Not only are mosquitoes a nuisance, they are also a public health threat. While most mosquito species in Marin and Sonoma counties are considered nuisance mosquitoes, their bites can cause severe allergic reactions in some people. High populations of mosquitoes can also have an impact on livestock, negatively affecting milk production in dairy cows.

More importantly, mosquitoes must be controlled because of the risk to human health. Mosquitoes have the capability of transmitting diseases such as West Nile virus, dog heartworm and western equine encephalomyelitis. The goal of the District's mosquito control program is not to eradicate all mosquitoes, but to successfully reduce nuisance mosquito populations and prevent mosquito-borne disease transmission.