Alcohol ingestion stimulates mosquito attraction.
Shirai O, Tsuda T, Kitagawa S, Naitoh K, Seki T, Kamimura K, Morohashi M.
Department of Biodefence Medicine, Faculty of Medicine, Toyama Medical and Pharmaceutical University, Sugitani, Japan.
Mosquito bites should be avoided because of the risk of contracting parasitic and viral diseases such as malaria, dengue fever, and several encephalitides. Although humans have been said to suffer more mosquito bites after ingesting liquor, little is known about whether that is true. Thirteen volunteers (12 men from 20 to 58 years old and a 24-year-old woman) were chosen as test hosts and a 30-year-old man was established as a control. We measured ethanol content in sweat, sweat production, and skin temperature before and after ingestion of 350 ml of beer (ethanol concentration 5.5%) by volunteers and compared them with a control subject. Our study demonstrated that percent mosquito landing on volunteers significantly increased after beer ingestion compared with before ingestion, showing clearly that drinking alcohol stimulates mosquito attraction. However, ethanol content in sweat and skin temperature did not show any correlation between alcohol ingestion and mosquito landings. This study shows that persons drinking alcohol should be careful about their increased risk to mosquito bites and therefore exposure to mosquito-borne diseases.
What Else Do Mosquitoes Like?
Entomologist Daniel L. Kline tested the attractiveness of socks worn by humans. The results: Mosquitoes love them. "I wore the socks for 12 hours on 3 consecutive days. When I wasn't wearing the socks, I put them in a sealed plastic bag," says Kline. "I also found mosquitoes love Limburger cheese. Interestingly enough, the main ingredient in the cheese is a bacterium that can be found on the human foot," he says. Incidentally, Limburger cheese was first made in The Netherlands by monks. They trampled the cheese with their feet to get the finished product.
Kline tested responses of female Aedes aegypti to the socks, human hands, and Limburger cheese. He used females because only they will take blood. They need it for reproduction. In field studies, Kline says the socks alone attracted very few mosquitoes. However, a significant increase occurred to CO2-baited traps when combined with a worn sock for most mosquitoes, including species of Aedes, Anopheles, Coquillettidia, Culex, Culiseta, and Psorophora-major disease-transmitting mosquitoes. Mosquito responses were favorable in the olfactometer, but the human hand still attracted the most.
Clothing Affects on Mosquito Attraction
Contrary to what you might think, blood is not the mosquito's favourite food. It feeds mainly on nectar from wildflowers. A researcher from Ontario observed that the emergence in spring of one of the most common species of mosquito in our region coincided exactly with the appearance of wild cherry blossoms, which it loves. A similar phenomenon can be seen at the Arboretum around the third week of May, when the blossoms on the fruit trees are at their peak and numerous newly hatched mosquitoes make their appearance. We then have a few days' grace during which the mosquitoes ignore us; they are more interested in looking for flowers and in mating. It is only after the female has mated that she starts searching for blood, for the bothersome biter is actually a pregnant mosquito looking for protein supplements for her offspring. A single meal of blood is sufficient to feed her whole brood, and enables her to lay hundreds of eggs.
Stimuli That Attract Mosquitoes
The factors involved in attracting mosquitoes to a host are complex and are not fully understood. Mosquitoes use visual, thermal, and olfactory stimuli to locate a host. Of these, olfactory cues are probably most important. For mosquitoes that feed during the daytime, movement of the host and the wearing of dark-colored clothing may initiate orientation toward a person. Visual stimuli seem to be important for in-flight orientation, particularly over long ranges, whereas olfactory stimuli become more important as a mosquito nears its host.
It has been estimated that 300 to 400 compounds are released from the body as by-products of metabolism and that more than 100 volatile compounds can be detected in human breath. Of these odors, only a fraction have been isolated and fully characterized. Carbon dioxide and lactic acid are the two best-studied mosquito attractants. Carbon dioxide, released mainly from breath but also from skin, serves as a long-range airborne attractant and can be detected by mosquitoes at distances of up to 36 meters . Lactic acid, in combination with carbon dioxide, is also an attractant. Mosquitoes have chemoreceptors on their antennae that are stimulated by lactic acid. These same receptors may be inhibited by N,N-diethyl-3-methylbenzamide (DEET)-based insect repellents.
At close range, skin temperature and moisture serve as attractants. Different species of mosquitoes may show strong biting preferences for different parts of the human body (such as the head or feet), which may be related to local skin temperature and eccrine sweat gland output. Anhidrotic (low sweat) persons show markedly decreased attractiveness to mosquitoes. Other volatile compounds, derived from sebum, eccrine and apocrine sweat, or the cutaneous microflora bacterial action on these secretions, may also act as chemoattractants. Whole-host odors are more attractive than carbon dioxide and lactic acid alone. Floral fragrances from perfumes, soaps, lotions, and hair-care products may also attract mosquitoes. The attractiveness of different persons to the same or different species of mosquitoes varies substantially. In general, adults are more likely to be bitten than children, although adults may become less attractive to mosquitoes as they age. Men are bitten more readily than women. Larger persons tend to attract more mosquitoes, perhaps because of their greater relative heat or carbon dioxide output.
Chemical Insect Repellents
Previously called N,N-diethyl-m-toluamide, N,N-diethyl-3-methylbenzamide (DEET) remains the gold standard of currently available insect repellents. This substance was discovered and developed by scientists at the U.S. Department of Agriculture and was patented by the U.S. Army in 1946. It was subsequently registered for use by the general public in 1957. DEET is a broad-spectrum repellent that is effective against mosquitoes, biting flies, chiggers, fleas, and ticks. Twenty years of empirical testing of more than 20 000 other compounds has not resulted in another marketed chemical product with the duration of protection and broad-spectrum effectiveness of DEET. The U.S. Environmental Protection Agency (EPA) estimates that more than 38% of the U.S. population uses a DEET-based insect repellent every year and that worldwide use exceeds 200 000 000 people annually.
Formulation of Available Products with DEET
In the United States, DEET is available in 5% to 100% concentrations in multiple formulations, including solutions, lotions, creams, gels, aerosol and pump sprays, and impregnated towelettes.
Until 1989, the standard-issue insect repellent of the U.S. military consisted of 75% DEET in an alcohol base. Complaints about the aesthetic feel of this product and concerns about potential toxicity under long-term daily use led to U.S. Army-sponsored studies to produce new formulations. The 3M Company (St. Paul, Minnesota) developed a slow-release, polymer-based product containing 35% DEET; this has become the repellent provided to all U.S. military personnel. This product is available to the general public exclusively through the Amway Corporation (New York, New York) under the brand name HourGuard. If lower-strength formulations of extended-release DEET are desired, Minnetonka Brands (Eden Prairie, Minnesota) offers products containing 6.5% and 10% DEET. (Update: The polymer-based product mentioned as Hour Guard is now being marketed by 3M as 3M UltraThon. It is available in specialty stores like REI.)
As a general rule, higher concentrations of DEET provide longer-lasting protection. Unfortunately, no guidelines are available to help consumers decide what concentration of DEET is appropriate for their specific needs. The number of variables that affect a repellent's effectiveness precludes assigning an "insect repellent factor" to individual products.
Mathematical models of the effectiveness and persistence of mosquito repellents show that the protection offered by a repellent is proportional to the logarithm of the dose (concentration of the product). This curve tends to form a plateau at higher repellent concentrations, providing relatively less additional protection for each incremental dose of DEET that exceeds a 50% concentration. In one laboratory study, 50% DEET provided about 4 hours of protection against Aedes aegypti mosquitoes, but increasing the DEET concentration to 100% provided only 1 additional hour of protection. In another study, 12.5% DEET provided over 6 hours of protection against Aedes albopictus; doubling the DEET concentration to 25% increased the protection time only to about 8 hours.
Extended-release formulations of DEET have made it
possible to reduce the repellent concentration without sacrificing duration of
action. When tested under laboratory and several different environmental and
climatic field conditions, the 35% DEET polymer formulation by the 3M
Corporation was as effective as 75% DEET in repelling mosquitoes. The polymer
formulation provided up to 12 hours of more than 95% protection, depending on
the environmental conditions and species of mosquito tested.
One study showed
that Minnetonka Brands' 6.5% liposphere microdispersion of DEET was effective
for up to 2.5 hours and that their 10% product was effective for about 1 hour
Citronella is the active ingredient most commonly found in "natural" or "herbal" insect repellents marketed in the United States. It is registered with the EPA as an insect repellent. Citronella oil has a lemony scent and was originally extracted from the grass plant Cymbopogon nardus. Limited data are available from studies that directly compared the efficacy of citronella-based products with that of DEET-based products. In one study, 0.01 Ámol of DEET per L of air was sufficient to prevent 90% of mosquitoes from landing on their targets; a 1000-fold higher concentration of citronellol (one of the active chemicals in citronella oil) was required to achieve a similar effect.
Studies show that citronella can be an effective repellent, but it provides shorter complete protection time than most DEET-based products. Frequent reapplication of the repellent can partially compensate for this. The manufacturer of Natrapel (Tender Corp., Littleton, New Hampshire) has laboratory data showing that their 10% lotion reduced mosquito bites by 84% during a 4-minute test period. In contrast, 14% DEET reduced biting by 96% in the same test period. Buzz Away (Quantum, Inc., Eugene, Oregon) with 5% citronella oil provided an average protection time of 1.9 hours against Aedes aegypti. In field testing, Buzz Away Oil provided an average of 88% repellency during a 2-hour exposure. In general, the repellency of Buzz Away was greatest within the first 40 minutes after application and decreased over the remainder of the test period.
Citronella candles have been promoted as an effective way to repel mosquitoes in the backyard. One study compared the ability of commercially available 3% citronella candles, 5% citronella incense, and plain candles to prevent bites by Aedes mosquitoes under field conditions. Persons near the citronella candles had 42% fewer bites than controls, who had no protection (a statistically significant difference). However, burning ordinary candles reduced the number of bites by 23%. The efficacy of citronella incense and plain candles did not differ. The ability of plain candles to decrease biting may result from their action as a decoy source of warmth, moisture, and carbon dioxide.
The citrosa plant (Pelargonium citrosum 'van Leenii') has been marketed as being able to repel mosquitoes through the continuous release of citronella oils. Unfortunately, when tested, these plants offer no protection against bites.
Bite Blocker (Consep, Inc., Bend, Oregon) is a plant-based repellent that was released in the United States in 1997. Bite Blocker combines soybean oil, geranium oil, and coconut oil in a formulation that has been available in Europe for several years. Studies conducted at the University of Guelph, Ontario, Canada, showed that this product gave more than 97% protection against Aedes mosquitoes under field conditions, even 3.5 hours after application. During the same period, a 6.65% DEET-based spray afforded 86% protection, and Avon Skin-So-Soft citronella-based repellent gave only 40% protection. A second study showed that Bite Blocker provided a mean ▒ SD of 200 ▒ 30 minutes of complete protection from mosquito bites.
Pyrethrum is a powerful, rapidly acting insecticide, originally derived from the crushed and dried flowers of the daisy Chrysanthemum cinerariifolium. Permethrin is a human-made synthetic pyrethroid. It does not repel insects but works as a contact insecticide, causing nervous system toxicity that leads to the death or "knockdown" (out of the air) of the insect. The chemical is effective against mosquitoes, flies, ticks, and chiggers. Permethrin has low toxicity in mammals, is poorly absorbed by the skin, and is rapidly inactivated by ester hydrolysis.
Permethrin should be applied directly to clothing or other fabrics (such as tent walls or mosquito nets, not to skin! The spray form is non-staining, nearly odorless, and resistant to degradation by heat or sun and maintains its potency for at least 2 weeks, even through several launderings. The combination of permethrin-treated clothing and skin application of a DEET-based repellent creates a formidable barrier against mosquito bites. In a field trial conducted in Alaska, persons wearing permethrin-treated uniforms and a polymer-based 35% DEET product had more than 99.9% protection (1 bite/h) over 8 hours, even under conditions of intense biting pressures; unprotected persons received an average of 1188 bites/hour!