- Author: Nadia Zane
Mosquitoes are often associated with summer, but recent warm weather has started mosquito season early. The itchy bites and annoying buzzing are usually all that concerns us, but mosquitoes can also transmit harmful pathogens to
In order to propagate, mosquitoes need water. Depending on the species, they may lay their eggs on standing water or on soil that will eventually be inundated. Eggs can be single or in “rafts” of 100 or more. Eggs laid on water will usually hatch within a few days; eggs on soil can stay viable for several years, waiting for a time when water floods the area, creating a favorable environment for the larvae to grow in.
When mosquito eggs hatch, they go through 3 larval stages. After the fourth molting they become pupae; they emerge from the final molt as adults. The process from hatching to adult is rapid, taking as little as 7 days. Males emerge and start mating whereas females, who are typically the bloodsuckers of the mosquito family, search for blood and the protein it provides for egg production.
California has over 50 species of mosquito, most of which do not bother humans. There are several genera, however, that carry harmful diseases:
• Culex spp: vector for West Nile Virus (see below)
• Anopheles spp: carrier for human malaria, vector for canine heartworm
• Aedes spp: carrier of Dengue hemorrhagic fever, Japanese encephalitis, others
West Nile Virus (WNV) has been a major concern since its arrival in New York in 1999, and subsequent spread westward to cover the entire continental United States. Culex spp act as vectors, transmitng WNV between birds, humans, and domestic animals. Roughly 80% of people infected with WNV develop mild symptoms such as fever, headache, nausea, or skin rash. However, about 1 in 150 of infected people develop severe neurological symptoms, which can lead to death.
• Birdbaths should be emptied at lease once a week.
• Unused planting saucers should be stored upside-down; those in use should be dumped on a regular basis.
• Roof gutters can become clogged with debris, so clean a couple of times a year to keep water running smoothly.
• Pools are great places for mosquitoes. Chlorine does not kill mosquitoes, so maintain water quality and keep covered. Do not let water collect on top of the cover.
• Poorly drained areas such as low spots in your lawn or on hard surfaces can collect water. Install drainage or fill in areas as necessary.
Taking personal precautions will also help to prevent bites. Avoid being outside when mosquitoes are most active. For Culex spp this is typically at dawn and dusk. If you must be outside, wear long sleeves, pants, and a hat. Use mosquito repellant containing DEET, Picaridin, or oil of lemon eucalyptus. Prevent mosquitoes from entering your home by maintaining screens on windows and doors.
Biological controls at the larval stage (larvicides) are the most selective method, meaning they target the mosquitoes while being safe for other wildlife, humans, and domestic animals. Bti (Bacillus thuringiensis ssp. israelensis) is a commonly used bacteria in water features without flowing water (moving water suffocates mosquito eggs and larvae). The mosquito larvae eat the bacteria, which kills them before they can grow into adults. It comes in solid and liquid forms.
Another biological control agent is the use of mosquitofish (Gambusia affinis) in ponds or unused pools. These little minnows will eat mosquito larvae and are available free from the San Joaquin County Mosquito and Vector Control District. In favorable conditions they can become established and provide long-term mosquito control.
There are also pesticides to kill mosquitoes at the adult stage, though these tend to be short-term solutions. Before using any product, always read and carefully follow all precautions and safety recommendations given on the container label.
Wild birds act as a reservoir of WNV, so the California Department of Public Health has set up a hotline to report suspicious dead birds and squirrels. Be sure to collect information on the location of the animal, a description (color, size, type), when you found it, and condition of the animal when found (intact, run over, etc). The hotline phone number is 1.877.968.2473, or visit the West Nile Virus and Dead Bird website at:westnile.ca.gov/report_wnv.php.
Other helpful resources are listed below:
UC Integrated Pest Management
SJ County Mosquito and Vector Control District (free mosquitofish)