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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.

Beware of Mosquitoes and West Nile Virus


Yikes! This Asian tiger mosquito better pull out before it explodes.


With many parts of the nation suffering moderate to extreme drought, some folks are wondering why mosquitoes and diseases they carry, such as West Nile Virus (WNV), are such a huge problem this year. In many parts of the nation, especially states bordering the Gulf of Mexico and along the eastern seaboard, rashes of thunderstorms have dumped more than 10 inches of rain during the last 60 days. This is ample water to fill natural vessels like tree holes and artificial containers like birdbaths and wheelbarrows to create perfect nurseries for breeding mosquitoes. A five-gallon pail in my backyard is now home to more than 300 mosquito larvae, called wrigglers, and pupae, called tumblers. 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 temperatures soaring in this year of record warmth, mosquitoes are completing their life cycle in record time. Hot wet weather is the recipe for spawning huge numbers of mosquitoes. During the first several days of adulthood, both male and female mosquitoes consume carbohydrate rich food such as plant nectar or aphid honeydew. For male mosquitoes, sweets are the sole source of food, but the gal has a blood lust. Female mosquitoes use animal blood as the source of protein to produce eggs. The pregnant mosquito lays her eggs in a water-filled container such as a pail or birdbath, or in pools of standing water on the ground. Some, like the ferocious Asian tiger, Aedes albopictus, lay eggs near the water line of a container. When the vessel fills with rainwater, eggs hatch and larval development begins. Others, such as the Northern house mosquito, Culex pipiens, lay eggs in clusters called rafts that float on the surface of the water. Each raft can contain more than 150 eggs.


Hundreds of larvae will hatch from these three egg rafts floating on the surface of the water.


West Nile Virus (WNV)

Several species of mosquitoes feed on birds and high numbers of mosquitoes increase the likelihood that WNV will be transmitted from bird to bird. However, many of these bird-feeding mosquitoes bite humans as well. As the incidence of infection increases in the bird population and populations of mosquitoes burgeon in an area, the chances of humans being bitten by an infected mosquito go up. This perfect storm of infected birds, hungry mosquitoes, and vulnerable humans is currently underway in several parts of our country. WNV has killed more than 1000 people in the United States since it was first detected in New York more than a decade ago. At the time of this posting, more than 700 human cases of WNV and 26 deaths have occurred this year, mostly in states in the southern and central areas of the US. Fortunately, approximately 80% of infected people shrug off the virus with no symptoms. In 20 % of infected individuals, WNV causes minor illness such as fever, headaches, and nausea. The real problem arises in about 1 in 150 people who develop severe symptoms when the virus attacks the brain and the membrane covering it and the spinal cord. This neuroinvasive form of the disease can cause high fevers, headaches, body aches, disorientation, confusion, comas, and in some cases death. Symptoms may arise 3 – 14 days after the bite of an infected mosquito. Seniors and others with compromised immune systems may be at greatest risk. Recent research helps explain why this may be so. Our immune system plays a vital role in preventing diseases carried by mosquitoes 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 WNV, 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 virus before the virus can multiple and make us seriously ill. Researchers suggest that some seniors and people with compromised immune systems may lack sufficient TLRs and related immune system proteins to thwart WNV. 

Tips to avoid mosquito bites

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. To learn more about repellents, please visit the previous episode entitled “Mosquito redux”

If you dine outdoors, place a small fan on your patio. The light breeze created by the fan will greatly reduce the number of mosquitoes flying and biting. Many traps are also available to capture and kill mosquitoes. Some rely on a light source to attract blood seekers. Many types of moths, flies, and beetles are attracted to light, but mosquitoes, unfortunately, do not use light to find their meals and are not readily attracted to light traps. One study demonstrated that less than 1% of the insects attracted to light traps were biting flies such as mosquitoes. This study estimated that light traps kill billions of harmless and beneficial insects each year. Actually, mosquitoes are attracted to odors emanating from the host. As we move about the earth, we release many odors, including carbon dioxide from our lungs and lactic acid in our sweat that help hungry mosquitoes 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 WNV so readily moves from one of the common reservoir hosts, birds, to humans. 

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, emptying the wading pool, and getting rid of water-filled containers. If you have an aquatic water garden or standing water on your property that breed mosquitoes, you can use a product containing the naturally occurring soil microbe known as Bacillus thuringiensis israelensis, a.k.a. Bti. Bti comes formulated in doughnut-shaped tablets that can be placed in water to kill mosquito larvae. With plenty of thunderstorms and hot weather forecast for the foreseeable future, battalions of biters are about to make their presence known. Get ready to protect yourself as you work and play outdoors, or prepare to give blood.


Several interesting articles were used in writing 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; “Acute olfactory response of Culexmosquitoes to a human- and bird-derived attractant” by Zainulabeuddin Syed and Walter S. Leal; and “Temperature-dependent development and survival rates of Culex quinquifasciatus and Aedes agypti (Diptera: Culicidae)” by L. M. Rueda, K.j.Patel, R. C. Axtel, and R. E. Stinner.

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