January 2005

Tiny Mosquitoes Cause Big Fuss :West Nile intensifies debate over pest-control methods

By Gail Binkly

In the dead of winter, few people are thinking about mosquitoes. The 2004 season ended, at least in Southwest Colorado, on a positive note, with experts happy that the West Nile Virus did not strike more drastically than it might have.

According to the Centers for Disease Control, in 2004 Colorado reported 276 human cases of West Nile and three deaths, down dramatically from 2003, when the state had 2,947 cases and 63 human deaths.

Some call for resumption of DDT use

At the time it was formulated in the 1940s, DDT seemed like a miracle. Long-lasting, it kills by touch. A thin film of DDT on interior house walls, for instance, will kill any mosquito or other bug that alights there. The pesticide is still used that way in Africa, where malaria is rampant.

Malaria is no longer a serious threat in the United States, but the mosqui-to species that carry it remain here. What’s fortunately lacking, experts say, is a “reservoir” of humans with the disease who could give it to the mos-quitoes that bite them.

DDT was used in the United States for far more than killing mosquitoes. It was sprayed on cotton and other crops and used widely as a general pesti-cide. Later it was linked to deformities in the eggs of birds such as bald eagles and hawks, and was banned in 1973.

Because of the threat of West Nile, some people have called for ending the ban in the United States. They say the judicious use of DDT to kill mosqui-toes rather than agricultural pests could be done safely and effectively. Joe Conlon, technical advisor for the American Mosquito Control Association, believes DDT did get something of an unfair reputation, but he also says there is no need to use it in the United States.

“DDT for West Nile control would be ridiculous,” he said. “We have many other weapons we can use here.”

However, he said it should continue to be used in places where malaria is a major threat. “Banning DDT in Africa could condemn millions of children to death,” he said. Although DDT can accumulate over time in fatty tissues, he said, it’s never been proven to harm humans, and he disputes the validi-ty of the link to bird deaths. “It’s one of the safest pesticides known.”

In Montezuma County, there were four cases of West Nile in humans. La Plata County had 18 cases, Dolores County none. By far the county hardest-hit was Mesa, with 125 cases. “I think we’re all breathing a little sigh of relief,” said Jason Carruth, district manager of the Montezuma Mosquito Control District.

But spring is just a few short months away, and the mosquito eggs are out there, waiting to hatch and launch a new generation of whining, biting pests.

And once again, controversy will swirl over the best methods of com-bating these creatures. When is aerial spraying appropriate? Are the pesticides used in fogging as safe as they’re claimed to be, or do they contribute to a toxic environment that causes human health problems? On the other hand, does the very real risk of West Nile Virus outweigh the subtler risks of the chemicals?

Related to nerve gas

Settlers struggling to tame the buggy swamps of the East Coast and the South during the nation’s early days would doubtless have welcomed any form of effective mosquito control. It may be difficult to imagine now, but malaria, which is carried by mosquitoes, was common in parts of the United States up until the 1940s. The disease killed or sickened countless thousands of people and did not really disappear until the development of DDT, a long-lasting, highly effective pesticide that was later banned because of apparent links to dwindling bird populations. (See sidebar.)

Mosquitoes can transmit other is-eases, however. There are some 2,800 mosquito species worldwide, some of which carry illnesses such as dengue and yellow fever. In Colorado, three viruses have traditionally been carried by mosquitoes: Western equine encephalitis, St. Louis encephalitis, and California encephalitis (a viral group). The arrival of West Nile Virus in the United States in 1999 added new urgency to the debate over mosquito control. The virus, first identified in 1937 in Uganda, appeared on the East Coast of the U.S., but now is present in at least 47 states. In the East, it’s carried mainly by a mosquito species called culex pippiens; in the West, mainly by culex tarsalis.

The virus can sicken a host of animals, including horses, llamas, cattle, sheep, skunks, harbor seals and bird species from whooping cranes to hummingbirds. It reportedly killed 300 alligators at a Florida farm. It also strikes humans – though it should be noted that most of the people bitten by infected mosquitoes will not experience any noticeable symptoms. In 2004, the United States had a reported 2,432 human cases, 87 of them fatal. Throughout the early part of the 20th Century, people used a variety of clumsy methods to battle mosquitoes, such as draining wetlands wholesale and pouring oil on ponds. But pesticides used after the DDT ban proved effective in keeping mosquito populations in check. One of the most common is malathion, an organophosphate distantly related to nerve gas.

Most residents of rural areas in the West are familiar with the mosquito-control trucks that emit clouds of the foul-smelling chemical. Malathion is an “adulticide,” meaning it kills adult mosquitoes. Although many people find its smell obnoxious (it vaguely resembles cat urine), the pesticide is relatively safe, experts contend.

Malathion is a general-use pesticide, effective on insects from flies to lice to crop pests. It’s highly toxic to honey-bees, though pesticide-applicators try to spray it after dark when bees aren’t about. It’s also toxic to many fish and amphibians. Whether it’s a human carcinogen is uncertain, but the Natural Resources Defense Council, an environmental group, says organophosphate are the pesticides most likely to cause cases of acute poisoning in humans.

Many mosquito districts have switched to permethrin, a synthetic version of a pesticide present in chrysanthemums. Permethrin’s better odor is probably the major reason, but permethrin is also considered less toxic, although it is still harmful to fish and bees and at high levels can cause impaired breathing, headache, nausea, and convulsions in humans.

“Permethrin is less toxic, but frankly malathion is not any more toxic than table salt,” said Joe Conlon, technical adviser to the American Mosquito Control Association, a non-profit trade association with about 1,700 members. “There’s a hundredfold safety factor built in – you would have to use 10 times what’s recommended” to suffer ill effects.

However, he noted, “malathion has baggage because it’s an organophosphate. It’s related to German nerve gases. But that’s kind of silly, because it’s several generations of chemistry removed. In point of fact, it’s been used for 40 years and we’ve never had any problems with it.”

Not everyone agrees. Rebecca Thomas, owner of a three-acre tract in La Plata County with a small organic garden, said she is greatly concerned about the spray-in of chemical pesticides. Thomas said she is very sensitive to chemical exposure. Her grandfather owned a chemical company that manufactured fungicides, herbicides and pesticides; her father worked there. He came home reeking of chemicals, including malathion. Thomas said her brother eventually died of an environmental illness related to chemical exposure. Since she was 17, she likewise has been sensitive to chemicals, she said. One day she woke up aching; “the next morning, I couldn’t walk,” she said. “I went to the hospital and had different tests.” The only thing for which she tested positive was chemical poisoning, particularly for organophosphate.

‘Life on Planet Earth’

Thomas tries to avoid all chemical exposure, but can’t entirely. She suffers periodic bouts of tingling, numbness, paralysis, and aching. For years she has fought with the Florida (Colo.) Mosquito Control District about spraying done in her area that drifts onto her property. Although the district now uses permethrin, she said, tests have found traces of malathion in her soil – “it’s still in the soil after years, which is scary,” she said.

Thomas said she understands the fear of West Nile, but that people need to realize that pesticides have deleterious effects, too. “I don’t want anyone to suffer from anything,” she said, “but this is life on Planet Earth.” She believes people ignore the potential health effects of chemicals if they can’t be felt immediately. She also believes West Nile prevention should focus on educating people about avoiding exposure to mosquitoes. “I’m not saying people shouldn’t have their homes sprayed, but I don’t think I should have to be sprayed with chemicals.”

2004 Human West Nile Cases
and Fatalities (in parentheses)

Arizona 389 (14)
Colorado 276 (3)
Montezuma County 4
La Plata County 18
Dolores County none
Mesa County 125
New Mexico 87 (4)
San Juan County 30
Utah 11 (0)
U.S. 2,432 (87)

2003 Human West Nile Cases
and Fatalities

Arizona 13 (1)
Colorado 2,947 (63)
New Mexico 209 (4)
Utah 1 (0)
U.S. 2,866 (264)

Susan Cowan of Dolores has a diametrically opposed view. “I’ll take every-body else’s spray,” she said. “Give me the extra. I’m not really an environmentalist, I’m sorry.” Cowan was bitten by a mosquito in September 2003 while she was dining out in Cortez. Two weeks later, she came down with what she thought was a bad case of the flu. When it lingered, she went to her doctor and tested positive for West Nile.

Although this was well over a year ago, Cowan said, “I never really got over it.” Cowan, who is in her 40s, still has days when she is forced to stay in bed, suffering from sweats, occasional fever, and deep muscle pain. “I call it my layers of pain,” she said. “When you were healthy your whole life and now you get tired from grocery-shopping, it affects you mentally. I get very frustrated.”

‘If it flies, it dies’

Mosquito-management is handled by thousands of different entities nationwide - municipalities, counties, or special taxing districts. Each mosquito-control district has to judge how to balance concerns about pesticide spraying with how many mosquitoes people will tolerate.

“It’s not a matter of, if it flies, it dies,” Conlon said. “There is a certain algorithm to follow in mosquito control. It’s all based on surveys to find out what species you have, how many. When you find larvae, you can eliminate their habitat, which is most effective, but often you can’t do that if it’s a wetland. Maybe you can remove vegetation around the borders of the water to eliminate hiding places for larvae from dragonflies and indigenous fish.

“But if you can't modify the habitat, you go to larvicides.”

The Montezuma Mosquito Control District, a special district completely separate from the county, switched its approach in 1998 from lots of spraying of malathion to what’s known as “integrated pest management,” which means a more biologically balanced approach.

The district contracted with Colorado Mosquito Control, a Brighton-based company, to manage its efforts. Like many other districts nationwide, Montezuma now concentrates on controlling mosquitoes in their larval stage, before they develop wings and start biting. (It’s only the females that bite, of course; they need blood to develop their eggs. The males feed on nectar and actually pollinate plants.) The primary product used is Bti, a naturally occurring soil bacterium that is virtually non-toxic to animals, birds and fish and readily breaks down in the environment. It works by blocking a receptor in the gut of mosquito larvae, preventing them from feeding.

Bti can be distributed in pellets or larger “dunks,” or mixed into a liquid formulation and sprayed. Workers travel the 300-square-mile district (which covers most of the county) looking for mosquito larvae in bodies of standing water. If they find them, they apply Bti.

Larvicides offer up to a month of mosquito control vs. a few hours for fogging with adulticides, according to experts. “You can spray and then more mosquitoes would emerge the next day,” from their pre-winged stage, Carruth said. “Then you’d have to spray again.”

Fogging is done when the adult mosquito population is found to be high. If Carruth gets a call from someone complaining about mosquitoes, he sends someone to set out a mosquito trap, a cylinder with netting that draws mosquitoes by emitting carbon dioxide. If 60 mosquitoes are caught in one night, he’ll order the area sprayed. “That's actually on the low end,” he said. “In other areas they'll wait for it to be 100.”

Since adopting the integrated approach, the Montezuma district has cut the amount of adult pesticide it uses from 50 drums a season to about three drums.

“You’re never going to make every-body happy,” Carruth said. “You’ve got people that want napalm if it gets rid of their bugs, and then you have people that don’t want you to step on an ant. But this is the best way to put as few toxins as possible into the environment and still kill mosquitoes.

You’re never going to kill them all. This is mosquito control, not mosquito eradication.

But some longtime residents of the county still put their faith in fogging. Benny Gordanier, a member of the Montezuma district board since 1976, would like to see more spraying himself. “We all argue about it,” he said. “I think they're doing a good job. But I would like to see a little more spraying. I wish they were not so picky about where they spray.”

He said it’s difficult to juggle the demands of people who want more spray with those who ask not to be sprayed. “In town is the worst place.” But he believes in the integrated approach. “I really feel like we’re doing a better job than we used to,” Gordanier said.

Mosquito control remains hugely controversial in many places. In 2003 in Paonia, Colo., where spraying is highly unpopular, someone bombed the mosquito district’s warehouse, where malathion was stored. No one was injured. In La Plata County’s Animas Mosquito Control District, the May 2004 board election drew 725 voters and five candidates; the two victors were both write-ins.

The big picture

Environmentalists say it’s important to see the big picture. They believe the spread of West Nile may be exacerbated by global warming, which allows the mosquitoes to survive in more places; and by the decrease in bird diversity. Common birds such as crows, starlings and Canada geese seem to be more susceptible to infection, and their numbers are increasing, while many other birds are declining.

Carruth said he'll start his spring program in April, spreading larvicide along the Dolores River. He said the district board wants to remain flexible in its approach, because it’s impossible to say that sort of year it will be. “We haven’t been able to really predict West Nile,” he said. “We don’t know what the heck it's going to do.”