By Barbara Kessler
Green Right Now

Mosquito-borne West Nile virus has spiked this summer, causing more serious illnesses than any year since 2004 — a total of 390 confirmed and presumed cases as of Aug. 7, according to the Centers for Disease Control.

Culex mosquitoes carry and transmit West Nile virus. (Photo: James Gathany, CDC.)

Those cases include 8 deaths from the disease confirmed by the CDC; a number certain to rise as local health departments report to the agency. (The hard-hit Dallas area, for instance, has attributed 11 deaths to West Nile, only 3 of which are included in the CDC figures.)

West Nile  has been especially devastating in 2012 because the weather provided ideal conditions for the mosquitoes that carry the virus. A mild winter without hard frosts appears to have set the stage for outbreaks in North Texas and other regions, stirring fears and cranked up city insecticide fogging operations. Another factor was the wet spring, which facilitated the mosquitoes ability to breed.

Research by a team of epidemiologists even suggests that this summer’s heat wave may have exacerbated the situation. Their paper, “Infectious Diseases in a Warming World: How Weather Influenced West Nile Virus in the U.S.“, found that hot temperatures raise the incidence of West Nile virus in humans. Higher humidity also is a contributor, though findings about the types of rainfall events associated with the disease incidence are inconclusive, according to the research team.

West Nile, which is transmitted by infected mosquitoes that get the disease from infected birds, usually presents as a fever and flu-like illness in humans, marked by headaches, nausea and vomiting.

In about one in 150 cases it can escalate into a neuro-invasive disease with brain-swelling, muscle paralysis and coma being possible outcomes. The best way to avoid West Nile is to limit your exposure to the mosquitoes carrying it.

The CDC recommends taking these steps:

  • Use a mosquito repellent when you are outdoors.
  • Wear protective clothing — long sleeves and pants — during night time hours, from dusk to dawn when mosquitoes are most active.
  •  Use a mosquito repellent on your skin and even over clothing, for extra protection during the evening and recognize that mosquitoes can bite through thin fabric. The CDC recommends repellents that use either DEET (N,N-diethyl-m-toluamide) or Picaridin (KBR 3023) as being the most effective. It also acknowledges that the EPA-registered plant-based repellent ingredient oil of lemon eucalyptus can be effective, providing protection similar to repellents with low concentrations of DEET.

    Oil of Lemon Eucalyptus, a plant-based repellant considered effective against mosquitoes.

    See more about repellent useat the CDC website.  (While DEET has been controversial, it’s also been shown to be safe at lower concentrations that are typically sold today; oil of eucalyptus is a natural compound, but expire quicker, requiring reapplications.)

  • Use a permethrin repellent on top of clothing — but not on the skin.
  • Reapply mosquito repellent if you are perspiring or getting wet.
  • Keep mosquitoes from breeding in your yard by emptying any containers holding still or stagnant water. Refill bird baths frequently, or use a water feature with movement to deter mosquito larvae.

Other ways to reduce the mosquito population around you

Naturalists are fond of pointing to the mosquitoes natural predators, mainly birds, bats and to a lesser extent, dragon flies, as potential helpmates in reducing mosquito populations.

The National Wildlife Federation even carried a blog on this topic this week, identifying a few specific insect-eating birds.

Developing a backyard that supports native wildlife is a great idea if you want to enjoy that wildlife, but it’s not a panacea for mosquitoes says, Boris Kondratieff, a professor of entomology at Colorado State University.

While it’s helpful to invite mosquito predators, such as birds, bats and dragon flies, into your landscape, don’t count on them to keep your yard safe.

Nature is complex, and the same pond you install to service dragon flies could very well harbor and produce mosquitoes, says Dr. Kondratieff.

Dragon flies are lovely acrobatic predators of insects and they may swoop over your pond, he said, providing a visual show. But they’ll also likely be performing during the heat of the day, in the blazing sunlight, when they’re most active — and the exact time when mosquitoes are at rest.

Bats eat mosquitoes, it's true. (Photo: USDA)

Bats and many birds, which also consume mosquitoes, can help make your backyard “wildlife friendly,” and bats have the advantage here being nocturnal just like mosquitoes,  but don’t count on them as your natural mosquito zappers either, Kondratieff says.

Yes, bats will eat the insect pests, but it’s not as if they’re house pets feeding from a finite bowl of food in your backyard.

Your carefully cultivated bats and birds may eat the mosquitoes in their immediate path, or they may swoop off into another area, leaving but a flood of the blood-sucking mosquitoes in your yard.

For that matter, many mosquitoes themselves travel broadly, and may invade your space from miles away.

One helpful attribute of the Culex female mosquito, the breed that carries and transmits West Nile, is that they generally breed and feed within a one-quarter mile radius, according to entomologists at Purdue University. (See Purdue’s extensive information on mosquitoes here.)

“Female Culex mosquitoes are noted for very short flight ranges. They feed and lay eggs within close proximity, usually within a quarter mile or so of the water in which they completed larval development,” they report. So you have a better chance of squelching this type of mosquito with a neighborhood-wide or yard-specific approach to control.

So while mosquitoes and dragon flies, can migrate over great distances, the mosquitoes you want to target, the Culex species are relative homebodies.

Dragon flies eat mosquitoes too, but don't count on them. (Photo: ASU.)

Still, Kondratieff said, don’t expect to dictate which and whether insects hang in your yard, or another.  The landscape is interconnected, and again, it’s complex.

“If you enjoy the dragon flies, you can do some things to attract them,” said Kondratieff, who also directs the C.P. Gillette Museum of Arthropod Diversity. “But saying that, once you attract them, you can’t say scientifically that you’ll have fewer mosquitoes.”

Kondratieff reiterated the CDC’s advice to rid your yard of stagnant or still pools of water, which can become mosquito breeding grounds.

If you have standing bodies of water, like a stock pond or bird bath, use mosquito “dunks” which target “filter feeding” insects, i.e., mosquitoes, with a bacterium, but do not harm other species, he said.

There’s some concern, he added, that extensive mosquito control could be overkill and have a cascading negative effect on the ecosystem, given that other animals do feed on mosquitoes and need them around.

Bottomline: Heed the experts, use repellants, cover up, clean up pooling water.

Which brings us back to long sleeves and avoidance.

Here’s some advice from Kondratieff that you might not expect to hear from someone who studies nature: “As it gets dark you may not want to be outside.”

“…and (you will need to) monitor people to not keep breeding pools and bird baths”

“You need to take some proactive action”

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