Marin/Sonoma Mosquito & Vector Control District
800-231-3236 (toll free)
707-285-2200 (office)
595 Helman Lane, Cotati, CA 94931-9736

PHONE
800-231-3236 (toll free)
707-285-2200 (office)
707-285-2210 (fax)

ADDRESS
595 Helman Lane
Cotati, California
94931-9736

HOURS
Monday through Friday
7:00AM to 3:30PM

3. THE TREE HOLE MOSQUITO (OCHLEROTATUS SIERRENSIS)

The treehole mosquito is widely distributed in western North America from Mexico to British Colombia, and throughout California. Its legs and abdomen have narrow bands of white scales, while the probosis and wings are mostly dark.

In the late spring, Ochlerotatus sierrensis lays its batches of 200 to 300 eggs in holes in the trunks and branches of more than 20 different kinds of trees. Coast live oak, California bay-laurel and black walnut are the trees most often used in the Bay area. The eggs hatch when fall storms fill the holes with water. Development proceeds slowly and the adults emerge between late February and June. Usually there is only one brood per year.

Ochlerotatus sierrensis is an important pest mosquito. Females feed on a wide variety of mammals, and can be a vicious biter of man. In bad years, they generate a flood of complaints in parks, recreation districts and urban and rural areas near woodlands with older trees or mature fruit orchards. It is not an important vector of the more common human disease agents, but it is the main carrier of Dirofilaria immitis, the parasite that causes heartworm in dogs.

The mating behavior of Ochlerotatus sierrensis is unusual. After a long period of larval development, adult males begin to emerge some two weeks before the females. They obtain nectar or other carbohydrates and then begin to form "mating swarms" around a variety of mammals, including man. Adult females start to appear later, and when attracted to a mammal host for a blood meal they are beset by the waiting males. This is the only mosquito in the northern San Francisco Bay area with this behavior.

Sometimes residents, when they are surrounded by flying Ochlerotatus sierrensis and not paying much attention to whether they are male or female, feel they are being attacked by blood feeders. In fact, they are only being used as a decoy by anxious males. District personnel have collected Ochlerotatus sierrensis with sweep nets and found mostly males in the two weeks after first emergence. By the end of July the two sexes are present in equal numbers.

Last updated: October 2, 2004