Aedes vexans mosquito [Image courtesy CDC]

Publisher’s note: This story was updated on July 2 to reflect that the way in which former Moab Mosquito Abatement District manager Bob Phillips assisted in the spraying operation was by calibrating the fogging machine.

Publisher’s note: This story was updated on June 26 to reflect that the Moab Mosquito Abatement District has modified their plans to use a chemical fog to combat the mosquito nuisance problem.  

Heather Palmer is a homeowner in a neighborhood near the Scott M. Matheson Wetlands Preserve on the western side of Moab. She said the mosquitoes around her home have been so bad lately that she and her family have been trapped indoors.

“It’s really frustrating when even in the daytime you can’t go outside (due to the mosquitoes),” she said.

On June 25, the local mosquito abatement district said it is planning to combat the mosquito nuisance problem in Moab by spraying a chemical fog, Biomist 4+4, on Wednesday, June 26, beginning just after 9 p.m. if conditions allow (higher winds would prevent application, as would temperatures below 50-degrees Fahrenheit).

Biomist 4+4 is a synthetic mixture that includes permethrin, a compound derived from the chrysanthemum plant, and a petroleum distillate. Permethrin is found in medications to treat lice and scabies in humans.

“It’s a quick knockdown,” Moab Mosquito Abatement Manager Libby Nance said. “It knocks down the adults and prevents them from laying eggs.”

The primary focus of the fogging is planned to be the wetlands on the western side of town, which is where the mosquito population and breeding grounds are concentrated.

“We’re trying to knock them down in their habitat, in the trees and the shrubs,” she said.

When the mosquito abatement district first announce its plan to use a chemical fog, the fogging was going to come close enough to homes that the abatement district advised residents to close windows, turn off swamp coolers and take other precautions to prevent the insecticide from wafting into the home. However, the district revised its plan after receiving feedback from the hospital and members of the public with concerns about air intake.

Nance said that, due to those concerns,“We are concentrating on the parcels that are abutting the eastern boundary of the sloughs.”

Nance said that the spray will be truck- or UTV-mounted and will be dispersed in a fine aerosol mist as the truck travels back and forth from one side of the wetlands to the other. She said there is a fire road called Higgins Lane that goes along the eastern border of the sloughs and the current plan as of midday June 26 was to mount the fogger on a UTV and use that road.

“The [fogging] machine is incredibly loud. People will know we’re in the neighborhood,” she said.

Nance could not say precisely how long the fogging process would take once it commenced, since the district has not done this before, but expected that even with the change in plan it would likely take until around midnight. She said Bob Phillips, the previous mosquito abatement district manager, would be assisting with this fogging operation by calibrating the fogging machine.

Previously, the district’s pest management plan had only authorized fogging after detecting West Nile virus, a potentially deadly disease, in the local mosquito population. West Nile virus has not been detected so far this year. The decision to include high numbers of mosquitoes as a reason to spray came in response to public feedback on the impact of the mosquito infestation that has plagued Moab in recent weeks.

A wetter-than-average year has resulted in higher river flows and an unusually large population of mosquitoes, what Nance calls a “major nuisance hatch.”

“It’s just critical mass right now,” Nance said.

Nance said that her office has been flooded with phone calls, emails and in-person feedback that something needed to be done to mitigate the bloodsucking swarms.

With some members of the public requesting the use of spray and others against it, Nance said she feels the district has made a fair compromise with their decision to stay out of neighborhoods and concentrate on the eastern boundary of the wetlands. She also said that mosquitoes already in neighborhoods will have to come back to the sloughs to breed, and she is hopeful that the pesticide will get those mosquitoes when they fly back in.

“We’ve had to change our policy in order to [fog]” she added.

Nance said she contacted the state to confirm what was required to change their pest management plan (officially called a Pesticide Discharge Management Plan, or PDMP). She said the mosquito abatement board reached consensus on adding a clause to their pest management plan to say that they will use a chemical spray when 250 mosquitoes of the mosquito species Aedes vexans are found in their traps, which is the average threshold for spraying in Utah. Nance said that they are regularly finding around 10,000 mosquitoes in the Moab traps.

“That shows us what the pressure is,” she said. “It’s exorbitant.”

This change to the pest management plan does not require the mosquito abatement district to alert the public, nor vote on the proposed change at a publicly noticed meeting, she said. The board has been communicating about the change via email.

“There’s nothing about the PDMP that has to be run through the public,” Nance said.

But, Nance added, she is aware of and sympathetic to the public’s concerns about pesticides, including the potential impacts to bee populations and other pollinators.

“I do understand the beneficial insects. I’m an entomologist,” Nance said. “But people are suffering.”


Biomist 4+4 is “highly toxic to bees exposed to direct treatment on blooming crops or weeds,” according to the product’s label.

Nance said she has conferred with Grand County Beekeeper Jerry Shue and determined that there are five hives located in areas likely to be affected by the fogging. She said the beekeepers are “on board” and will be covering their hives during the fogging.

Shue said he appreciates her diligence and communication.

“I realize this is an exceptional situation,” Shue said. “You have to weigh a lot of things. The abatement board is under a lot of pressure from the public … people are demanding action. I can live with that and I’m just happy we are communicating. The beekeepers will take certain steps to minimize any issue.”

In addition to posing hazards for bees and other pollinators, the product is “extremely toxic to aquatic organisms, including fish and invertebrates,” the label states.

Nance said she is aware of this and the district will be minimizing danger to the local aquatic life.

“We spray in the trees and the shrubs,” she said. “I will not be applying over bodies of water.”

However, Nance acknowledged this treatment is not completely without risk — no insecticide is— but she believes that this is the best option given the circumstances.

“This is the least toxic. And that’s why I picked it,” she said.

Nance said that if there was a silver lining to be found, it would be that the district will have some practice with using Biomist 4+4 and the fogging machine if West Nile virus is detected at a later date and the district has to fog under more urgent circumstances, with people’s lives potentially at risk.

“Hopefully, we won’t have to do it often,” Nance said of the fogging. “But we’ve got to do something.”

It was uncertain as of press time whether the spraying would take place as planned due to higher wind speeds.

Sheer number of pests drives decision

“Hopefully, we won’t have to do it often. But we’ve got to do something.”

The mosquito abatement district advises that residents living near the sloughs should stay inside while the fogging is taking place on Wednesday, June 26, from 9 p.m. until approximately midnight. They should also turn off their swamp coolers, close windows and otherwise prevent the possibility of the insecticide entering their home.