Moab Mosquito Abatement District Biologist Michael Carlson shows a mosquito larva collected from a pool of standing water in the wetlands. [Rachel Fixsen / Moab Sun News]

Moab may be a desert, but that doesn’t mean there aren’t mosquitoes. In the summer of 2019, mosquitoes were rampant in the city as the Moab Mosquito Abatement District was short-staffed. .

“It was just clouds of mosquitoes,” remembers Shannon Amsberry, technician supervisor for the Moab Mosquito Abatement District. He said he looked at a photo of himself taken at work in 2019 and counted 40 mosquitos perched on him in the picture.

This summer hasn’t been like that, which left some residents wondering why the district decided to use its truck-mounted fogging machine to disperse pesticides on two evenings in early September. MMAD Manager Michele Rehbein explained that the move was meant to be a quick, aggressive reaction to the detection of a concerning invasive species of mosquito called Aedes Aegypti, which can be a vector for diseases.

Moab Mosquito Abatement District

Rehbein joined the crew as district manager in May. She holds a PhD in environmental science and has been researching mosquitoes since 2014. In Illinois, she studied the Culex genus of mosquito and hopes to conduct similar research in the Moab area.

Biologist Michael Carlson, Amsberry and two technicians complete the district staff. The district is not a county department, but operates as its own governmental entity funded by property taxes within a 25 square mile boundary (its boundaries are not the same as those of Moab City.)

At the MMAD office, next to the Moab Recycle Center on Sand Flats Road, a garage shelters vehicles, bags of pesticides, and equipment. There’s a canoe hanging from one wall in the garage: the team uses it for occasional river monitoring. During mosquito season, which can begin as early as February and usually starts by April, MMAD staff monitor known mosquito breeding habitats. Amsberry recalled a recent year when the district was receiving bite complaints and couldn’t find the source of the mosquitoes; they investigated the interior of a river island and found that flood waters had filled basins on the island, leaving standing water in which mosquitoes could breed.

Staff use plastic cups mounted on long handles to dip into standing water to check for larvae. Adult mosquitos are caught in traps, which look like a cylindrical tent a little smaller than a five-gallon bucket and are baited with carbon dioxide. Black-and-white contrasting elements visually attract the bugs and some traps are baited with a “stink lure,” which particularly attracts Aedes Aegypti.

The traps are set and dips conducted wherever there’s breeding habitat. Carlson said there are about 150 trap sites. Some are on commercial properties or private properties with the cooperation of land owners, others are on city or county property and the Scott and Norma Matheson Wetlands Preserve.

Data on how many adults and larvae are found, their species and sex, and where they are found is logged at the office. Recently, over 20 years worth of paper entries have been entered into an online database called VectorSurv.

Among the desks, computers, files and reference books in the office, there are clear plastic containers with live larvae squirming in brownish water in the bottom, and a separate chamber in the top with dead adult mosquitoes. These containers allow the larvae to hatch into captive adults so technicians can gather data on what kinds of mosquitoes are breeding where the larvae were collected.

In a corner of the office is “the lab.” There’s a wooden desk with a microscope, a centrifuge, and vials of mosquito samples. There’s also a rapid analyte measurement platform, or “RAMP” machine, which can be used to detect the presence of viruses.

Carlson demonstrates: a vial of mosquitoes is put in the centrifuge to create a “mosquito smoothie.” The resulting goop is put onto a special slide that contains a tracer for West Nile virus and the slide is inserted into the RAMP machine, which gives a reading. Samples may be sent to state labs for more detailed information.

Some adult mosquitoes are also stored in freezers to be sent to state labs for DNA analysis. The district hopes to find out through that analysis whether the Aedes Aegypti detected in Moab this summer is related to the mosquitoes of that species that were found in Moab in 2019, or if they are a new introduction.

Aedes Aegypti

Aedes Aegypti are native to Africa and have spread all over the world. They can spread from place to place via human transport—on boats, trains, planes, or automobiles—and once in a habitable place, can become established.

“As temperatures are changing with climate change, these mosquitoes are increasing their range,” said Rehbein. Aedes Aegypti adapts well to developed, human-inhabited areas. They like to lay their eggs in containers, and may do so when the container is dry. The eggs stick to the containers’ sides and can wait for up to years for as little as a teaspoon of moisture to trigger them to hatch. The eggs are difficult to identify, staff said.

“They look like dirt,” Rehbein said, the size of a pencil nib and black.

Aedes Aegypti is active throughout the day, in contrast to most species which are most active in the mornings and evenings, and are particularly known for biting around the ankles. They prefer biting humans to other animals. In addition to being peskily comfortable in close quarters with humans, Aedes Aegypti is a disease vector.

“This mosquito can transmit diseases like Zika, dengue, chikungunya, and yellow fever viruses,” says a notice on the MMAD website.

MMAD staff detected the species in late August this summer, on a date close to when they were detected in Moab in 2019. Rehbein said MMAD quickly decided to fog to try to prevent Aedes Aegypti from getting established in Moab.

West Nile

West Nile virus has been detected in Aedes Aegypti, but it’s not the primary vector species for the disease, Rehbein said. Several species of mosquitoes found in the Moab area are also capable of carrying West Nile virus, and are more likely vectors. The virus was detected in some mosquitoes in the district last year, and also in 2019, when a resident was diagnosed with West Nile. Usually West Nile is unnoticeable or mild in humans, but it can occasionally be severe and even fatal.

According to the Utah Department of Health, 11 residents of Salt Lake, Davis and Weber counties had been diagnosed with West Nile virus as of Sept. 8. One death had occurred in the Weber/Morgan health district, and five more potential cases were pending confirmation. No West Nile virus has been detected in the MMAD area this year.

“Utah is now seeing the highest number of mosquito trap sites test positive for West Nile virus than we’ve had in the history of West Nile surveillance in the state,” said Hannah Rettler, the vectorborne/zoonotic epidemiologist for the Utah Department of Health. “West Nile virus is an annual presence in Utah and it isn’t going away.”


The best way to keep mosquito populations down and prevent disease is to reduce mosquito habitat and avoid getting bit. Aedes Aegypti especially is adept at making homes in easy-to-overlook places, like the drain trays of indoor or outdoor potted plants. MMAD staff ask residents to inspect their properties and empty any standing water that may collect in those trays or other containers like toys, tires, tarps, holes in trees, bird baths, pet dishes, and any other areas where water could collect, even in small amounts. The containers should be scrubbed to remove the sticky eggs that can persist even when the container is dry.

Fogging is one of the last measures the district will turn to in efforts to keep down mosquito populations. MMAD focuses on larvicide, which can be conducted on a very small area. One form of larvicide the district uses is based on the chemical methoprene, a growth regulator that disrupts the growth cycle and prevents mosquitoes from reaching their adult stage. It’s applied to water where larvae are found, but it can be toxic to other wildlife like fish.

Rehbein emphasized that all chemicals the districts use are applied strictly according to labels, which are regulated by the Environmental Protection Agency, the Centers for Disease Control, and various federal laws and regulations.

Fogging, which is aimed at adult mosquitoes, uses a chemical called permethrin, a synthesized version of a chemical found in chrysanthemum flowers. The chemical kills mosquitoes by entering their central nervous systems and interrupting their nerve channels.

“The most toxic thing that comes out of that is from the engine itself,” Amsberry said, referring to the fogging truck’s exhaust.

According to the EPA, pyrethroids (of which permethrin is one), “can be used for public health mosquito control programs without posing unreasonable risks to human health when applied according to the label. At high exposure levels, such as those resulting from accidents or spills, pyrethroids can affect the nervous system.”

Regarding pyrethroids and wildlife, the EPA website says, “When applied according to label directions, pyrethroids used in mosquito control programs do not pose unreasonable risks to wildlife or the environment. Pyrethroids are low in toxicity to mammals and are practically nontoxic to birds. However, pyrethroids are toxic to fish and to bees.”

Those risks, the higher effectiveness of larvicide, and public opposition to fogging are reasons the district fogs infrequently.

“This district has taken pride in how little we fog,” said Carlson.


Some Moab residents and other activists around the country question the use of fogging at all. Kaki Hunter is a longtime Moab resident and she questions the EPA’s approval of the chemicals used in pesticides. [See “Opinion: Reconsider mosquito fogging,” Sept. 16 edition. -ed.] She pointed to a report from the nonprofit Utah Physicians for a Healthy Environment, a Salt Lake City-based organization that advocates for environmental health, which calls the EPA’s research inadequate and outdated. The report references studies that link pyrethroids to impaired brain development, brain disorders and diseases, and autism and behavioral disorders in children.

The MMAD used to fog with a chemical called malathion. Malathion is in a group of chemicals called organophosphates, which the UPHE report says may cause impaired neurological development to mammals, including humans, even at low exposures. However, the EPA says that ground application (meaning fogging equipment mounted on trucks) “does not raise concerns.”

Longtime Moab resident Andrew Riley remembers that in the mid-1990s, the MMAD would fog twice a week throughout the mosquito season. He and his neighbors on Powerhouse Lane asked the district not to fog their street but were ignored. Riley still remembers sleeping with the windows open on hot summer nights when the MMAD truck went by.

“It was shocking to wake up and have your whole home just blown full of chemicals,” he said.

Riley and his neighbors barricaded their street one night to keep the fogging truck out. The MMAD director at the time told the truck operator to call the police, who arrived and removed the barricade. Riley and his wife stayed outside, but the truck operator sprayed anyway—directly onto them.

“It was completely against all the protocols for using these chemicals in a populated area,” said Riley. A recent label available for malathion reads: “Causes temporary eye injury. Harmful if absorbed through skin. Do not get in eyes, on skin or on clothing.”

Riley said they didn’t get sick from the incident, though they did worry about it. Riley joined the MMAD board shortly afterwards, and the district got a new manager, Bob Philips, who held the position for over a decade. Philips employed fogging very infrequently. Hunter said this past month was the first time she remembers fogging near her home in over a decade.

Riley said he understands the mandate that governs the MMAD, and appreciates the difference in practices now compared to decades ago.

“We’re vastly improved, and I think the town is much healthier for it,” Riley said.

Hunter said that she invited the MMAD staff to inspect her property for mosquitoes. MMAD staff set a trap that caught 14 mosquitoes, five of which were the Aedes Aegypti species. Still, Hunter is not convinced of the efficacy of fogging at all, favoring an education campaign to alert people to the Aedes Aegypti habitat: small containers that periodically collect moisture.

Hunter also said notification of the fogging was inadequate. She heard about it through a newsletter from a city council member and told neighbors, who she said were surprised to learn about it.

“This just came out of nowhere,” she said of the treatment.


Monitoring suggests that the fogging was effective at reducing the Aedes Aegypti population, Rehbein said.

“Before the fog there were over 200 Ae. Aegypti collected,” she said. “The week following the second fog, MMAD collected only four Ae. Aegypti adults.”

She noted that other methods, including larval surveillance and inspecting locations with standing water, also contributed to the reduction.

Once the mosquito season is over—likely in October but can be as late as December—Rehbein said district staff will spend time updating the agency’s Pesticide Discharge Management Plan. The document includes guidelines for taking certain actions like fogging. Rehbein also hopes to increase the district’s outreach and education efforts.