Structure and seasonality of nearctic Culex pipiens populations.

The abundance and structure of urban autogenous and anautogenous populations of Culex pipiens mosquitoes were documented systematically in Boston, MA, during three successive years. Autogenous larvae become abundant mainly in enclosed sites and anautogenous larvae in sites that provide free access and egress. Both populations begin to proliferate when the water temperature exceeds 15 degrees C during June. Larval anautogenous mosquitoes increase in abundance 10-fold in two weeks and autogenous in three weeks. Although anautogenous larvae rapidly disappear after mid-August when winter diapause commences, the abundance of autogenous larvae continues to increase until mid-October. The forms generally are reproductively isolated in nature but occasionally hybridize during August and thereafter. Anautogenous females feed mainly on birds; autogenous females generally never feed on blood; and hybrid females appear to feed indiscriminately on avian or mammalian hosts. Such northern C. p. pipiens mosquitoes range as far south as 33 degrees N. Taken together, these observations suggest that C. p. pipiens-borne pathogens may proliferate in the northern United States until mid-August and affect human hosts thereafter. Intensity of transmission decreases toward the south.