Mosquitoes are delicious food for fish and other aquatic creatures, but...

The emergence of West Nile virus has focused public attention on mosquitoes. Fear may cause us to reach for a pesticide spray can, but this is an ineffective control. Pesticide sprays reach relatively few mosquitoes, and outside, they probably cause more harm to beneficial insects. Residents can have a greater effect on the numbers of mosquitoes in urban areas by following the tips in this fact sheet.

The young (or larvae) of mosquitoes live in water and feed on microorganisms and organic matter. Just about any area or container that can hold water for more than a few days can produce a large crop of mosquitoes. Only adult female mosquitoes bite humans and other animals to feed on blood. Adult male mosquitoes feed on flower nectar.

There are many different species of mosquitoes. Some bite during the day, while others feed at night. Although some mosquitoes can fly long distances from their watery breeding sites, others travel only a short distance to find their blood meals. Your bites may be coming from mosquitoes you are raising in your own backyard.

• Eliminate standing water in containers around the home, including water in cans, plastic containers, potted plant saucers, buckets, garbage cans, barrels, wheelbarrows, and any other container that holds water for more than a few days. Empty the water and then either invert, cover, punch drainage holes in, or dispose of these containers.
• Change water in birdbaths and pet water dishes at least once a week, preferably every 2 to 3 days.
• Fix leaky outdoor faucets and sprinklers, and don’t overwater your yard. Any standing water can produce mosquitoes.
• Recycle tires or store them so they do not collect water. Tires are extremely hard to drain, and each one can produce thousands of mosquitoes.
• Keep roof gutters clean so water drains; otherwise mosquitoes can breed in the leaf and water mixture.
• Don’t dump yard waste into street gutters, storm drains, or creeks. It can impede the flow of water, allowing mosquitoes to breed. The decaying organic matter then provides food for dense numbers of growing mosquito larvae.
• Drain plastic wading pools or fountains when not in use, or cover tightly to deny access to mosquitoes. If the fountain is large enough, stock with mosquitofish (see below).
• Keep swimming pools and hot tubs chlorinated and filtering. When not in use for extended periods, cover pools or tubs tightly or stock with mosquitofish (see below). One untended pool or hot tub can breed enough mosquitoes to affect a whole neighborhood.
• Use mosquitofish (Gambusia affinis) in backyard ponds or water gardens, watering troughs, and stockponds. These fish are available, at no cost, from your mosquito and vector control district (see inside). Gambusia are cannibalistic, so be sure to provide rocks and plants in your backyard pond to help shelter young fish, but not so many plants that the pond becomes heavily shaded. Gambusia do not require supplemental food. Overfed fish may not feed on mosquitoes, and excess food in the water may cause bacterial blooms that harm the fish. Do not release mosquitofish into the wild. Caution: Check with your water district before using tap water to fill your pond. Water containing chloramines is toxic to mosquitofish and must be chemically treated first. Products (such as Aqua Plus®) are readily available at pet stores.
• Fill tree holes with a polymer such as Soil Moist® or Broadleaf P4®. In the winter, the granules absorb water and eliminate mosquito breeding habitat. The granules can last for many years, absorbing water in winter and drying out in the summer. If necessary, consult a certified arborist about the condition of the tree. The western tree hole mosquito is the primary vector of canine heartworm in this area. Keep your dog’s heartworm medicine up to date.

Note: If you are experiencing serious or persistent mosquito problems, call your mosquito and vector control district (see inside) before considering the use of pesticides.

Propane powered traps, such as Mosquito Magnet®, work by attracting adult mosquitoes with carbon dioxide, heat, and a natural mosquito attractant. These devices can be very expensive and they only remove adult mosquitoes without addressing their source. Energy and money might be better spent reducing breeding habitat and purchasing products containing Bacillus thuringiensis (see left) that can provide more effective control.

Don’t use electric bug zappers because they kill far more beneficial and neutral insects than mosquitoes.

The products described below can be used in ponds and water gardens, birdbaths, fountains, pools, tree holes, and other standing water where mosquitoes lay eggs. Apply when mosquito larvae are first noted in the spring and continue at the intervals recommended on the package.

• Bacillus thuringiensis subsp. israelensis (Bti) is the active ingredient in a number of mosquito control products (Mosquito Dunks®, Mosquito Bits®, Vectobac®) that are used in water. This bacterium is a stomach poison and must be consumed by mosquito larvae to be effective. Only mosquitoes, black flies, and some midges are susceptible. Other aquatic life are unaffected. Follow all label directions.
• Methoprene (Pre-Strike®) is an insect growth regulator that interferes with the normal development of mosquitoes. It must be present in the larval habitat to be effective. Larvae continue to grow until they reach the pupal stage, at which point they die. Do not apply to waters that drain into public waterways.

Other Pesticides

• Do not use pesticide sprays to control adult mosquitoes. Use a combination of the techniques listed above or call your mosquito and vector control district.
• Do not treat street gutters or storm drains with pesticides. Storm drains are connected directly to the Bay, and pesticides cause serious problems for aquatic life. Call your mosquito and vector control district if you suspect mosquitoes are breeding in the storm drains or catch basins.

West Nile virus made its appearance in the U.S. in 1999. Most of the small number of serious cases in this country have been among the elderly. According to the Centers for Disease Control and Prevention, only 1% of people bitten by infected mosquitoes become seriously ill and the vast majority of those recover.

Birds serve as a host for this virus. Mosquitoes acquire the virus from infected birds and then transfer the virus to people. The Department of Health Services is encouraging anyone who finds a dead bird (especially a crow, raven, magpie, jay, or hawk) to report it by calling: 877-WNV-BIRD (877-968-2473)

Do not pick up the bird with your bare hands.

Web Sites For More Information:

California Department of Health Services:
www.westnile.ca.gov

Mosquito & Vector Control Association of California:
www.mvcac.org

Examples of trade names of products listed in this fact sheet.

Polymer for filling tree holes (also for use in soil to reduce plant waterings):

• Soil Moist®
• Broadleaf P4®

Bacillus thuringiensis subsp. israelensis (Bti):

• Mosquito Dunks®
• Mosquito Bits® Quick Kill Mosquitoes
• Vectobac®

Insect Repellents

• OFF!® (active ingredient: DEET)
• Bite Blocker® (active ingredients: soybean, coconut, and geranium oils)

Product for making chloramine-treated water safe for fish:

• Aqua Plus®

Methoprene (insect growth regulator):

• Pre-Strike®