seal  Purdue News

July 11, 2003

Flood benefit: onslaught of disease-carrying mosquitoes delayed

WEST LAFAYETTE, Ind. – The heavy Midwest rains have provided one temporary benefit, according to Purdue University experts. The floods have temporarily reduced the disease-carrying mosquito population, but people need to take precautions for later in the summer and in the fall.

Floods have washed away most eggs or larvae that were already developing this year, and the water has been flushing out potential mosquito breeding areas continually, said Ralph Williams, a Purdue entomology professor.

However, the wet weather has created the potential for a heavy onslaught of West Nile virus-carrying insects later. How soon and how many depend on how fast waters recede and how fast temperatures rise. Then, the key to mosquito control will be to drain any stagnant water.

"We don’t have a problem now because the flooding has cleaned out most mosquito breeding areas," Williams said.

Most people would expect the deluge of floodwater to produce more mosquitoes. Williams said this isn't the case with the types of West Nile-carrying mosquitoes because these bloodsuckers generally breed in places, such as containers, where standing water remains for a period of time,.

"We’re seeing some nuisance mosquitoes but reduced numbers of Culex mosquitoes that carry West Nile," Williams said. "We won’t have West Nile mosquitoes again until it starts drying out and we have isolated pockets of stagnant water in the usual places again."

He does expect the number of nuisance mosquitoes to increase rapidly over the next couple of weeks.

Already this year, 28 states, including Indiana, have found birds that tested positive for West Nile, according to the Centers for Disease Control and Prevention. Eight states, including Indiana, have found West Nile-positive mosquitoes. South Carolina has confirmed one human case of the disease contracted by a 70-year-old man in late May. Twenty-two horses have tested positive; the closest ones to Indiana were in Kentucky and Wisconsin. (These figures are as of July 9, 2003.)

Mosquitoes transmit West Nile by biting infected birds, then biting people or horses. This sends virus-laden saliva into the victim’s bloodstream. According to the U.S. Department of Agriculture, sickness develops one to two weeks after being bitten.

William said that though many people may be worried about floodwater on their property becoming disease-carrying mosquito breeding grounds, that’s unlikely.

"Mosquitoes don’t breed in flood water," he said. "They pick places that always have standing water."

He advises against using spraying larvicides as a control because not all standing water harbors mosquito larvae, and the insects can fly up to five miles,, Williams said. "Right now, there are so many wet places that spraying will only provide a minimal relief from mosquitoes."

It’s important to know the mosquito life cycle and where they breed in order to use larvicide effectively. In addition, larvicide can be dangerous to humans and animals if incorrectly used.

Williams said a better approach to mosquito control is to drain ditches, tires, buckets and wheelbarrows. Also clean gutters, regularly clean or drain water troughs, ornamental ponds and swimming pools. Stocking ornamental pools with bluegills or sunfish also can keep down the mosquito population.

Williams suggests that those who think they have a mosquito problem they can't solve by draining and cleaning potential breeding areas should call their county health department. Depending on the county, a health department official will either come look at the situation or recommend a pest-control company.

In the meantime, Purdue veterinarians are reminding people to vaccinate their horses and take precautions to minimize other animals’ exposure to mosquitoes. Michel Levy, associate professor of large animal medicine and a member of the Indiana Governor’s West Nile Virus Task Force, said that the equine vaccine now has full FDA approval and appears to be at least 95 percent effective in preventing the virus in horses.

No vaccine is available for humans or other animals. Some sprays are now on the market for dogs and cats that are supposed to repel West Nile-carrying mosquitoes. However, it’s not known whether these are effective, said Lynn Guptill, Purdue small animal veterinarian. She advises not using sprays made for people or horses on other animals.

Williams said that people should use repellants containing diethyl-meta-toluamide (DEET) and always follow the directions on the label. It’s also important to wear light-colored clothing with long-sleeves and long pants, socks and hats.

Although mosquitoes can bite anytime, the chances of being bitten can be lessened by staying inside during dawn and dusk, Williams said. This is recommended for horses. They also can be covered with lightweight, light-colored sheets to help prevent bites.

"We’re not out of the woods yet, when it comes to mosquitoes," Williams said. "The worst time of year is always in August and September, and the season can extend through October.

"How bad the mosquitoes are this year and how many West Nile cases we have depends on the weather. If it changes to a drying trend and warms up, then we’ll have more isolated pockets of stagnant water like last year. With that and the active virus in the birds, we do have the potential for another West Nile outbreak."

Writer: Susan A. Steeves, (765) 496-7481,

Sources: Ralph Williams, (765) 494-4560,

Michel Levy, (765) 494-8548,

Lynn Guptill, (765) 494-1107,

Ag Communications: (765) 494-2722; Beth Forbes,;

Related Web sites:
Purdue Department of Entomology
Purdue University West Nile virus Website
USDA Animal and Plant Health Inspection Service
US Geological Survey West Nile virus statistics and maps
Indiana/Purdue Animal Disease Diagnostic Laboratory
Purdue Veterinary Department of Pathobiology
Indiana Board of Animal Health

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