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.

Treehole mosquitoes can bite aggressively during the day and evening. The good news is that they are not known to transmit any human diseases. However, they can spread dog heartworm, a serious and potentially fatal disease in dogs. You should ask your veterinarian about how to prevent dog heartworm in your dog.

People experiencing treehole mosquitoes biting at their homes sometimes ask whether an adulticiding treatment ('spray' or 'fog') is possible. The relief from such a treatment would be very temporary, since more adults are emerging from the water each day. Preventing treehole mosquitoes is complicated by the fact that some holes are quite high up in trees - out of reach for our technicians to use our usual larviciding preventative methods. However, there are some things you can do to reduce the mosquitoes and bites in these situations:

• Inspect treeholes, including small holes (mosquitoes don't need much water to lay their eggs), Contact a licensed arborist to discuss possibilities for filling the holes so water does not accumulate.
• Dump and drain any buckets, toys, tarps, or other items holding water.
• Check your gutters for clogs and standing water.
• Contact the District for a mosquito inspection on your property.
• Use an EPA-registered repellent when outdoors.
• For temporary relief while outdoors, try using one or more oscillating fans around seating areas. Treehole mosquitoes are weak fliers and have difficulty flying against the wind generated by fans.

Importantly, there is an invasive mosquito species in California, Aedes aegypti, that also bites during the day.  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. Although this mosquito is not known to occur in San Mateo County, Aedes aegypti was recently detected in Santa Clara County, and we are very interested in keeping this mosquito out of San Mateo County. Learn more about Aedes aegypti HERE.

Page last reviewed: May 1, 2024