The District conducts intensive surveillance for invasive Aedes mosquitoes. The surveillance program mainly targets two mosquito species, Aedes aegypti and Aedes albopictus, which have been spreading throughout California over the last several years.

These mosquitoes are aggressive biters and also can transmit diseases including dengue, chikungunya, yellow fever, and Zika.

Invasive Aedes mosquitoes are very good at using human-created environments to lay their eggs. They can lay eggs in very small amounts of water, including many things in and around our homes. The District surveillance program therefore consists of special traps that target mosquitoes that lay eggs in containers. 

The majority of these traps are located at cemeteries, nurseries and other areas of elevated risk of introduction or colonization, distributed throughout San Mateo County. Additional traps are concentrated in areas of past invasive Aedes mosquito detections in Menlo Park and Atherton. Traps are set year-round, and additional traps are set from May through October.

Ovicup traps, the most commonly set traps in this surveillance program, are small black cups or buckets filled about half-way with water. A sheet of seed germination paper lines the inner surface of each cup. The water and black coloration of the trap attracts container-breeding invasive Aedes mosquitoes. They deposit eggs on the seed germination paper. The papers are collected weekly, replaced, and checked in the laboratory for eggs.

When eggs are collected in an ovicup, the District can determine the species of mosquito that laid them using an in-house DNA “barcode” test. In this test, DNA is extracted from the eggs and purified using a process called DNA extraction. Once the DNA is extracted, it can be identified using quantitative Polymerase Chain Reaction (qPCR) with primers that are specifically designed to detect Aedes aegypti. The District can also confirm the identification by sending the DNA out for sequencing. This DNA sequence can be matched to an online database which allows us to determine the species of the mosquito with a high degree of certainty. These tests have greatly increased the speed in which we can identify invasive mosquito eggs, as it doesn’t require raising mosquitos to maturity, which has a high failure rate and can take several weeks. The District lab has conducted these DNA barcode tests to assist other neighboring vector control districts.

Another important aspect of the District’s invasive mosquito surveillance program is imported disease case investigation. When the District is sent a report of a travel-related mosquito disease case from the San Mateo County Health Department, we conduct additional mosquito surveillance in the neighborhood. This surveillance includes both ovicups and BG-Sentinel traps. A BG-Sentinel trap is an adult mosquito trap that includes a scent lure attractive to mosquitoes that prefer human blood.  These traps target invasive mosquito species that are capable of transmitting imported diseases.  Any invasive mosquitoes collected in a BG sentinel trap can be tested for the presence of imported pathogens.

To date, the District has not detected any invasive vector mosquitoes in the vicinity of travel-related disease cases.

 

 

Page last reviewed: July 27, 2026