'Above Average' Mosquito Populations Likely Due to Heavy Summer Rains

Your financial contribution empowers watchdog reporting that protects and informs us all. Since the late 1960s, researchers at Iowa State University documented and studied mosquito species, diseases and population trends in Iowa. Data for 2025 show mosquito populations have remained high this summer. Ryan Smith, an associate professor at ISU and director of the Iowa Mosquito Surveillance, said there are more "nuisance" mosquitoes, or the type that like to bite an exposed ankle at an evening barbecue, persisting into the beginning of fall because of the heavy rainfall Iowa saw in July. Smith explained that of the approximately 56 types of mosquitoes in Iowa, the majority tend to be the nuisance variety that, while annoying, are not the type of mosquitoes carrying diseases like West Nile virus. These nuisance varieties, he said, are also "really influenced" by rainfall. Mosquito population trends in 2024 vs 2025. May = weeks 20-22, June = 23-26, July = 27-30, August = 31-35, September = 36-39, and the beginning of October =40. (Graph courtesy of Iowa Mosquito Surveillance) West Nile virus West Nile virus, which is the primary mosquito-borne disease in the country according to the U.S. Centers for Disease Control and Prevention, is carried by Colex species of mosquitoes. Smith said the lab at ISU has observed there are fewer Colex mosquitoes this year, but the prevalence of West Nile virus in that population is at a "really high level." Most infected individuals do not develop symptoms, but those that do, can develop mild symptoms like a fever, headache, stiffness or vomiting. Neck stiffness, high fever, confusion, muscle weakness or tremors can be a sign of severe illness from the virus, which can affect the central nervous system. Individuals with these symptoms should see a doctor, according to the CDC. In the U.S. approximately 1,200 develop severe illness, and approximately 120 people die from West Nile virus annually. Already, Iowa has observed one case of West Nile virus this year. Iowa Department of Health and Human Services reported Aug. 14, the infected individual was in their early 40s and from northern Iowa. Neighboring states, like Minnesota, South Dakota and Nebraska, already have between 15 and 25 cases each, according to CDC data. According to Iowa HHS, 21 Iowans were diagnosed in 2024 and one death was associated with the virus. Late August and early September is typically when the number of infected individuals peaks. "It's important to remind people to stay vigilant … especially outside in the evening, when these Culex mosquitoes are typically the most active," Smith said. Preventive measures can include removing standing water from around a home or backyard, wearing insect repellants that contain DEET or wearing long-sleeved shirts, pants and socks when outdoors in the evenings. Mosquito data Iowa Mosquito Surveillance sends out traps to various partners, like parks and recreation staff or county public health departments, who are taught how to use the traps and then send them in to the lab, where Smith and his team analyze each trap. Smith said while it's hard to compare data from year to year, based on several factors, this year has been an "above average" year compared to a 20-30 year average. Smith said when there are instances of heavy rainfall, or flooding in areas, it can lead to stagnant water that mosquitoes can "can take advantage of" to "really thrive and amplify their population." Precipitation accumulation in Iowa from April 1, 2025 through Aug. 15, 2025. (Map courtesy of Iowa Environmental Mesonet)The Iowa Mosquito Surveillance does more than provide a baseline of mosquito data for the state. Smith said it also helps inform public health entities that alert the public of potential mosquito-related risks and it can help municipalities decide if they're going to spray for mosquitoes and how to manage the spray. Several cities across the state spray regularly for mosquitoes, but notably, the City of Des Moines stopped its spraying program this summer to save an estimated $300,000, according to reporting from Axios. Precipitation accumulation in Iowa from April 1, 2025 through Aug. 15, 2025. (Map courtesy of Iowa Environmental Mesonet) The City of Des Moines said the decision was made due to budget constraints. The last spray it completed was at the beginning of May when it sprayed for mosquitoes in the larval stage. Smith said while he's sure the decision not to spray leads to higher populations of mosquitoes in an area, there are too many other factors impacting mosquito populations to say definitively the city's decision had an impact on Polk County mosquito populations. Knowledge is power! Stay empowered by subscribing to the free newsletter, packed with valuable insights, stories of local newsmakers and ideas to get your weekend started.