As we begin to move from winter into early spring, you will notice increased mosquito activity when outside temperatures reach 60 degrees.

While cold weather kills most species of adult mosquitoes, the adult female Anopheles freeborni hibernates during the cold winter months.  The majority of these mosquitoes were bred during the previous summer in agricultural lands throughout the west and south areas of the District.  When the female mosquitoes are done laying their eggs, they look for a place to hibernate or over-winter.  These locations include garages, outbuildings, thick grass, etc.  These hibernating mosquitoes generally are not a problem during the winter months until a warming trend occurs.  A warming trend (in excess of 65°) in January, February, or March sounds a wakeup call to these mosquitoes.  They are extremely hungry and are looking for a blood meal that will nourish their developing eggs.  Biting females are most bothersome during the afternoons and early evenings. Fortunately, this problem usually lasts only as long as the warm days persist.

This is not a phenomenon.  This happens each year when a warm spell grabs hold of Butte County during the late winter months.  Residents can experience these mosquitoes in some foothill communities as well as every place in the valley.

The overwintering Anopheles freeborni mosquitoes are not easily controlled during the cool weather months.  The District usually cannot "fog" for mosquitoes in the winter months due to its ineffectiveness compared to the warmer spring and summer months.  Certain conditions have to be met in order to effectively control adult mosquitoes.  During the warmer months, the District sprays for mosquitoes before the sun rises in the mornings and after it has set in the evenings.  This is not usually possible in January, February, or March since the temperature is not warm enough to spray during those times.  The overwintering mosquitoes are usually most active during sunlight hours.  The District does not spray during sunlight hours due to improper weather conditions such as the lack of an inversion layer, UV rays reducing the pesticide effectiveness, and the risks to non-target beneficial insects.

Can these mosquitoes transmit disease?  The Anopheles freeborni mosquito is considered unlikely to transmit West Nile virus.  These mosquitoes historically transmitted malaria and were involved in the malaria epidemics during the late 1800’s and early 1900’s in northern California.  Although malaria is not currently a concern locally, the potential for disease transmission exists if malaria is reintroduced through an imported human case.  The District works closely with state and local health departments to monitor new and emerging vector-borne diseases.  To summarize, these overwintering pests are primarily a nuisance, especially during winter and spring months.

Individuals can reduce their risk of mosquito-borne disease by following the 7 Ds of prevention.