Culex pipiens mosquitoes do not lay their eggs in summer or fall in the United States, but overwinter as adults and lay their eggs in the spring. They live in caves or hollow trees in the winter to stay warm when it snows. In warmer climates, they can be active year round. These mosquitoes thrive in stagnant water like rain barrels, flower pots, rain gutters, and storm drains.
They will bite humans when the mosquitoes enter buildings, but they prefer bird blood. They are competent vectors of the West Nile virus, along with many other mosquito species. These mosquitoes amplify the virus and spread it among birds and humans. Sixty two other mosquito species have been found to carry West Nile virus in the US. The genera include Aedes, Anopheles, Coquillettidia, Culex, Culiseta, Deinocerites, Mansonia, Orthopodomyia, Psorophora, and Uranotaenia.