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West Nile Virus Season in the Bay Area
Summer is West Nile virus season in California, and the Bay Area is no exception. In June, samples of adult mosquitoes infected with West Nile virus were collected in Alameda and Santa Clara Counties. To date, no infected mosquitoes have been found in San Mateo County, but the District has been watching carefully for any sign of the virus.

In the meantime, residents and visitors should continue to observe the '3 Ds' to reduce their risk of West Nile virus infection:

DEET – Apply insect repellent containing DEET, picaradin, oil of lemon eucalyptus, or IR3535 according to label instructions.

DAWN & DUSK – Wear long pants and sleeves and repellent if outside at dusk and dawn when the mosquitoes that can transmit West Nile virus are most active. Keep windows and doors screened or tightly closed.

DUMP & DRAIN – Eliminate all sources of standing water on your property, by emptying flower pots, old car tires, buckets, and other containers. 

Read more about West Nile virus.
When Moles, Voles, and Gophers Dig In
Moles, voles, and gophers are exclusively outdoor pests; they don't infest homes. This doesn't stop them from creating a major nuisance as they burrow through lawns and gardens and consume your favorite plants.

Identifying your pest is always the first step of integrated pest management. Moles rarely come to the surface, but leave long ridges along the ground as they tunnel just below the surface. Gophers often emerge partially from their burrows to feed on vegetation, but never venture far. Their burrows are 6-12" below ground, and they often leave horseshoe-shaped dirt mounds at the surface. Voles create surface runways through vegetation and may been seen scurrying about above ground.

Excluding moles, voles, and gophers from lawns and gardens can be difficult and labor-intensive. If you decide to protect some areas of your garden, you will need to create a barrier at least two feet deep made of wire mesh with openings less than 1/4". Wire mesh can also be used to protect individual trees or plants, or to exclude burrowing pests from raised beds.

Read more about moles, voles, and gophers from the University of California Integrated Pest Management Program.

West Nile Virus Update

As of June 29, 2018, there have been 176 dead birds reported in San Mateo County. Of those, 52 have been suitable for testing and two have tested positive (4%) for West Nile virus (WNV). No mosquito pools or sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018. Throughout California, there have been nine human cases of WNV, as well as detection of WNV in birds and mosquitoes in 15 counties. Read more.

Mosquito Population Update

Populations of most mosquito species increased in abundance during June. However, the number of collected Culex pipiens, the most abundant summer mosquito, remains below average. This is likely a result of pro-active seasonal catch basin control, utilizing mapping software in the field for more effective treatments. There were no invasive mosquito detections in San Mateo County during June. Read more.

Our Work by the Numbers

In June, technicians responded to 359 service requests, including 101 reports of mosquitoes, 155 yellowjacket and wasp requests, 44 rodent inspections, and 15 insect identifications. Total service requests are higher this June than the three-year average, although the number of mosquito service requests is somewhat lower than average. Most of the increase is due to the spike in yellowjacket and wasp service requests, which are a seasonal problem. Read more.
Board Meeting Materials
Annual Reports
Calendar of Events
Finance Reports
Request Service
Yellowjackets and Wasps
During the summer months, the District receives numerous calls regarding yellowjackets and paper wasps. These social wasps both have yellow and black markings and are common in San Mateo County, but they have important behavioral differences. The term “yellowjackets” actually includes a number of wasp species of the genera Vespula and Dolichovespula. They can nest underground or in empty areas such as tree holes or openings in a wall. Their nests are made of wood, mixed with saliva. Unlike bees, they can sting more than once and will also bite. In contrast to yellowjackets, paper wasps are in the genus Polistes. They often construct nests under the eaves of houses or in trees. These wasps are much less aggressive than yellowjackets. Read more about yellowjackets and wasps.
Laboratory Updates
It’s summer and that means peak season for the dog tick and Pacific coast tick (collectively called Dermacentor ticks)! These large, brown-colored ticks are very prevalent this time of year along the edge of trails and in brushy areas with tall grass all throughout California. Unfed, they can be nearly three times as large as the black-legged ticks that are prevalent during the winter and spring and can engorge to the size of a grape if left attached to repletion.  Read more.
District laboratory staff conducted a rodent survey for hantavirus with California Department of Public Health (CDPH) staff on June 4-5 in La Honda. Peromyscus truei (pinyon mice) and Peromyscus californicus(parasitic mice) were trapped and blood samples were taken for testing. Kidney samples from any mice with positive blood samples will also be tested by the CDPH to be used for a study of the geographical distribution of different hantavirus strains in California.  Read more.
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