Rainfall, flooding produce more late summer ‘nuisance’ mosquitoes in Iowa

Rain showers bring flowers, but a few months later they also produce “nuisance” insects for Iowa — the mosquito — which have come out in droves over the last month and will continue biting into September.

Summer flooding in the northern areas of the state and increased rainfall this season have provided the ideal breeding conditions for mosquitoes, Ryan Smith, Iowa State University associate professor of plant pathology, entomology and microbiology, said. The flooding and rain left standing or stagnant water, which allows the young mosquitoes to mature and survive, resulting in a larger adult population.

Smith, who runs the Mosquito Surveillance Program at the university, said 110,547 mosquitoes were counted through July 31 in all three types of traps which are set up in 15 counties across the state. In one type of trap alone, the program counted 93,458 through July 31.

The largest numbers were found in Polk and Black Hawk counties. This is higher than the end-of-year totals over the past five years. The highest count for mosquitoes was in 2018 with 184,000.

Smith said this year’s larger population was the result of early warm temperatures and significant rainfall.

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Mosquitoes may serve as food for birds and bats, but Smith said he doesn’t know of any other purpose for them in the ecosystem. Mostly, they are a “nuisance” for humans who are bitten, resulting in itching and swelling. Some mosquitoes also carry disease, and those are the ones “we’re concerned about,” Smith said.

There is an elevated risk at this time of year — end of August and first of September — for West Nile virus, he said.

The State Hygienic Lab confirmed the first case of West Nile virus this year in Harrison County, according to the Iowa Department of Health and Human Services. The infected person was an older adult, between 61 and 80 years old.

West Nile virus is mostly spread through the bite of an infected mosquito, according to the Centers for Disease Control and Prevention. The virus circulates in the environment between mosquitoes, primarily the Culex species, and birds.

Symptoms include fever, headache, body aches, vomiting, diarrhea or rash.

Black Hawk County Public Health officials said last week the levels of the virus in adult mosquitoes have increased in the county. Because the increased levels of the virus are common at this time of the season, there likely will be more positive tests. They advised residents to take precautions to avoid mosquito bites.

“Black Hawk has been one of the hot spots in the last few years,” Smith said.

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The county department suggested using repellents containing DEET, wearing protective clothing, using air conditioning and keeping windows and doors shut. The public also is advised to remove or empty containers holding water, change water in bird baths and wading pools once a week and clean out rain gutters to prevent standing water.

Smith also said residents should avoid being outside at dawn and dusk, which are the most active times for mosquitoes.

Mosquito program monitors 100 traps in 15 counties

The Iowa Mosquito Surveillance Program has been tracking the species since the 1960s. One of the country’s most active and long-running programs led by a university traps mosquitoes to learn population trends by year, by county, by the sites where the traps are set up and what species are in the state.

The university staff scientists and students working in Smith’s lab count, sort and identify about 100,000 mosquitoes over the season but the program monitors about 100 traps in 15 counties, according to the program.

Smith said those areas are a good cross section of the state, which tends to have more open fields as opposed to the urbanized areas.

The location where the traps are placed are somewhat dependent on the willingness of local trained partners who would be available to monitor the traps, Smith said. Many of the partners are county health officials and the State Hygienic Lab, which tests the mosquitoes for disease.

Funding for the program comes from the state department of health, which receives funding from the CDC, he said.

Smith said more than 55 types of mosquitoes have been found in Iowa. The most common mosquito, Aedes vexans, isn’t a carrier of human disease.

According to the surveillance program, 19 mosquito samples have tested positive for West Nile virus so far this year. The only human case reported was the one in Harrison County.

In 2023, 96 samples tested positive for the virus and 17 human cases were identified.

The surveillance program raw data show the largest numbers of mosquitoes trapped at a given time have been in Polk County with about 42,675 and Black Hawk County had 34,573 this year. Other counties with mosquitoes trapped are Story, Johnson, Woodbury, Sioux and Page.

Comments: (319) 398-8318; trish.mehaffey@thegazette.com