Mosquito Abatement Efforts On San Leandro Creek

The following information has been provided to the Friends of San Leandro Creek by John R. Rusmisel, the District Manager for Alameda County Mosquito Abatement.

Alameda County Mosquito Abatement District regularly inspects and controls mosquitoes in and along San Leandro Creek, from the Willow Park Golf Course where the creek flows into Chabot Reservoir to the tidal interface near the San Leandro Bay. Our control efforts along the creek are nearly 100 percent larval control and take place in early spring in the area around Willow Park and Lake Chabot to control the woodland mosquito, Ochlerotatus washinoi. This mosquito breeds in the overflow areas of the creek downstream from the golf course where water collects under the willow and alder trees. This is usually a single larvicidal treatment that typically is done with Bacillus thuringiensis israelensis (Bti) granules. This dead bacteria is highly selective for mosquito larvae.

The portion of San Leandro Creek from its Lake Chabot outlet to San Leandro Bay is inspected when the water flow diminishes to a point where the flow stops and potholes start to appear. This can be as early as April and continues until the rainy season begins in November. The creek is inspected in sections from one road crossing to the next (i.e. From Highway 580 to Bancroft Avenue). The water is sampled for mosquitoes and if larvae are present the water is treated with a larvicide that includes Bacillus sphaericus (Bs), Altosid or GB 1111 in addition to the Bti described earlier. Bti and Bs are naturally occurring bacteria; Altosid is an insect growth regulator with an active ingredient methoprene, which is a synthetic mimic of an insect juvenile hormone. GB 1111 is a petroleum-based product that provides a surface film that prevents the mosquito larvae from breathing. We use GB 1111 in waters with few or nonexistent beneficial insects to aid in natural predation. Mosquitofish(Gambusia affinis) are not used in San Leandro Creek due to the fact that the water usually does not persist throughout the summer and because some areas of the creek have populations of stickleback fish that provide natural mosquito control. The areas that seem to produce mosquitoes in San Leandro creek are places where water enters the creek from underground storm drain systems and under the road crossings where the impervious concrete bridge footings hold the water throughout the year.

The mosquitoes produced in lower San Leandro Creek include: Culiseta incidens, commonly called the fishpond mosquito; Culex pipiens, the house mosquito; and Culex tarsalis, the encephalitis mosquito. The two Culex species are vectors of West Nile Virus and are being targeted for control by the District. The tree hole mosquito, Ochlerotatus sierrensis, is often found in trees and containers along San Leandro Creek from mid March sometimes as late as June. The tree hole mosquito is a vector of canine heartworm.

If you would like more information on our District’s control program, please visit our website at There is a button on the right side of the home page called “control program” that links to a document that describes our program in detail.