THE MOSQUITO LIFE CYCLE
Around 90% of the adult mosquito population in Montana is made up of two species, depending upon habitat. The first, and most vicious biter, is the “floodwater” mosquito, Aedes vexans. This mosquito and other closely-related floodwater species are responsible for nearly all severe outdoor annoyance. The second, an important disease vector, is Culex tarsalis. These two species, along with all other mosquitoes, must have water for their early stages, and all undergo the same four-stage life cycle: egg, larva, pupa, and adult. The larval and pupal stages are always aquatic.
Depending on the particular species, the female mosquito lays her eggs either individually or in attached groups called rafts. The eggs are placed either directly on the surface of still water, along its edges, in treeholes, or other areas that are prone to flooding from rain, irrigation or overflow. In some species, the eggs may hatch within a few days of being laid, with the exact amount of time dependent on temperature. But if the egg is laid out of water and is subject to intermittent flooding, the embryo may lay dormant for many years until the ideal natural hatching conditions are met. Mosquitoes frequently overwinter in the egg stage, but may also overwinter as larvae or adults.
The Larval Stage
Once the egg hatches, the larval stage begins. The larvae of most mosquito species hang suspended from the water surface. An air tube extends from the larva’s posterior to the water surface and acts as a snorkel. The larvae filter-feed on aquatic microorganisms near the surface. As a defense mechanism, when alarmed, the larvae can dive deeper into the water by swimming in a characteristic “S” motion. As they feed, larvae outgrow their exterior covering and form a new larger skin, casting off the old ones. The stages between these molts are called instars. The larval stage has four instars. The length of the larval stage ranges from 4 to 14 days, varying with species, water temperature, and food availability.
The Pupal Stage
In the pupal stage, no feeding occurs. Like the larvae, the pupae are sensitive to light, shadows and other water disturbances. Pupae are also physically active and employ a tumbling action to escape to deeper water. The pupal stage lasts from 1 1/2 to 4 days, after which the pupa’s skin splits along the back allowing the newly formed adult to slowly emerge and rest on the water surface until taking flight.
The male mosquito will usually emerge first and will linger near the breeding site, waiting for the females. Mating occurs quickly after emergence due to high adult mortality rates. As much as 30% of the adult population can die per day. The females compensate for this high rate by laying large numbers of eggs to assure the continuation of the species. Male mosquitoes will live only 6 or 7 days on average, feeding on plant nectars. Females with an adequate food supply can live up to 5 months, while the average female survives for a few weeks. To nourish and develop her eggs, the female usually must take a blood meal in addition to feeding on plant nectars. Females locate hosts by the carbon dioxide and other trace chemicals exhaled, and the temperature patterns they produce. Mosquitoes are highly sensitive to several chemicals including carbon dioxide, amino acids, and octenol. The average female mosquito’s flight range is normally between a few hundred yards and 10 miles, but some species can travel up to 40 miles before taking a blood meal. After each blood meal, the female will oviposit (lay) her eggs, completing the life cycle. Several ovipositions per female are possible.