Spring is the prime time for the emergence of Aedes sierrensis, the western treehole mosquito. Aedes sierrensis, the western treehole mosquito, frequently lays its eggs in the water-filled cavities of trees such as oak, laurel, madrone, and eucalyptus. They may also breed in other standing water, such as containers and roof gutters. Residents living in areas with dense populations of trees or adjacent to recreational areas and open space may experience large numbers treehole mosquitoes.

Western treehole mosquitoes are very small, bite aggressively during the day, and are most common between March and August in areas where many trees are present. They often feed outdoors, but may enter homes. Western treehole mosquitoes can transmitted the parasite that causes dog heartworm. The mosquito does not pose a risk to human health; we do not control adults of this species.

Importantly, another mosquito species that bites during the day, Aedes aegypti, is not native to San Mateo County.  If you are being bitten during the day, especially in a non-forested area, please report the mosquitoes to us by calling 650-344-8592. Learn more about Aedes aegypti HERE.

Like all mosquitoes, the western treehole mosquito needs standing water. Most western treehole mosquitoes will hatch from water-filled cavities in tree trunks. These cavities can be filled with sand or cement, or you may choose to consult an arborist about eliminating tree cavities on your property.

Although this mosquito species prefers tree cavities, it may reproduce in other available areas of standing water as well. The best way to prevent the western treehole mosquito, as well as other mosquitoes, is by eliminating standing water where mosquito eggs, larvae, and pupae are found, including:

• Turn over, cover, or dispose of small containers
• Screen/cover rain barrels or other water collection devices
• Inspect and repair sump pumps
• Stock ponds, fountains, water troughs, and unused pools with mosquito fish
• Change water in birdbaths and pet bowls weekly
• Keep gutters clear of leaves and debris
• Repair leaks that create standing water
• Avoid over-watering lawns and gardens

Page last reviewed: June 6, 2023