DuPage County has launched its annual battle against mosquitoes capable of infecting humans with West Nile virus.
Forest preserve officials say they've started their 12th season of monitoring and managing Culex mosquito populations. The effort will continue until the first hard frost, which is usually in October.
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"We've always had a program over those years focused on reducing the threat to public health," said Tom Velat, the district's insect ecologist.
DuPage last year had six confirmed cases of West Nile virus, which causes illnesses that include flu-like symptoms. In 2012, there were 56 confirmed cases, including five West Nile-related deaths.
The DuPage County Health Department started its West Nile virus monitoring efforts last month. It has nearly 40 mosquito traps strategically placed around the county.
At its sites, the forest preserve is focused on catching adult Culex mosquitoes. Tests then are done to determine if the insects are carrying West Nile virus.
If an infected mosquito is found in a forest preserve, workers return to where it was caught and treat the site with a biologically derived mosquito larvicide that kills the insects but doesn't harm the surrounding environment, officials said.
The forest preserve also shares its data with the Illinois Department of Public Health and the county health department.
In addition to testing adult mosquitoes, forest preserve employees are monitoring about 130 sites for the presence of Culex mosquito larvae.
"If we find Culex, we put larvicide in those places," Velat said.
The areas, which include ponds, marshes, wetlands and ditches, are regularly checked because they're where Culex mosquitoes tend to breed.
Culex mosquitoes breed in warm, stagnant water, officials said. They also pick spots where there's no fish or other animals to eat the mosquito larvae.
Velat said most of the district's wetlands are healthy and support good predator populations.
"We rely heavily on those areas that have predators to suppress mosquito populations naturally," he said.
Velat said the forest preserve doesn't spray insecticides to kill adult mosquitoes because those products could kill other insects.
"People need to understand that we're an ecologically based organization," Velat said. "We can't spray (insecticides) because we have a responsibility to the native insects and wildlife on our property."
The forest preserve is focused only on controlling the Culex mosquito. It doesn't target so-called "nuisance" mosquitoes because they don't transmit the West Nile virus to humans and are an integral part of the ecosystem.
"A lot of things eat mosquitoes," Velat said.
In addition to monitoring, the health department once again is offering a "personal protection index" to inform the public about the amount of West Nile virus activity in the area.
The index, which appears as a widget at dupagehealth.org, uses a scale of zero to three, with zero being no activity and three announcing that multiple human cases of West Nile virus have been confirmed.
Health officials said the index advises residents about what precautions they should take, such as draining standing water, using insect repellent and being careful at dusk and dawn. When the index reaches its highest level, it suggests residents do all those things in addition to wearing long sleeves, pants and closed shoes while outdoors.
The potential to contract West Nile virus is greatest after stretches of hot weather in July and August, officials said.
Still, Velat said people should always protect themselves against mosquito bites. He also urges residents to check around their homes and drain any items that could hold standing water.