1 mile. Psorophora species have flight ranges of at least 5 miles.
• Some salt-marsh mosquitoes in the genera Aedes and Ochlerotatus can disperse with the prevailing winds for 20 to 40 miles or more away from the larval development sites.

You need to recognize these flight distances in order to find the source of mosquito problems and choose the appropriate management strategies. If the source of the mosquitoes is not on your property, you may need to cooperate with others to be able to control them. In some cases, you may be unable to control them at all.

Identifying Mosquitoes

All mosquitoes are classified in the order Diptera (true flies) and in the family Culicidae, which has more than 2,500 recognized species in the world. Adult mosquitoes are small, long-legged flies that have two wings like most other true flies.

All adult mosquitoes have three characteristics in combination, which separates them from all other flies:

• Long, many-segmented antennae
• A piercing and sucking mouthpart system elongated into a distinctive beak or proboscis, at least in the females
• Scales on the wing veins and margins

To control the mosquitoes in your area, first you need to know what kinds are living there. At least 84 species of mosquitoes are known to occur in Texas. These species have considerable variation in their larval breeding sites, time of day when they bite, and flight distances of the adults.

Table 1 provides a summary of this information for some common species in Texas. Identification of larvae or adult mosquitoes to species is complicated and generally requires considerable expertise and training. If there is a question about the species involved, it is best to send samples to an identification lab. A mosquito control district, a university, or a pest control operator should be able to help.

Table 1. Biological data on 20 common species of mosquitoes found in Texas

Groups of Mosquitoes

Mosquito species are often divided into groups based on where the females lay their eggs and where the larvae
develop. There are different control strategies for each group. The Center for Disease Control and
Prevention (CDC) has developed a convenient grouping of four mosquito types according to the habitats in
which the larvae generally develop:

• Permanent pool group
• Transient water group
• Floodwater group
• Artificial container and tree-hole group

Understanding these groups will help you determine the source of the mosquitoes and what control measures
are likely to help.

Table 2 provides a summary of these groups, the more problematic genera and/or species, larval breeding habitats and general management approaches.

Table 2. Mosquito groups, their breeding sites and management suggestions for each group.