Buzz Off
Control mosquitoes, or conserve wildlife?


Wetlands are essential fertile ecosystems for breeding and feeding many species of fish and wildlife, but they are also the nurseries for one of nature’s greatest nuisances: mosquitoes. Right now there is a conflict between saving wetlands for wildlife and public health concerns over mosquito-spread diseases such as West Nile virus.

Mosquitoes, of course, are the airborne mini-vampires that swarm on us each spring and summer. They drive us indoors or make us use foul-smelling repellents to ward them off. When they don’t bite, they still buzz in our ears and drive us crazy. When they do bite, they extract our blood and leave itchy red welts. Sometimes — and in the worst-case only — they carry deadly diseases.

There are at least 12 mosquito-borne viruses found in the U.S. Technically they are called “arboviruses” (arthropod-borne). They attack a variety of warm-blooded animals, and include various forms of encephalitis, dengue fever, yellow fever, Rift Valley fever, and West Nile virus (WNV) — the new virus on the block.

Two notes here: 1) the AIDS virus cannot be spread by mosquitoes as it does not survive in them; and 2) to the delight of conspiracy theorists, according to the CDC, the form of WNV that has become established in the U.S., for reasons no one understands, seems to be closest to strains found in the Middle East. Although it is most likely that a mosquito stowaway on a plane or ship is the guilty party, bioterrorism cannot be ruled out.

West Nile virus was first isolated in the blood of a Ugandan woman in 1937. Human and equine (horse) outbreaks of WNV have since occurred in Africa, India, Russia, eastern and southern Europe, the Middle East, and — most recently — the U.S. WNV made its appearance in New York in l999, and has since spread rapidly across the country, thanks to a transmission system of mosquitoes and the birds that the mosquitoes bite. WNV does not seem to be spread through sexual contact, but there is evidence to suggest that it can be spread through blood transfusions, organ donations, or mother’s milk.

The virus can attack the brain, or membranes of the brain, causing encephalitis or meningitis. It kills lots of birds — more than 110 species have fallen victim, with crows, jays, magpies, ravens, and domestic chickens the most susceptible. It can also kill livestock, pets, and humans.

Roughly 80 percent of the people who are exposed to WNV do not show any symptoms. Of the remaining 20 percent who develop “West Nile fever,” one of every 150 require hospitalization. Nonetheless, in 2002, 3,399 people in 39 states became ill enough with WNV flu-like symptoms to warrant medical attention, and 193 died — in part because there is no specific treatment available. (For perspective, over 20,000 people die from the flu each year.) Incidentally, those who recover often suffer from polio-like symptoms for a year or more. Louisiana, Illinois, Mississippi, and Ohio have reported the largest numbers of victims.

WNV is one of a seemingly endless series of things to worry about, thanks to better monitoring and media coverage, as well as modern transportation systems that can take illnesses global in hours. Its significance for this column is two-fold: People who enjoy being outside are more likely to be exposed to mosquitoes carrying WNV, and so they need to take extra precautions, which — according to the Center for Disease Control — include reducing time outdoors, particularly in the early evening hours, wearing long pants and long-sleeved shirts, and applying mosquito repellent to exposed skin areas.

An equally important issue is control. The best way to prevent the spread of WNV is mosquito control, and since mosquitoes breed in water, that means targeting wetlands. All across the U.S. there are local and county mosquito and vector control agencies involved in an ongoing battle to control mosquitoes. This is not an easy task — some varieties, such as flood mosquitoes, can reproduce in four days and then fly over 20 miles. And even if water is drained from wetlands, mosquito eggs may lay dormant in the ground for years and then hatch in hours when a flood occurs.

Dragonflies, fish, spiders, bats, and birds eat mosquitoes, but if public health workers detect the presence of WNV, either through dead birds, infected humans, or in samples of mosquitoes collected in regular monitoring, a public health emergency can be called, and that supercedes wildlife conservation. That’s where the mosquito wars come in.

In California, where over 90 percent of the state’s original wetlands acreage is gone, 50 percent of the state’s endangered and threatened species are wetland-dependent, as well as the majority of the state’s resident breeding waterfowl. The problem becomes more vexing in California because 60 percent of the remaining wetlands in the state are privately owned, and these are either rice fields, or duck clubs, which the state says are liable to foot the bill for mosquito control. Last spring duck-club owners in the Butte Sink area were shocked to receive letters for local mosquito-abatement districts asking them to cough up $18 an acre for mosquito control.

California is the second largest rice producer in the U.S., and rice fields also represent wetlands. But the fields are only flooded part of the year, pesticides are used to control insects, the vegetation is removed after harvest, and the fields are drained in February. Consequently studies show that flooded woodlands (ideal habitat for feeding and nesting ducks) hold 300 to 1000 times as many mosquitoes as flooded rice fields.

Still, the California Health and Safety Code declares the production of mosquitoes a public nuisance and enables mosquito-control districts to take whatever action is necessary to control the little vampires, and recover all costs. If private landowners do not comply, the abatement districts can assess civil penalties of $500 a day and even put a lien on property to insure abatement costs are paid.

Mosquito control includes water management, draining breeding grounds, removing vegetation that hide adult mosquitoes, and chemical use — such as malathion in aerial sprays and pyrethrums, larvicidal oils, surfactants, and bacterial insecticides from the ground. The problem with nearly all chemical methods is that they will also impact non-target species, like bees, midges, etc. Mosquito control may also include the introduction of bacteria to infect mosquitoes as well as the strategic planting of mosquito-eating fish.

All these things are done in the name of public health. But on the other side of the balance-scale remember that mosquitoes, especially the larvae, are an important food item for wildlife. Adult ducks would probably prefer wetlands with fewer mosquitoes, but for juvenile ducks, mosquito larvae are a primary source of protein, especially in the first two weeks after hatching, which coincides with the peak of mosquito breeding.

So, which is more important: controlling mosquitoes, or conserving wildlife? The guy who invents a better mousetrap is supposed to become a millionaire, but the genius who comes up with a way to prevent West Nile virus without disrupting ecological balance will not only be wealthy, he will make a lot of fish, birds, bats, dragonflies, and people very happy.

— James Swan is a contributing editor of He also writes for the Outdoor Channel’s Engel’s Outdoor Experience, which just won a Golden Moose for the category “Best Waterfowl Shows 2002.”


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