Mosquito Larval Control
Why is Mosquito Control Important?
Mosquitoes can pose a serious problem to our health. Only female mosquitoes bite because they need a blood meal to develop their eggs. When mosquitoes bite, they inject their saliva into the the body of the host. A host is the human or animal the mosquito bites. Not every mosquito is infected and different species can transmit different diseases like West Nile virus, Western equine encephalitis, Zika, and parasites like malaria. Other mosquito species can be aggressive bitters but don't spread diseases or germs to people. These mosquitoes are categorized as nuisance mosquitoes.
Mosquito Life Cycle
When you think of a mosquito, you probably picture the flying insect that bites you. However mosquitoes go through four life stages just like a butterfly.
- A female mosquito lays her eggs in stagnant water.
- The eggs then hatch into larvae which are known as "wigglers." Larvae are aquatic and have a large head and a narrow, wormlike body.
- After the larvae go through their four stages of molting, they turn into a pupa. The mosquito's pupal stage does not feed and is just wiggling while going through metamorphosis (similar to when a caterpillar goes into its cocoon).
- After a short period, an adult flying mosquito emerges from the pupa and the cycle starts again.
Warm temperatures help mosquitoes complete their life cycle faster. This is why we see more of them highly active during the summer time. This is also why climate change has been increasing mosquito problems. However, the larvae of the winter salt marsh mosquito (Aedes squamiger) hatch and go through their aquatic stages (larval and pupal stages) between October and March. Winter salt marsh mosquitoes only have one generation per year.
Because one adult mosquito can lay hundreds of eggs at a time, the best method is to control them at the larval stage. The majority of our control efforts are focused on preventing the aquatic stages of mosquitoes (larvae) from reaching the adult biting stage. We are also responsible for monitoring the presence of diseases like West Nile virus and conduct adult-mosquito-control treatments only when we detect a positive West Nile virus mosquito (meaning biting mosquitoes infected with the virus have been found in an area of the County). Our goal is to manage vectors like mosquitoes that can cause discomfort to our community members and minimize the chance of a human being bitten by an infected mosquito.
How Do We Check for Mosquito Breeding?
We inspect areas like creeks, catch basins, curbs, ponds, swimming pools, marshes, and anything that can hold water for mosquito breeding year-round. We control mosquito breeding using Integrated Pest Management practices. During our inspections, we use a tool known as a "dipper" to collect a water sample (picture a plastic cup attached to a long stick), and identify the mosquito species found. We analyze the amount of breeding in the water sample collected with the dipper and base the severity of breeding depending on how many mosquito larvae and what larval stages we see. Depending on the area we find breeding, we will use different tools and methods to proactively control the mosquito breeding. This can range from using physical control like managing the land, biological control like the use of mosquitofish, or even chemical control like using pesticides.
We use different methods to apply pesticides. This can be done on foot by using backpack sprayers, on land by using all-terrain vehicles (ATVs), or by air using drones and helicopters. ATVs, drones, and helicopters are only used in non-residential locations for pockets of water that are hard to reach on foot. There is no potential exposure during or after the treatments.
Pesticides Used
We use several pesticides for larval control but mainly use a naturally occurring soil bacteria Bacillus thuringiensis var. israelensis (Bti), Lysinibacillus sphaericus (Bacillus sphaericus, Lsph), or Saccahopolyspora spinosa (Spinosad) that activates when consumed by mosquito larvae. We also use an insect hormone regulator (methoprene) that prevents mosquito larvae from becoming adults, and a highly refined mineral oil as the sole method of pupae control.
Aerial Treatment Activity
More information on the pesticides used during an aerial treatment is listed below.
| Treatment Date | Treatment Location | Press Release | Pesticide Used | Pesticide Safety Data Sheet (SDS) | Pesticide Label |
| 2/14/2024 | Palo Alto Flood Basin | Press Release Link | Vectobac12AS Altosid SR 20% |
