Mosquitoes in the Forest Preserves

(This page explains Forest Preserve District efforts to monitor specific types of mosquitoes and ways to avoid bites in your own backyard. For information on mosquitoes in general, click here.)

2017 District Weekly West Nile Virus Monitoring Report

Mosquitoes are an important part of the food chain, so under normal conditions the Forest Preserve District does not attempt to eliminate them. But with the arrival of West Nile virus, the District’s fundamental concern for the health of county residents and forest preserve visitors spurred the start of a comprehensive mosquito monitoring program that continues today.

What is the Forest Preserve District doing to address West Nile virus?
The District’s mosquito monitoring program focuses on Culex mosquitoes, the species most likely to transmit West Nile virus in northern Illinois. The program follows Illinois Department of Public Health guidelines, which advise public and private landowners alike to focus first on monitoring areas for Culex larvae and using a larvicide where they’re found. The larvicide is made from a natural bacterium that targets and kills Culex larvae before they can become adults; very few other insects are affected.

Culex are “drought-driven” mosquitoes. Rather than reproducing in healthy wetlands or after heavy rains, they lay their eggs in warm, stagnant waters formed after long dry spells. To prevent breeding grounds from developing, District employees drain water from trash bins, buckets, gutters, truck beds, anywhere it can collect. They also treat stormwater catch basins in forest preserve parking lots with larvicide.

In addition, the District monitors select natural waters with very few fish or other larvae-eating animals. Each week employees licensed by the Illinois Department of Agriculture look for Culex larvae at more than 130 such sites that are close to neighborhoods, parking lots, visitor centers, picnic grounds and other high-use areas. If they find Culex, they treat the water with larvicide. They also test adult Culex for the presence of West Nile virus to identify areas with an increased risk of transmission — and a need for closer larval monitoring. The District shares its findings on an ongoing basis with regional, state and county health departments and continues to collaborate with agencies outside of the mosquito breeding season.

In 2015 the District produced a video on its mosquito monitoring program, which you can watch here. You can also click here to see slides from the 2013 presentation to the Board of Commissioners.

What about sprays?
The District does not use fogs or sprays to kill adult mosquitoes. The chemicals can kill any insects they contact, including butterflies, moths and lightning bugs, and dissipate after only 48 hours. Larvicides can be effective for up to four weeks and when paired with monitoring are not only the most ecologically sound option for DuPage wildlife but also the most cost-effective way to reduce Culex mosquito populations.

Where can I find more information West Nile virus?
The DuPage County Health Department provides updates on West Nile virus activity in the county and posts a “personal protection index” on its website at The Centers for Disease Control and Prevention offers a variety of detailed information at

What about Zika?
The Zika virus is spread primarily through the bites of infected Aedes albopictus and Aedes aegypti mosquitoes, both rare or non-existent in northern Illinois. (To date, people in Illinois infected with Zika acquired the disease outside of the U. S.) Both species prefer to lay eggs in tires and other abandoned or neglected items that hold water, breeding grounds not normally found in natural areas. 

Of the two mosquitoes, A. albopictus has a potential to be in Illinois, but scientists believe the few previous reports north of Peoria were rare, isolated incidents and not from established populations. Still, because even a remote potential exists, the Forest Preserve District is using a special trap designed to effectively capture Aedes mosquitoes. It’s the same type of trap being tested by the IDPH, the DuPage County Health Department and other agencies. As with its efforts related to West Nile virus, the District is collaborating with these government offices.

For more information on the Zika virus, including health-related news, visit the CDC website at

What can I do to avoid mosquito bites?
Homeowners are often surprised to learn their own backyards offer ideal breeding grounds for Culex mosquitoes. We can all greatly reduce the risk of getting mosquito bites near our homes by keeping yards clean and dry.

• Empty standing water from toys, flower pot drip pans, gardening equipment and other containers.
• Clean clogged gutters.
• Tightly cover rain barrels with lids or 16 mesh screening.
• Empty and clean neglected swimming or wading pools and bird baths twice a week; empty pets’ water dishes daily.
• Repair faulty faucets or leaky pipes.
• Visit the IDPH website for more tips.

In addition, the CDC offer recommendations for avoiding bites no matter where you are.

• Stay indoors at dawn, dusk and in the early evening.
• Wear light-colored, long-sleeved shirts and long pants.
• Use insect repellent containing DEET when spending time outdoors. Be sure to read and follow the product directions, especially when using the products on children.
• Visit the CDC website for more recommendations.

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©2016 Forest Preserve District of DuPage County