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Type: Western Tree Hole Mosquito
entific Name: Ochlerotatus sierrensis

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

The female Ochlerotatus sierrensis is commonly referred to as the “tree hole” mosquito because the immature stages occur most often in water collected in the rot holes in trees.

The adults are easily recognized by their small size, dark (almost black) color, pointed abdomen and brilliant white leg bands.

This species is common throughout California, Oregon, Washington and parts of Idaho, Nevada, Montana and Utah. It occurs in locations ranging from near sea level to over 9,000 feet in elevation.

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Mosquitoes have four distinct life stages: egg, larval, pupal, and adult as seen in the illustration at the bottom. The larval and pupal stages are dependent on water for their survival and development.

The female Oc. sierrensis deposits its eggs singly (up to 150) on the damp sides of the tree hole in the late spring and summer. The eggs remain unhatched until soon after the tree hole retains enough water to flood the eggs. Some eggs hatch into larvae (wigglers), which feed on small organic particles and microorganisms suspended in the water. Feeding takes place 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. At the end of the larval stage, the mosquito molts and becomes a pupa (tumbler). 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. During this time (usually 3 days) feeding does not occur. After this transformation has been completed, the adult splits the pupal skin and emerges.

Oc. sierrensis is usually single brooded, but two broods may occur under unusual climatic conditions (i.e. El Nino conditions). The adults usually emerge in March or April, but can appear as early as February and usually persist into June. It is possible to find them as late as September. Water temperatures determine all mosquito developmental times.

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Female Oc. sierrensis are vicious biters of man. They prefer to feed outdoors during the daytime, usually in shady areas. Biting also occurs during the early evening hours. In most areas, biting females stay relatively close to the tree hole that they emerged from. Males do not bite, instead they feed on nectar and plant juices. Females may also feed on plant juices, but must usually have a blood meal in order to develop their eggs.

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Tree hole mosquitoes are frequently pests in residential and recreational areas, especially where large numbers of trees are present. Although tree hole mosquitoes can be a severe nuisance, they are not known to transmit any disease to man. This species is the main vector of dog heartworm (Dirofilaria immitis).

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The most important method of controlling this species is to modify the tree hole so that it no longer holds water. Tree holes can be filled with an absorbent material and crotches may be drained by cutting wood away from the side. If valuable trees are involved, consultation with a qualified tree surgeon may be advisable.

There are no known effective biological control agents at this time.

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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