Bulletins & Announcements

Increased mosquito activity in western Placer county

It is that time of the year again!

We have been receiving a few reports of increased mosquito activity in western areas of Placer County. Below is a status update, which we hope answer some questions folks may have about the current mosquito situation:

What is the current situation?
Western Placer is seeing a lot of mosquitoes out lately. These mosquitoes are locally known as “rice field mosquitoes”, which are also known as Anopheles mosquitoes. These mosquitoes are special because they overwinter as adult mosquitoes, which is a kind of hibernation.
These mosquitoes are an issue for any agricultural or ag-adjacent areas in the region.
These mosquitoes are coming out of their shelter because of the recent warmer daytime temperatures (65+ degrees) and are NOT emerging from standing water sources. They are coming out of bushes, trees, under decks, or other areas that provide shelter from the winter cold. Since the warm weather has “tricked” them into coming out of hibernation early, they will be hungry and aggressive.
These mosquitoes will try to enter your garage or home when the temperatures drop in the evening.

What is the good news?
These mosquitoes are NOT competent vectors for West Nile virus or other common human diseases transmitted by mosquitoes.
The winter emergence of these mosquitoes is usually a short period of time (3-4 weeks) and is cut short when daytime temperatures get colder again or with rain.

What does the District do to manage Anopheles mosquitoes?
The District monitors Anopheles mosquito activity year-round. At this time of year, and with this particular type of mosquito, environmental conditions are not favorable for wide-area adulticide treatments. The bulk of Anopheles mosquito management happens in the spring and summer, when we treat surrounding rice fields for mosquito larvae, and in the summer and early fall, when we treat the adult Anopheles mosquitoes in the rice fields and surrounding areas, before they find their winter shelters.
Since these mosquitoes are also capable of traveling up to 15 miles, localized treatments would only provide a 1-2 days of relief.

What does the District do to help prevent West Nile virus?
For West Nile virus prevention, the District monitors standing water sources year-round. When environmental conditions are suitable, we start placing mosquitofish in standing water sources. This typically happens around March, depending on the year.
Once we identify mosquito larvae for Culex species mosquitoes in water sources, we begin to treat those areas with mosquito larvicides. This typically begins in the late spring, depending on temperatures. Culex species mosquitoes are the ones that transmit West Nile virus and other mosquito-borne encephalitis viruses.
In the spring, the District also starts setting over 50 adult mosquito traps throughout the county to monitor adult mosquito numbers and to see if any mosquitoes are testing positive for West Nile virus infection.
The District also begins to test dead birds for West Nile virus infection.
When we start to see increased numbers of West Nile virus vector mosquitoes, or West Nile positive dead birds or mosquitoes, this is usually the indication when we need to do wide-area adulticide treatments.

What can residents do?
Dumping or draining standing water is always a good idea, even if mosquitoes are not quite laying eggs or emerging from water sources right now. It will get you in the habit of doing it all the time, especially when the risk for West Nile virus is high (summer and early fall).
Make sure your doors, windows, and garage doors remain closed and window screens are in good condition. When temperatures drop in the evening, Anopheles mosquitoes will aggressively try to seek warm shelter either in your garage or your home.
If you will be outside, wear thick long sleeves and long pants, and apply a CDC-recommended repellent to any exposed skin.