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Tree Hole Mosquitoes







The Western Tree Hole Mosquito, Aedes sierrensis, is a common pest mosquito in Alameda County and the most important vector (carrier) of Canine (Dog) Heartworm. The mosquito is found in areas where older trees have had time to develop rot cavities or pockets between limbs which can hold rain or irrigation water. Occasionally it is also found in containers in which organic debris and leaves have accumulated.

Adult mosquitoes of this species are characteristically very small, dark insects with brilliant white bands on their legs. The adults are long-lived (up to six months) and are found from March through June and often appear as swarming white spots. 

Tree hole mosquitoes are persistent biters of man and animals. They most commonly bite in the evening although they will readily bite all day in shady areas. Both the male and the female are attracted to potential hosts, giving the impression of many more biting mosquitoes. Only the female bites for a blood meal. The male feeds on plant juices and does not take a blood meal. Occasionally the males will form a swarm in shady areas. Tree hole mosquitoes are normally outdoor biters, but may enter homes on occasion.

The life cycle of this species is centered around tree holes and containers. Female mosquitoes lay eggs on the damp surfaces just above the water line. The eggs remain dormant until the container is refilled with water by rain or irrigation. The eggs hatch shortly after submersion, producing young larvae which progress rapidly through four instars. Larvae transform to pupae when temperatures have reached levels that are suitable for adult mosquito activity. The pupa is a transition stage between larva and adult. An adult mosquito emerges from a pupal case in just a few days. If an adult female mosquito is successful in obtaining a blood meal, she will return to a tree hole to lay eggs, beginning the cycle again.


The best method to control tree hole mosquitoes is to eliminate tree holes and containers that hold water around your home. Tree holes, especially those which hold water, are causing damage to the tree. Although very effective for controlling mosquitoes, filling, drilling, or cutting may expose new wood or hold moisture causing the rot to spread. For the health and preservation of the tree, it would be ideal to keep cavities and tree holes dry by installing deflectors to prevent water entry. Old stumps can be removed, filled, or buried. It is recommended that you consult with a tree specialist or nursery advisor for information on stopping the damage caused by rot cavities. This is especially important if the affected tree is a valuable part of your landscaping.

There is no adequate, fully effective, long term biological control for this species. There are a number of experimental techniques now being used to provide partial control.

The Alameda County Mosquito Abatement District uses a biological agent to treat larval sources to reduce the number of mosquitoes for one season. The District treats the larval source and does not treat for adult mosquitoes. It is not possible for the District to eliminate all of the the tree hole mosquitoes, because of the thousands of potential sources and the difficulty in locating and reaching them.


Carefully check your property for containers and tree holes.

Eliminate containers and locate water-holding tree holes.

Call the District to treat water-holding tree holes you are unable to eliminate.   Treatment is after the first rains.


Canine heartworm disease is a clinical condition in dogs caused by the nematode parasite Dirofilaria immitis which resides within the dog's heart. This disease is a serious veterinary problem having become widespread throughout the United States and tropics and is primarily associated with dogs. Canine heartworm can only be transmitted by the bite of mosquitoes. The Western Tree Hole Mosquito, Aedes sierrensis, is the most important vector (carrier) of canine heartworm in the San Francisco Bay Region.

The life cycle of the nematode parasite causing canine heartworm disease has three basic stages. The adult female worm, while residing in the dog's heart, releases immature worms, known as microfilariae, into the dog's blood stream. Further development of the microfilariae requires that a mosquito bite the infected dog. The ingested microfilariae then continue their development to the "infective stage". An adult mosquito may then, when biting another dog, transmit the heartworm disease. The infective larvae migrate to areas under the dog's skin where they continue their development for the next three months, reaching a length of approximately three inches. The worm then migrates to the heart to complete its growth and release more microfilariae to start the cycle all over again.

Studies have shown that the adult worms live an average of five years and the microfilariae can persist for three years. This means that an infected dog can be a source of infection to other dogs for many years. It may be advisable to have your dog on preventative medication if you feel your dog will be at risk from vector mosquitoes or will travel to high risk areas (see risk chart).

Severe cases of dog heartworm result in general weakness, coughing and labored breathing. In advanced cases treatment is difficult and some dogs may die from cardiopulmonary failure. Visible symptoms do not occur in the early stages of the disease, although your veterinarian can diagnose and treat this disease. It is recommended that you check with your veterinarian about early detection and treatment.


Dog Heartworm Risk Chart
  Outdoor Dog Indoor Dog
Heavily Wooded Area (wet)
Heavily Wooded Area (dry)
Urban Area (not wooded)