Why mosquito season came early this year

Editor’s note: This story originally appeared in RED in July 2019 and has been updated to ensure that all information is relevant and accurate. 

Colorado’s strong snowpack and a wet spring eliminated drought conditions statewide for the first time since 2019, but that combination may also contribute to a banner season for mosquitoes.

It’s all part of the complicated collections of ecosystems that Coloradans — and 40-plus species of mosquitoes — call home. But with mosquitoes in Arapaho, Delta, Boulder, Larimer and Weld counties testing positive for West Nile virus weeks earlier than in previous years, it’s more important than ever for residents to understand the life cycle of the insect to avoid bites and potential infection.

At lower elevations across the Front Range, the mosquito species Culex pipiens and Culex tarsalis are the state’s principal vectors of West Nile virus, and they could be on the verge of intense outbreaks, said Robert G. Hancock, Ph.D., a professor in the Department of Biology at Metropolitan State University of Denver and an author of many scholarly articles on bloodsucking insects.

Culex tarsalis primarily lay eggs in wetlands, especially cattail marshes such as those formed from water that seeps out below the dams of Colorado’s reservoirs. And they mature much more quickly after hatching than other mosquito species. “It can be on your arm in 12 days after hatching,” Hancock said.

Culex species aren’t the only mosquitoes with elevated populations this year. This month, Hancock told Fox31 he had seen “epic numbers” of all species of mosquitoes in his traps.

RELATED: What’s behind Colorado’s West Nile virus surge?

Mosquitoes have a reputation for reproducing by laying their eggs in standing water, but some species, such as the Aedes vexans or “summer floodwater mosquito,” hatch after hot thunderstorms. They are more likely to have started out life in depressions in the ground that fill up with rainwater, allowing the eggs to hatch. Females can keep stockpiling eggs in the same depression between storms, then the larvae hatch all at once.

“That’s when we can get incredible numbers of these kinds of floodwater mosquitoes,” Hancock said. “We’ve gotten to some hot, thunderstormy times, and I think that means we’ll get some hot spots and lots and lots of mosquitoes coming out.”

Then there’s Aedes hexodontus and Aedes pullatus, a.k.a. snowmelt mosquitoes. In the Sangre De Cristo Mountains, locals and hikers have reported swarms of skeeters stretching to the treetops. Each of those mosquitoes started as an egg laid in the earth last summer. Winter snows bury the eggs, which are protected by a thick outer layer, and the more the better, Hancock said. In fact, the more snow and the longer it takes to melt, the better the chances that the eggs of this species of mosquito will hatch as larvae and mature into adults.

“The rate of development from egg to the pupal stage is entirely temperature-dependent,” he said. While snowmelt mosquitoes bite, they’re not known to be a vector for any human diseases.

For Eric Reeve, a Biology major at MSU Denver who helped Hancock perform experiments on several mosquito species in Colorado, studying the bloodsucking arthropods in the lab is a “dream come true,” he said.

While he respects that mosquitoes are vectors of serious human diseases, Reeve said the research has helped him gain more respect for mosquitoes as “an integral part of the local ecosystem,” with other species relying on mosquitoes as a food source.

“They became less of a nuisance bug to me,” he said, “and became a creature that has an actual purpose.”

Still, it’s best to keep the mosquito population down, especially in densely populated cities.

Hancock’s suggestions for mitigating mosquitoes:

• Flip over waterlogged, slime-coated tarps and buckets where mosquitoes often breed, particularly in urban areas.
• Be aware that mosquito activity peaks at dawn and, especially, dusk in the Front Range.
• Use insect repellent that contains DEET. “I’m a mosquito professional, and I’m 100% in favor of repellent containing DEET,” Hancock said.
• Wear breathable, long-sleeve, light-colored shirts and long pants; spray DEET on your clothes instead of directly onto your skin.
• Take a head net with you to the mountains.