Mosquito control

Managing disease comes with environmental concerns

James Gathany/Center for Disease Control

It’s an unfortunate fact of life that along with those endless dog days of summer — perfect for barbeques, biking, hiking and swimming — also comes the height of mosquito season. While these blood-sucking members of the fly family are often little more than nuisances, certain species of mosquitoes can carry dangerous diseases, such as West Nile.

The bird-borne virus hit Colorado hardest in 2003 when nine people died in Larimer County. In 2013, there were 322 reported cases of West Nile in the state, with seven of those resulting in deaths. The Colorado Department of Health and Human Services confirmed this year’s first two human cases of West Nile virus just last week in Saguache and Pueblo counties. And after a year of abundant moisture and early summer temperatures, conditions are ripe for mosquitoes to breed.

Cities can and do work to control mosquito populations, though strategies may vary among cities in the same county. There are two main methods for controlling mosquito populations: larviciding, which uses a naturally occurring soil bacteria to kill mosquitoes before they hatch, or adulticiding, which kills the bugs when they’re full grown via a chemical called permethrin.

Permethrin-based sprays concern some folks as the chemical can, according to toxicity data from the Environmental Protection Agency, kill beneficial insects such as bees. In 2013, Fort Collins City Councilman Bob Overbeck spoke for concerned residents and said that the city should review the health and environmental effects of mosquito treatments. This year, the city of Fort Collins announced it wouldn’t start spraying against West Nile virus until two humans are infected in the city.

Many vector control experts disagree with Fort Collins’ choice to wait until humans contract West Nile.

“When you are trying to stop transmission of [West Nile from] mosquitoes to humans and you’re waiting for humans to be your indicator as opposed to infected mosquitoes, you’ve already passed [the window to stop transmission],” says Jessica Schurich, Colorado Mosquito Control’s operations manager for Northern Colorado. Schurich manages mosquito abatement programs for the cities of Longmont and Fort Collins. She says that because some people don’t show symptoms of West Nile for as much as two weeks, there’s a delay in the investigation of reports.

“If you’re waiting to find humans, the likelihood you’re really going to stop transmission [decreases], especially late in the season,” she says. Last year, she adds, “[Fort Collins] basically waited until pretty much most of the cases had already occurred. I do not support [Fort Collins’ city] council’s decision. I think when you’re again trying to stop a public health emergency or an increase season for risk, waiting until you find [infected] humans is not the way to do so.”

Al Summers sat on Boulder County’s Mosquito Control Advisory Board from 1998 to 2011 and knows firsthand how contentious an issue mosquito control can be.

“How programs are financed is through taxes and that goes into a general fund for mosquito control programs,” Summers says. “The problem with that is those are tax-paying citizens and they feel they have right, and they do have legal right, to have a say-so in this. A lot of the meetings when I was on the control board dealt with issues about whose concerns take precedence over whose concerns.”

Summers, who has also been a beekeeper for 50 years, maintains that permethrin’s impacts to human and environmental health are almost nonexistent.

“If you drank the stuff or lived in it all the time, yeah, but the amount of exposure people get from mosquito control, it’s not in that same league at all,” Summers says. “There’s a tendency for over-reaction in this. Pyrethroids [the organic compound in permethrin] are not benign, but they are orders of magnitude less toxic and problematic than products they’ve used in the past.”

Cities apply permethrin around problem zones — standing bodies of water, such as ditches and reservoirs — using ultra-low-volume spraying. The tiny droplet size allows the spray to degrade quickly, posing fewer threats to the environment, according to Summers. Spraying is done at night, when bees are in their hives, and by the morning the spray has evaporated.

Tom Theobald, founder of the Boulder County Beekeeper’s Association, agrees that permethrin, used in ultra-low volumes at night, is likely not posing great risk to human and beneficial insect health. But with bees around the nation dropping in numbers — something referred to as colony collapse disorder, which scientific studies have linked to increased pesticide use — Theobald questions whether spraying is warranted at all.

“There are lots of opportunities for things to go wrong,” he says. “Since there’s no vaccine for West Nile, you are really dependent on your genes as to how you respond to it. You have two choices: you face [contracting West Nile] at some point in your life, or you avoid getting bitten by mosquitoes forever. Spraying just postpones that. We may be postponing the exposure for this small segment of the population [who contracts West Nile] and I sympathize with them — it’s tragic but unavoidable.”

While some municipalities in Boulder County have been spraying since the early summer — Longmont has sprayed weekly since June 18 — the City of Boulder does not use adulticides on the streets as other cities do.

“The whole goal of our program is to reduce public risk of contracting West Nile, trying to balance that with minimizing impacts to the environment,” says Rella Abernathy, integrated pest management coordinator for the City of Boulder. To do this, the city uses a naturally occurring larvicide called Bti. Tablets or so-called “dunks” of this soil bacteria are dumped in standing bodies of water where mosquitoes breed.

“The place to reduce numbers the most is when they’re larva,” says Abernathy. “They are all in one place, not flying around, and we’re using a product that has fairly minimal impacts to other organisms.”

Abernathy points to the fact that adult insecticides are broad-spectrum neurotoxins.

“The way they work, they use these little tiny droplets that have to come into contact with mosquitoes [to kill them]. There are a lot of areas where mosquitoes are never going to come into contract with the droplets,” she says. “Some experts believe you need to control 90 percent of mosquito populations to control West Nile, and there are various studies showing adulticides only result in about a 40 percent reduction.”

The city of Boulder contracts OtterTail Environmental to help with their mosquito abatement. This group of biological consultants map larval sites and then conduct testing to differentiate between nuisance mosquitoes — those that harass people but don’t carry disease — and the species known as Culex tarsalis, which does carry West Nile. Boulder only treats areas with disease-carrying bugs. OtterTail developed a vector index that acts as a predictor of whether human cases of West Nile will occur.

The decision to focus only on the presence of vector mosquitoes — those capable of carrying disease — was controversial in 2003 when the city’s abatement program was implemented, but the Centers for Disease Control eventually adopted Boulder’s approach. The vector index is now used in many municipalities across the country.

But Summers says he understands why cities choose to spray instead of focusing on larval killing.

“It’s cheaper for [cities] to do a widespread spraying for mosquitoes than it is to invest the extreme time and money into a [larval] surveillance program,” he says. “You can be proactive and eliminate about 80 percent of mosquito problems by monitoring and treating at the larval stage rather than waiting until spring, but it costs so much more in staff and time, and samples have to be brought back to a lab and checked and confirmed.”

Abernathy says that every mosquito abatement program she’s aware of on the Front Range uses both techniques.

The City of Boulder spends around $240,000 a year on mosquito abatement, Abernathy says. In 2013, Fort Collins spent around $77,000 on just two sprays. Reports from Longmont show that a single citywide spray there costs $40,000.

The bottom line, says Schurich, is that West Nile is not something to be taken lightly.

“One of my residents has told me that she’s unable, 10 years later, to cut her own food [after contracting West Nile],” says Schurich. “From where I stand, that’s devastating and that is the justification for doing surveillance, not even mass spraying or calendar spraying, but unfortunately we have areas where we’re trending 85 to 90 percent of the mosquitoes in our traps are West Nile vectors.

“We’re not just spraying to spray,” she adds. “You assess the population on a weekly basis because those numbers can change just from all these environmental changes — is it warmer, was there more rainfall, is it more humid, are they living longer? I don’t think a lot of folks realize that disease management is done on a daily basis. You’re constantly assessing the data to makes sure you’re using good science and good data collection to drive your applications of Bti or adulticides.”

Summers agrees. 

“We’ve got an issue with West Nile Virus in Colorado and as far as the actual incidence of it, it’s not too bad, but from an epidemiological point of view, telling the population that there won’t be 100 people dying, there will only be one or two, that’s not acceptable to the general public.”