Wet weather and a swollen Yampa River have left more than 1,000 acres of prime mosquito-hatching ground in Moffat County.
A puddle as small as a cow's hoof print can produce 200 to 300 mosquitoes, Moffat County Pest Management Supervisor Chad Sasges said.
The abundance of standing water has tripled the pest management workload.
"There's so much water out there that's hidden," Sasges said.
If the weather heats up, the mosquito population could grow. Hot weather speeds the hatching process.
"The cool weather has given us a chance to treat the larvae in the water," Sasges said.
Pest management workers use a powder to kill larvae before they hatch. Once the larvae hatch, workers use a fogger to kill them.
"We try to kill the adults before they move into populated areas," Sasges said.
Mosquito season should peak in Moffat County in the next few weeks, Sasges said.
Whether a high mosquito population increases the danger of West Nile virus is unclear.
Mosquitoes are the primary carriers of West Nile, but the virus isn't commonly a problem at higher elevations.
There were no reported cases of West Nile in Moffat County last year, but Rio Blanco County had one human case.
Gwen Lewis, supervisor for the Colorado helpline, said it is important that people at higher elevations don't assume they're safe from West Nile.
The helpline is a Colorado Department of Public Health program offering free information on public health matters.
Carrie Godes, director of Community Care, said while only a fraction of mosquitoes carry the West Nile, it is something the public should be aware of.
"It's always something we should take precautions with," Godes said.
Moffat County has tested 50 to 100 mosquitoes this year for West Nile and none had the virus, Sasges said.
The mosquitoes tested came from Craig and along the Yampa River.
Sasges said the county can use residents' help to keep the mosquito population down.
That means draining anything that can hold water for more than seven days, including fountains, old tires and buckets.