Below average rainfall during June put the brakes on populations of mosquitoes in the Washington-metropolitan area. However, with nearly three inches of rainfall already in the month of July, ample water has accumulated in 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. With temperatures rising and legions of female mosquitoes about to emerge from the drink, as Paul Thomas Anderson said, “there will be blood”.
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 remain 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 spawn 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.
Mosquitoes are more than just a nuisance. In some heavily infested areas, bite counts exceeding hundreds per hour can bring grownups to tears. Several species of mosquitoes carry humankind’s deadliest diseases such as malaria, yellow fever, dengue, and in North America West Nile Virus. West Nile virus has killed more than 1000 people in the United States since first detected in New York a decade ago. While most of us shrug off West Nile virus, it can be severe and lethal to seniors and certain others. 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 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 West Nile virus 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.
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. However, many types of moths, flies, and beetles are attracted to light. 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 hungry mosquitoes use 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 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 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 the return of thunderstorms and hot weather 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 referenced for this post, 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: