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The Truth About Mosquitoes And Backyard Ponds...

By Julie Bawden Davis

Ponds usually bring good things to the backyard -- like cool comfort, a beautiful view, and interesting wildlife. One visitor to ponds is not so welcome, though. No one wants mosquitoes around because they're a nuisance, and more importantly, because of the possibility that they may carry the deadly West Nile Virus (WNV).

When Problems Arise
Although ponds are definitely a concern when it comes to mosquitoes, there is no need to panic. As a pond owner, there are things you can do to prevent mosquitoes from becoming a problem in your yard. It's also important to realize that becoming infected with the virus is not that common.

"Because humans aren't hosts of the WNV, even though it's in the environment, it rarely infects them" says Harry Savage, research entomologist with the Centers for Disease Control and Prevention (CDC) in Fort Collins, Colorado. "Problems arise when the amount of virus in the environment gets very high, which happens when there are a lot of mosquitoes present and conditions are good for their reproduction. Then the virus spills over to humans. The key is to reduce the population of mosquitoes which will then interrupt transmission to humans."

No Need to Panic
Not all mosquitoes are a threat. "Of 174 species of mosquitoes in North America, WNV has been found in just 28 of them," says vector biologist Stephen Higgs, associate professor at the University of Texas Medical Branch in Galveston, who studies mosquitoes and the viral diseases that they transmit. "The main species involved in the transmission cycle of WNV in the U.S. seems to be Culex pipiens pipiens and Culex pipiens quin-quefasciatus. The good thing about these mosquitoes is that they prefer to feed on birds. And although many birds can be infected, relatively few species develop symptoms of disease and actually die. Notable exceptions are members of the crow family, including crows and blue jays. Often the first indication of the arrival of West Nile in an area is the die-off of these birds."

"The West Nile Virus is an "Old World" virus that has been in existence for ten thousand years," says Harry Savage of the CDC. It can infect birds, horses, mosquitoes, humans, and some other mammals. Although it has been in Africa, Eastern Europe, West Asia and the Middle East for many years, it was not detected in the Western Hemisphere until 1999. It is most closely related genetically to strains found in the Middle East.

According to the CDC, as of December 3, there have been 3,775 laboratory positive human cases and 216 deaths. Despite these numbers, the chance of becoming infected with the virus is low and most people who do contract the virus will not have any type of illness. The CDC estimates that 20% of people who become infected will develop symptoms of West Nile Fever, which include fever, headache, body aches, and swollen lymph glands. The illness usually lasts only a few days and does not appear to cause any long-term health effects.

Complications arise when a person contracts West Nile encephalitis or meningitis, which can cause severe headache, high fever, neck stiffness, disorientation, convulsions, paralysis and coma. The CDC estimates that 1 in 150 infected people will develop a more severe form of the disease.

West Nile Virus can be transmitted when an infected mosquito bites a human and takes in blood. Mosquitoes become infected when they feed on infected birds. If the virus reaches the mosquito's salivary glands, it can then be injected into humans. Not all mosquitoes can be infected by the virus, and some species are more likely to be infected than others," says vector biologist Stephen Higgs.

"Only female mosquitoes feed on blood because they need the blood to produce and lay eggs. Mosquitoes don't actually live in ponds, but lay their eggs in ponds and then go looking for a blood meal," says Thomas W. Scott, professor of entomology and director of the Davis Arbovirus Research Unit at the University of California, Davis.

Responsible Pond Owners
Considering that mosquitoes can breed in ponds, it's important that pond owners act responsibly and do their part to minimize their reproduction. "The less hospitable habitats mosquitoes have to inhabit, the less problem we'll have overall," says Scott.

Keep the following mosquito control measure in mind:

Get Fish. Fortunately, many fish , like mosquito fish (Gambusia affinis), carp, goldfish, and koi, like to eat mosquito larvae. A good population of fish is a pond's best line of defense against mosquitoes. Fish will consume the larvae before they become adult mosquitoes. Mosquito fish are inexpensive and self-sustaining, multiplying rapidly. Some cities or towns with mosquito abatement districts even give mosquito fish away for free to those with ponds and other water features. (Check with your county to see if you have a mosquito abatement district. It may be located under vector control.)
Try Biological control measures. If you don't wish to have fish in your pond, or you want to make extra sure that mosquito larvae don't survive, there are biological controls you can use such as Bacillus thuringiensis israelensis and Bacillus sphaericus. These right-shaped briquettes are often called mosquito dunks. They are sold in garden centers and on-line. These dunks break down over time in the water, releasing a bacteria that will kill mosquito larvae, but won't harm other life forms, such as fish, birds, and plants.
Encourage water movement. Waterfalls are a good mosquito deterrent. In general, mosquitoes breed in standing water or in protected still areas of slow-running water. (During last summer's outbreak, many mosquitoes were found in locations such as old, abandoned swimming pools.) Quickly running water is not desirable to mosquitoes because the larvae has nothing on which to cling and will wash away.
Inspect other water sources. Keep in mind that mosquitoes will readily reproduce in any collected water. Potted plant saucers, birdbaths, and buckets provide great breeding grounds for certain species of mosquitoes. Check your yard weekly and dump out all standing water. Completely change birdbath water on a regular basis and keep the basin clean and free of debris. Avoid storing tires in the backyard, as they are notorious mosquito breeders.
Use additional protection strategies. Make sure to apply insect repellent before enjoying time at the pond. Although some mosquitoes bite at any time, many prefer to feed in the evening. Mosquito traps are also effective at catching some species of mosquitoes. Strategically placed, they may lure the adults away from the pond -- and you.

Julie Bawden Davis is an Orange, California garden writer whose work appears in a variety of national and regional publications. She is the author of the book Houseplants & Indoor Gardening (Black & Decker Outdoor Home Series, 2002.)

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Naturalizing Tips...

To develop the look of a natural pond you can:

- Go with the native vegetation that thrives in your area.

- Repeat mass groupings using one or two kinds of plants, especially grasses and reeds.

- Surround your pond with trees and shrubs that naturally occur near water, like alders, river birches, dogwoods and willows.

- Plant vigorous natives in the shallow margins, and make sure the center of the pool is more than eighteen inches deep, so natives won't over run the pond.