CLARKE MOSQUITO CONTROL
May 2026 UPDATE
As we move into June, the 2026 mosquito season is underway across the Chicagoland region. Our program focuses on protecting public health by minimizing vector disease risk, particularly West Nile Virus (WNV), while proactively managing mosquito populations.
Spring 2026 has been a story of contrasts. April was the 7th wettest on record for the Chicagoland area, with precipitation running more than three inches above normal—a notable departure from the drought conditions that shaped much of 2025. May then swung in the opposite direction, ranking among the driest on record for both Chicago and Rockford. While the dry May tempered some of April’s surplus, the early-season moisture did create expanded breeding habitat for floodwater mosquitoes. With above-normal temperatures forecast through summer and WNV already detected in 8 Illinois counties, these are conditions worth staying ahead of as the season progresses.
TO KEEP UP WITH THE LATEST DATA, USE THE CLARKE CUSTOMER PORTAL
The exceptionally wet April has created prime conditions for Aedes vexans, the floodwater mosquito and key nuisance species in the Chicagoland area. With precipitation running more than three inches above normal, significant standing water has accumulated across the region, providing ample breeding habitat for multiple broods as temperatures warm through May and June. Residents should expect noticeable floodwater mosquito activity in the coming weeks, particularly following any additional rainfall events.
Culex pipiens, the primary vector of West Nile Virus, typically becomes more active as the season progresses and temperatures consistently warm. Above-normal temperatures forecast through the summer months suggest conditions will be favorable for Culex pipiens development — making early-season surveillance and larviciding efforts especially important for staying ahead of potential WNV transmission later in the season.
Clarke maintains and operates an online Customer Portal that program administrators can use to get up-to-date information on their mosquito management program.
Find Floodwater Mosquito Brood Predictions and full monthly data on the Clarke Customer Portal. If you’ve forgotten your password, simply use the reset feature to regain access. Once logged in, select “Custom Reports” from the left menu, click “Run,” choosing your account and the correct month, then “View Report.” [Step-by-step guide here] Click here to link to the Clarke portal.
MOSQUITO BORNE DISEASE UPDATE
West Nile Virus surveillance is already underway for the 2026 season in Illinois. As of May 28th, 618 mosquito samples have been collected, yielding 30 WNV positives—a 4.9% positivity rate across 8 counties. First detections appeared as early as MMWR Week 16 (mid-April), with positive counties spanning from the Chicagoland area (Boone, DuPage, Cook, and Will) to central Illinois (Peoria, Tazewell, and Menard) and the Metro East (Madison). No human cases have been reported.
At the same point in 2025, 1,116 samples had been collected with 38 positives across 9 counties—a 3.4% positivity rate. While fewer samples have been collected so far this year, the higher positivity rate suggests WNV is circulating efficiently in early-season mosquito populations. The 2025 season ultimately produced 150 human cases statewide.
These early signals, combined with April’s expanded breeding habitat and above-normal temperatures forecast through summer, point to conditions that could accelerate mosquito development and compress the timeline to peak Culex pipiens activity. Warmer temperatures also shorten the extrinsic incubation period of WNV, meaning the virus can be transmitted more quickly once mosquitoes are exposed.
Since its emergence in 1999, West Nile Virus has resulted in 58,682 human cases across the United States, including over 2,700 fatalities. Given its established presence in bird and mosquito populations throughout Illinois, WNV remains a significant annual public health concern. A county is considered WNV-positive when any bird, mosquito, human, horse, or other mammal tests positive, and Clarke will report on surveillance results as the season progresses.
These early signals, combined with April’s expanded breeding habitat and above-normal temperatures forecast through summer, point to conditions that could accelerate mosquito development and compress the timeline to peak Culex pipiens activity. Warmer temperatures also shorten the extrinsic incubation period of WNV, meaning the virus can be transmitted more quickly once mosquitoes are exposed.
Since its emergence in 1999, West Nile Virus has resulted in 58,682 human cases across the United States, including over 2,700 fatalities. Given its established presence in bird and mosquito populations throughout Illinois, WNV remains a significant annual public health concern. A county is considered WNV-positive when any bird, mosquito, human, horse, or other mammal tests positive, and Clarke will report on surveillance results as the season progresses.
OPERATIONS UPDATE
With West Nile Virus already detected in 8 Illinois counties and a positivity rate outpacing last year’s pace, early prevention is especially critical this season. April’s record rainfall created abundant breeding habitat, and while May was significantly drier, above-normal temperatures forecast through summer could accelerate mosquito development. Getting ahead of mosquito populations now—before activity peaks later in the season—is the most effective way to reduce community risk.
IDPH and Clarke urge the public to continue to Fight the Bite by practicing the three “R’s” – reduce, repel, and report:
*REDUCE – Ensure doors and windows have tight-fitting screens. Repair or replace screens with tears or openings, and keep doors and windows shut when possible. Eliminate or refresh weekly all sources of standing water where mosquitoes breed, including bird baths, ponds, flowerpots, wading pools, old tires, and any other containers.
*REPEL – When outdoors, wear shoes and socks, long pants, and a light-colored, long-sleeved shirt. Apply an EPA-registered insect repellent containing DEET, picaridin, oil of lemon eucalyptus, IR 3535, para-menthane-diol (PMD), or 2-undecanone according to label instructions. Consult a physician before using repellents on infants.
*REPORT – Report locations where water sits stagnant for more than a week, such as roadside ditches, flooded yards, and similar mosquito-producing sites. Residents can report these through the Clarke Portal at www.clarkeportal.com/hotline.