With a backpack full of specialized bacteria, a force of technicians patrols Boulder-area still waters for miniature pests that can metamorphose into airborne vectors of disease.
Acting like the Ghostbusters for mosquitoes, technicians seek out larvae and douse them with a spray of tiny bacteria called Bti — Bacillus thuringiensis israelenis. Bti is derived from naturally occurring soil bacteria and act as a larvicide that kills infantile mosquitoes before they can emerge from their watery breeding grounds.
Mosquito control accomplishes more than just improving the outdoor activities that Boulderites love, it helps prevent a sometimes-fatal disease: West Nile virus.
“[West Nile virus] is a preventable disease, and the goal of our program is to reduce that risk,” says Marshall Lipps, environmental health specialist with Boulder County Public Health.
Most people would never know they were infected with West Nile virus, but one in five will contract West Nile fever, which can last weeks to months. The rare few, less than 1 percent, will develop a neuroinvasive disease like encephalitis or meningitis, which inflame the brain or surrounding tissue. These severe infections can cause permanent damage and even death.
As of July 22, West Nile virus was found in mosquitoes in ponds at Stazio Ballfield and Christensen Park in the City of Boulder. The first mosquitoes to hit Boulder County were found in three areas in Longmont on July 9: Jim Hamm Nature Area, St. Vrain Greenway at Emory Street and Lefthand Creek at Creekside. Across the state, mosquitoes in Denver, Larimer and Weld counties tested positive for West Nile virus and a human case was reported in Mesa County on July 8.
“We’re not out to exterminate mosquitoes, but we’re there to minimize their numbers, which in turn reduces that risk of West Nile [virus] transmission,” Lipps says. “And then the rest of it can be taken up by individual residents through wearing repellant and avoiding those mosquito bites from the mosquitoes that are out there.”
The City of Boulder has an especially targeted program that aims to prevent West Nile virus while minimizing environmental impacts.
“The main difference between Boulder’s program and other programs in the state is that ours is really targeted towards West Nile virus instead of just mosquitoes in general,” says Rella Abernathy, integrated pest management coordinator with the City of Boulder.
Technicians working in the City of Boulder are trained to recognize the difference between nuisance mosquitoes — ones that bite, but don’t transmit disease — and vector mosquitoes that transmit diseases like West Nile virus. The city only treats areas where vector mosquitoes are breeding, with a few exceptions like around the golf course and some ball parks where both human and mosquito traffic are high.
Vector mosquitoes belong to the genus Culex. They can be identified with the naked eye thanks to distinguishing characteristics like long, up-curved siphons at the end of their tails that they stick out of the surface of the water to get oxygen.
While Bti is not toxic to bees, fish or amphibians, it can kill blackflies and some nonbiting midges.
“We don’t want to disturb the natural balance and biodiversity and impact those other species, those nonbiting midges, that are important to the ecosystem,” Abernathy says. “They are the foundation of the food web and they’re fed on by a whole lot of other animals.”
The City of Boulder also differs from other municipalities in its decision to abstain from commonly used adulticides. Larval treatments are much gentler on the environment than adulticides — synthetic chemical sprays for killing adult mosquitoes. The city lists adulticide as an option only in the worst-case scenario.
“In the past we’ve never been in the situation where the risk was high enough that [spraying] was justified,” Abernathy says.
Both the city and county of Boulder contract OtterTail Environmental to control the local mosquito population. The Colorado-based company developed the vector index, which uses the population of Culex mosquitoes and the infection rate to determine the likelihood of humans contracting West Nile virus.
If the city reached a high enough vector index, Abernathy says they might consider using targeted adulticide treatments where mosquitoes are likely to be found instead of fogging the streets. This would reduce the likelihood of humans, animals and nontarget insects being affected by the spray.
“Adult mosquitoes, during the day, tend to hang out in cooler areas and vegetation because they’re tiny little insects, and they dry out in the hot sun,” Abernathy says. “They tend to find these cooler areas of vegetation and that’s called harborage. So it’s possible that you could identify the hot spot areas where you had likely West Nile virus infections and you could just treat that vegetation.”
OtterTail monitors mosquito populations including number and species throughout the county on a week-toweek basis and tests the bugs for West Nile virus. Mosquitoes are attracted to the breath of their next blood meal, so OtterTail’s traps are rigged with a cooler full of dry ice.
“We put dry ice in a cooler and that emits carbon dioxide throughout the night, which attracts the mosquitoes,” says Joe Cox, biologist with OtterTail. “It mimics our breathing, or exhaling, so it attracts mosquitoes to the trap and then they’re further attracted to a light source and then blown down through a fan into the net that collects them.
“We bring those mosquitoes back to the lab, we freeze ’em, identify ’em and then depending on the numbers we may spray in those areas if there are high counts,” Cox says. Thresholds that trigger spraying are higher in the City of Boulder than elsewhere, where spraying is a preventive measure rather than emergency procedure.
When Boulder County turns to adulticide sprays to attack mosquitoes that have escaped larvicide treatments, they use a permethrin-based spray. Permethrin is a synthetic insecticide derived from chrysanthemum flowers.
“[Permethrin] disrupts the ability of the mosquito to fly,” Lipps says. “It starts to disrupt some of the chemical signals within the mosquitoes … that’s how the mosquito ends up dying is they just fall down to the ground.”
The county chose permethrin over organophosphates like malathion, which have been in use longer. The Environmental Protection Agency (EPA) lists both as toxic to fish and insects, including important pollinators like honeybees. High doses of either chemical are known to affect the human nervous system, but because pesticide applications use ultra-low volume spray, the EPA maintains that they don’t pose an “unreasonable risk” to humans or the environment.
“Even if you do come in contact with [permethrin], we have the ability to break this product down very quickly, so it doesn’t tend to leave any long-lasting effect,” Lipps says. “The biggest risk is probably to an applicator having an acute exposure if they’re mixing it or something like that rather than the general public as we’re out spraying in the neighborhood.”
It takes contact with only one diluted droplet of pesticide to kill an adult mosquito. In lieu of chasing mosquitoes with a squirt gun full of pesticides, Boulder County uses truck sprayers. The spray comes out in drops so small that they remain suspended in the air, so mosquitoes can fly into them. This method has been criticized as inefficient because mosquitoes have to fly into the sprayed area to be affected, while larvicide can be applied directly to mosquito-ridden waters.
Larvicide is also cheaper than adulticide, but larvicide requires a larger physical task force. Spraying the streets from a truck at night covers a lot more ground than someone checking each area of standing water and applying treatment by hand. The City of Boulder spends $240,000 a year on mosquito abatement, while the unincorporated areas of Boulder County have a $313,000 contract with OtterTail.
Both the city and county stress that preventing West Nile virus does not end with their programs, urging residents to take steps to protect themselves. Together they promote the four Ds of prevention. Drain standing water around your house. Limit outdoor activities at dusk and dawn when mosquitoes are most active. Dress in long sleeves and pants. Wear a DEETbased repellent or proven alternative like lemon-eucalyptus oil or picaridin.
Anything from a birdbath to a kiddie pool to an over watered lawn can become a breeding ground for mosquitoes. They will lay eggs in as little as one inch of standing water if it persists for a week or more.
“These Culex mosquitoes tend not to travel very far from the area that they emerge,” Abernathy says. “So if you have mosquitoes breeding in standing water in your yard, they’re not gonna travel very far, they’re gonna be hanging around your neighborhood.”
Culex populations spike in the end of July and throughout August when the warm weather accelerates their lifecycle. The time from egg to adult mosquito is usually one to two weeks, but can be as short as four days under the right conditions.
“If we continue to get this rain and then have several weeks of very hot weather, I would certainly expect to see some large increases in mosquito populations,” Lipps says.
“So it’s beginning, we’re there,” he says. “When our first positive will come in, it could be this week, it could be next week. There’s no way to predict that.”