The Fight Against Mosquitoes
Mosquitoes are not very popular with human beings. They suck your blood and can cause infections. Many ways have been devised to limit their attacks. Female mosquitoes are efficient carriers of deadly diseases such as malaria, dengue and yellow fever, resulting each year in several million deaths and hundreds of millions of cases. To find human hosts to bite and spread disease, these mosquitoes use exhaled carbon dioxide as a vital cue. A disruption of the carbon dioxide detection machinery of mosquitoes, which would help control the spread of diseases they transmit, has therefore been a long sought-after goal. Anandasankar Ray, an assistant professor of entomology at the University of California, Riverside, and colleagues report that they have identified in the lab and in semi-field trials in Africa three classes of volatile odor molecules that can severely impair, if not completely disrupt, the mosquitoes' carbon dioxide detection machinery.
There are many existing methods used to prevent the spread of disease, or to protect individuals in areas where disease is endemic due to mosquitoes. These include:
Vector control aimed at mosquito eradication.
Habitat change which removes stagnant water and other breeding areas.
Treat with pesticides or natural predators.
Prevent mosquito bites, with insecticides, nets and repellents.
Pesticide and replellants (such as DEET) have either known negative human or environmental effects or may have such effects.
Natural predators are good too if available. The dragonfly nymph eats mosquitoes at all stages of development and is quite effective in controlling populations. A number of fish eat mosquito larvae, including goldfish, catfish, and minnows.
The odor molecules that the researchers identified work by affecting the mosquitoes' carbon dioxide receptors, which are located in tiny, antennae-like appendages - called maxillary palps - close to the mouths of the mosquitoes.
The three classes of the odor molecules are:
Inhibitors: Odor molecules, like hexanol and butanal, that inhibit the carbon dioxide receptor in mosquitoes and flies.
Imitators: Odor molecules, like 2-butanone, that mimic carbon dioxide and could be used as lures for traps to attract mosquitoes away from humans.
Blinders: Odors molecules, like 2,3-butanedione, that cause ultra-prolonged activation of the carbon dioxide sensing neurons, effectively "blinding" the mosquitoes and disabling their carbon dioxide detection machinery for several minutes.
"These chemicals offer powerful advantages as potential tools for reducing mosquito-human contact, and can lead to the development of new generations of insect repellents and lures," said Ray, who led the study. "The identification of such odor molecules - which can work even at low concentrations, and are therefore economical - could be enormously effective in compromising the ability of mosquitoes to seek humans, thus helping control the spread of mosquito-borne diseases."
Female mosquitoes spread disease by first obtaining a blood meal from an infected person and subsequently finding an uninfected person to bite. Extremely sensitive to minute changes in carbon dioxide concentrations, they can sense carbon dioxide in our breath from long distances. Upon encountering a carbon dioxide plume, the mosquitoes orient and fly upwind, arriving eventually near us.
Most mosquito-trapping devices also use carbon dioxide to attract mosquitoes. But these devices tend to be expensive and bulky, and suffer from the usual difficulties associated with supplying carbon dioxide via gas cylinders, dry ice or propane combustion.
In the case of the "blinder" class of molecules, Ray's group found that even a brief exposure to these odor molecules (presented in a blend of four odors: 2.3-butanedione, 1-hexanol, 1-butanal and 1-pentanal) activated the carbon dioxide-sensitive neurons in mosquitoes for at least five and a half minutes, and evoked such a strong and prolonged response in the neurons that the mosquitoes' responses to subsequent carbon dioxide stimuli were severely reduced for several minutes.
In collaboration with Ring Cardé, a distinguished professor of entomology at UCR, Ray's lab tested the effectiveness of this blend in wind-tunnels, and found that the flight of the blend-exposed mosquitoes toward sources of carbon dioxide in the wind-tunnels was disrupted.
The research team released Culex quiquefasciatus females in a large enclosed greenhouse that contained two hut-like structures with carbon dioxide-emitting traps placed in each of them. The researchers then included in one of the huts a source of the ultra-prolonged blend in the form of a small fan-driven odor dispenser. They found that only a few mosquitoes entered this hut and made it to the carbon dioxide trap.
For further information: http://www.ucop.edu/sciencetoday/article/25709