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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

It's biting time: Mosquitoes, Culicidae


This Asian Tiger better pull out before it explodes!


Mosquitoes carry some of humankind’s deadliest diseases such as malaria, yellow fever, dengue, and West Nile Virus (WNV). Temperature plays a huge role in abetting mosquito outbreaks. One study found that Culex mosquitoes, one of the vectors of WNV, transformed from eggs to adults in about 27 days at a temperature of 59 degrees Fahrenheit. At 93 degrees, mosquitoes went from eggs to adults in only about 7 days. This means that at warmer temperatures, mosquitoes can complete several generations in the amount of time required to complete a single generation when it is cool. With ample rainfall and scorching temperatures over the past few weeks, mosquito populations are on the rise throughout our region and it is not surprising that officials have already reported the US’s first deaths from human cases of West Nile Virus in California, Mississippi, and Nevada.

Scores of larvae will soon hatch from egg rafts floating in a water bucket.

West Nile Virus has killed more than 1500 people in the United States since first detected in New York more than a decade ago. 2012 was a particularly bad year for West Nile Virus and the Centers for Disease Control reported more than 5600 cases of West Nile Virus nationwide with 286 deaths, making it the worst year ever for the disease. While most of us shrug off West Nile Virus with few or no complications, it can be severe and even lethal to seniors and certain others. Recent research helps explain why this may be so.

Our immune system plays a vital role in preventing mosquito borne diseases from infecting our bodies. Cells lining our skin and mucus membranes bear specialized virus-sensing proteins called Toll-Like Receptors, a.k.a. TLRs. TLRs have the critical function of detecting invaders like West Nile Virus.  If TLRs detect the West Nile Virus, they release additional proteins that stimulate production of chemical communication compounds called interleukins. Interleukins released into the bloodstream marshal cellular assassins called macrophages and direct them to hunt and kill cells infected with WNV before the virus can multiple and make us seriously ill. Researchers have suggested that some seniors and people with compromised immune systems may lack sufficient TLRs and related immune system proteins to thwart the West Nile Virus.

Protein in blood will be used by this hungry lady to make mosquito eggs. 

How can you reduce the chances of being bitten by these tiny vexing vampires?  Many species of mosquitoes prefer to feed at dusk and you can avoid being bitten by staying indoors in the evening. Unlike many of our native mosquitoes, the exotic Asian Tiger is a daytime biter, adding hours of itching, scratching, and swatting to days in the garden. Protect yourself from aggressive biters by wearing light-weight, long-sleeved shirts and pants when working outdoors. Certain brands of clothing are pretreated with mosquito repellents such as permethrin. I have worn these in tropical rainforests where mosquitoes were ferocious and they really did help. Many topical insect repellents can be applied to exposed skin before you go outdoors. Some will provide many hours of protection, while others provide virtually none.  Some repellents should not be applied to children and you should always help kids apply repellents. For safety sake, be sure to read and follow the directions on the label of the repellent before you apply it to people or clothing.

Wheelbarrows and pails can breed hundreds of mosquito larvae.

When dining outdoors, place a fan on your patio. Mosquitoes do not fly or navigate well in a breeze and the zephyr created by the fan will reduce the number of mosquitoes flying and biting, providing greater comfort for you and your guests. Many traps are also marketed to capture, and kill mosquitoes. Some rely on a light source to attract blood seekers. Mosquitoes, unfortunately, do not use light to find their meals. One study demonstrated that less than 1% of the insects attracted to light traps were biting flies such as mosquitoes. However, many types of moths, flies, and beetles are attracted to light. Scientists estimated that light traps kill billions of harmless and beneficial insects each year. Actually, mosquitoes are attracted to odors emanating from the animals they bite. As we move about, humans release many odors, including carbon dioxide from our lungs and lactic acid in our sweat. Mosquitoes use these chemical cues to find us. One recent study found that a nine-carbon aldehyde, nonanal, commonly produced by birds and humans, is highly attractive to mosquitoes. This may help explain how West Nile Virus moves so readily from one of the common reservoir hosts, birds, to humans.

Mosquito larvae filter food from the water. Both larvae and pupae hang beneath the surface and gather air through specialized breathing tubes called siphons and horns.

To reduce the chances of mosquitoes breeding around your home, eliminate standing water by cleaning your gutters, dumping your birdbath twice a week, inverting your wheelbarrow and getting rid of water-filled containers. If mosquitoes breed in an aquatic water garden or standing water on your property, then you can use a product containing the naturally occurring soil microbe known as Bacillus thuringiensis israelensis, a.k.a. Bti, to kill the larvae. Bti comes formulated in doughnut-shaped tablets that can be placed in water. If you have an uncovered swimming pool, with proper filtration and disinfection systems, it should not serve as a mosquito breeder. However, if your pool is covered and water collects in a non-porous tarp, then mosquitoes may breed in standing rainwater that collects in the cover. You may need to empty the cover regularly or treat it with a larvacide. Learn more about keeping mosquitoes from breeding in your pool at this web site:

One final thought to ponder: in mosquitoes, the female is the more dangerous of the genders; only she sucks human blood. Males sip nectar from flowers. 

Learn more about biting back at  mosquitoes with the Bug Guy!


Several interesting articles were used as resources for this Bug of the Week, including “How the body rubs out West Nile virus” by  Nathan Seppa; “Toll-like Receptor 7 Mitigates Lethal West Nile and Encephalitis via Interleukin 23-Dependent Immune Cell Infiltration and Homing” by Terrence Town, Fengwei Bai, Tian Wang, Amber T. Kaplan, Feng Qian, Ruth R. Montgomery, John F. Anderson, Richard A. Flavell, and Erol Fikrig; “Density and diversity of non-target insects killed by suburban electric insect traps” by Timothy B. Frick and Douglas W. Tallamy; and “Acute olfactory response of Culex mosquitoes to a human- and bird-derived attractant” by Zainulabeuddin Syed and Walter S. Leal.

To learn more about the mosquitoes and how to defeat them, please visit the following web sites: