ST. GEORGE – Southern Utah residents have been fighting off swarms of mosquitoes in the wake of recent heavy rains, and even those in charge of controlling the pesky insects are having trouble keeping up.
Thousands, if not millions, of mosquitoes have been reported in neighborhoods across Southern Utah, including Little Valley, Washington Fields, Dixie Springs, Hurricane, Bloomington and Bloomington Hills.
Paula Smith lives near the south end of Little Valley Road and said she was bitten by a mosquito last week – for the first time since moving to the St. George area in 2006.
Smith said she first noticed the problem when a landscaper showed up to begin work at her house.
“He kept swatting himself,” she said, “and I asked him what was wrong, and he goes, ‘I have been bit like six times since I’ve been here,’ which was only like 10 minutes.”
She brought the man some bug spray and said she thought nothing of it until two more neighbors reported getting bitten, including one who received 15 bites just while mowing the lawn.
After mentioning it on Facebook, Smith said, another dozen friends reported swarms of mosquitoes in their neighborhoods.
So what is going on? Is it dangerous? And will it ever stop?
The problem, it turns out, is an unusual hatch of “floodwater” mosquitoes in the wake of heavy rains two weeks ago, according to the Southwest Mosquito Abatement and Control District.
Within three days of the heavy rains, the district was getting calls about lots of adult mosquitoes flying, and the problem got worse as district crews searched for the sources of the infestation.
While the mosquitoes are aggressive biters that are active day and night, there is no health risk, District Manager Sean Amodt said.
“There’s a bright spot in this,” Amodt said. “You don’t have to worry about West Nile virus, because floodwater mosquitoes don’t carry the diseases.”
The district runs regular routes throughout the county, checking for mosquito larvae in all the places standing water is known to occur and then treating the source of infestation. However, the recent heavy rains have left Amodt and his small crew scrambling.
“Those last rains were quite fruitful – the perfect temperature, the perfect amounts of water,” Amodt said. “There were quite a few new places that were being filled up (with water) that we had no idea about.”
Most species of mosquitoes lay eggs in standing water and are more easily controlled by the district, but floodwater mosquitoes lay their eggs in mud, where they can lie dormant in dry washes, fields or depressions for 10 years or longer. Each female floodwater mosquito can lay 100-200 eggs.
“They are perfect for this area,” Amodt said.
When will it end?
Relief is in sight, Amodt said – at least for the current wave of floodwater mosquitoes. While Amodt and his staff have been finding and treating every infested source they can find, there is also a natural limit to the infestation.
“We’re past the peak,” Amodt said. “They have a life cycle of two to three weeks, and within a week they will start disappearing.
New breeding grounds
While dry washes, slickrock ponds and other natural depressions are prime spots for floodwater mosquitoes, some breeding grounds are man-made.
Flood irrigation, which is common in agricultural areas of the county, can be a big problem for the mosquito district, Amodt said. Irrigation water runs through fields and pools at the bottom, leaving puddles for several days, which is long enough to hatch generations of mosquitoes.
Recently, Amodt noticed retention ponds being built in new developments, and as the ponds retain rain water, they also allow mosquitoes to thrive. Some of the newer parks, including the Treasure Valley Park in Washington, have grass-lined retention ponds.
These ponds are difficult to treat because the grass interferes with the bacterial larvicide used to kill mosquito larvae. The surfactant normally used to spread the larvicide over the surface of the water will not work if grass lines the bottom, and other pest control options can kill the grass.
“It’s a little troubling. We’re not getting the kill rate we had hoped,” Amodt said. “We can’t use the larvicide as effectively.”
West Nile virus
Floodwater mosquitoes are not carriers of disease, and the other types of mosquitoes are well under control, Amodt said.
No West Nile virus has been detected in mosquitoes in Washington County so far this year, he said.
The district sets 36 traps across the county each week using carbon dioxide and light. All the collected mosquitoes are tested immediately for West Nile and Western Equine Encephalitis.
Last year, 12 or 13 traps tested positive for West Nile.
Only a small percentage of people bitten by a mosquito carrying West Nile virus will get any symptoms at all, but for 1 to 2 percent it can cause serious, even fatal problems. Anyone with concerns about a mosquito bite should contact a health care professional.
The mosquito district is the only one in the state that has its own equipment to test for West Nile disease, Amodt said. He can get immediate results, rather than waiting two weeks to hear back from the state lab. This is a huge advantage in control efforts because mosquitoes can breed so quickly.
What to do around your home
Mosquito traps that use CO2 are not recommended by Amodt. The traps attract a lot of mosquitoes but not all of them will be caught, and the remainder will bite. Here are some other tips:
- Empty or cover anything outside that can hold water.
- Filter permanent ponds regularly or put fish in them. Fish eat mosquito larvae.
- Keep pools chlorinated or drained completely.
- Clean clogged rain gutters and storm drains regularly.
- Manage irrigation water effectively.
- Store old tires inside, or cover them.
- Report other mosquito breeding sites to the Mosquito District.
- Report mosquito problems to the Mosquito District by calling 435-627-0076 or by email
- Southwest Mosquito Abatement and Control District website
Click on photo to enlarge it, then use your left-right arrow keys to cycle through the gallery.
- Public hearing: Southwest Mosquito Abatement and Control District
- Rains come down, floods come up; STGnews Videocast, Photo Gallery
- When it is stormy, be flood smart; what to do after flooding
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