Study on current insect repellants confirms need for better insect repellants...

In continuation of update on  mosquito repellents developments and DEET

Now, Johns Hopkins scientists have discovered what it is in the bugs' molecular makeup that enables citronellal (the aromatic liquid used in lotions, sprays and candles) and DEET, to deter insects from landing and feeding on you. A better understanding of these molecular-behavioral links already is aiding the team's search for more effective repellants.

Researchers claim that, three taste receptors on the insects' tongue and elsewhere are needed to detect DEET. Citronellal detection is enabled by pore-like proteins known as TRP (pronounced "trip") channels. When these molecular receptors are activated by exposure to DEET or citronellal, they send chemical messages to the insect brain, resulting in an aversion response.

"DEET has low potency and is not as long-lasting as desired, so finding the molecules in insects that detect repellents opens the door to identifying more effective repellents for combating insect-borne disease," says Craig Montell, Ph.D., a professor of biological chemistry and member of Johns Hopkins' Center for Sensory Biology.

As per the lead researcher, when a mosquito lands, it tastes your skin with its gustatory receptors, before it bites. Researchers suggests that,  one of the reasons DEET is relatively effective is that it causes avoidance responses not only through the sense of smell but also through the sense of taste.

The team filled feeding plates with high and low concentrations of color-coded sugar water (red and blue dyes added to the sugar), allowing the flies to feed at will and taking note of what they ate by the color of their stomachs: red, blue or purple (a combination of red and blue). Wild-type (normal) flies preferred the more sugary water to the less sugary water in the absence of DEET. When various concentrations of DEET were mixed in with the more sugary water, the flies preferred the less sugary water, almost always avoiding the DEET-laced sugar water.Flies that were genetically engineered to have abnormalities in three different taste receptors showed no aversion to the DEET-infused sugar water, indicating the receptors were necessary to detect DEET. 

"We found that the insects were exquisitely sensitive to even tiny concentrations of DEET through the sense of taste," Montell reports. "Levels of DEET as low as five hundredths of a percent reduced feeding behavior."

To add to the evidence that three taste receptors (Gr66a, Gr33a and Gr32a) are required for DEET detection, the team attached recording electrodes to tiny taste hairs (sensilla) on the fly tongue and measured the taste-induced spikes of electrical activity resulting from nerve cells responding to DEET. Consistent with the feeding studies, DEET-induced activity was profoundly reduced in flies with abnormal or mutated versions of Gr66a, Gr33a, and Gr32a. 

In the second study, Montell and colleagues focused on the repellent citronellal. To measure repulsion to the vapors it emits, they applied the botanical compound to the inside bottom of one of the two connected test tubes, and introduced about 100 flies into the tubes. After a while, the team counted the flies in the two tubes. As expected, the flies avoided citronellal. 

The researchers identified two distinct types of cell surface channels that are required in olfactory neurons for avoiding citronellal vapor. The channels let calcium and other small, charged molecules into cells in response to citronellal. One type of channel, called Or83b, was known to be required for avoiding DEET. The second type is a TRP channel. 

The team tested flies with mutated versions of 11 different insect TRP channels. The responses of 10 were indistinguishable from wild-type flies. However, the repellent reaction to citronellal was reduced greatly in flies lacking TRPA1. Loss of either Or83b or TRPA1 resulted in avoidance of citronellal vapor.

The team then "mosquito-ized" the fruit flies by putting into them the gene that makes the mosquito TRP channel (TRPA1) and found that the mosquito TRPA1 substituted for the fly TRPA1. Researchers found that the mosquito-version of TRPA1 was directly activated by citronellal. Montell's lab and others have tallied 28 TRP channels in mammals and 13 in flies, broadening understanding about how animals detect a broad range of sensory stimuli, including smells and tastes.

Hop this discovery now raises the possibility of using TRP channels to find better insect repellents.

Ref 1: http://www.cell.com/current-biology/abstract/S0960-9822%2810%2901006-7
      2 : http://download.cell.com/neuron/pdf/PIIS0896627310005441.pdf?intermediate=true

Predator-released hydrocarbons repel oviposition by a mosquito - nature's insect repellents ?

                                                                   N-Heneicosane 

                                                                    N-Tricosane

Many animals use chemicals to communicate with each other. Pheromones (most of us are familiar with these class of semi-synthetic compounds-used mainly as insect repellents) which influence social and reproductive behaviors within a particular species, are probably the best known and studied. Kairomones are produced by an individual of one species and received by an individual of a different species, with the receiving species often benefiting at the expense of the donor.

Cohen and his Israeli colleagues focused on the interaction between two insect species found in temporary pools of the Mediterranean and the Middle East: larvae of the mosquito C. longiareolata and its predator, the backswimmer N. maculata. When the arriving female mosquitoes detect a chemical emitted by the backswimmer, they are less likely to lay eggs in that pool.

To reproduce conditions of temporary pools in the field, the researchers used aged tap water with fish food added as a source of nutrients. Individual backswimmers were then placed in vials containing samples of the temporary pools, and air samples were collected from the headspace within the vials. The researchers used gas chromatography-mass spectrometry to analyze the chemicals emitted by the backswimmers.

Cohen and his colleagues identified two chemicals, hydrocarbons called n-heneicosane and n-tricosane (see structures), which repelled egg-laying by mosquitoes at the concentrations of those compounds found in nature. Together, the two chemicals had an additive effect.

Since the mosquitoes can detect the backswimmer's kairomones from above the water's surface, predator-released kairomones can reduce the mosquito's immediate risk of predation, says Cohen. But they also increase the female mosquito's chance of dying from other causes before she finds a pool safe for her to lay her eggs in.

Researchers conclude that, these newly identified compounds, and others that remain to be discovered, might be effective in controlling populations of disease-carrying insects. It's far too soon to say, but there's the possibility of an advance in the battle against infectious disease.

http://www3.interscience.wiley.com/journal/123572945/abstract?CRETRY=1&SRETRY=0

Yellow fever mosquito's resistance to DEET !.....

In continuation of  my update on mosquito repellents,  I found this info  interesting to share with. We know that N,N-Diethyl-meta-toluamide, DEET (see structure), is the most common active ingredient in insect repellents. It is intended to be applied to the skin or to clothing, and is primarily used to repel mosquitoes. In particular, DEET protects against tick bites, preventing several rickettsioses, tick-borne meningoencephalitis and other tick-borne diseases such as Lyme disease. It also protects against mosquito bites which can transmit dengue fever, West Nile virus, eastern equine encephalitis, and malaria. 

Now researchers from Swedish University of Agricultural Sciences (SLU) and associates in the UK, for the first time in laboratory tests have shown that yellow fever mosquito has developed a resistance to the mosquito repellent DEET.

"Through testing, we have found that yellow fever mosquitoes no long sense the smell of DEET and are thereby not repelled by it. This is because a certain type of sensory cell on the mosquito's antenna is no longer active" says Rickard Ignell, a researcher at the Division for Chemical Ecology at SLU in Alnarp....

Rickard Ignell performed the research in collaboration with Rothamstead Research in the UK. The scientists have thus seen that the sensory cell on the mosquito's antenna has stopped reacting to DEET. This have many explanations, such as the protein that binds in to DEET having mutated. The researchers are now urging restrictiveness in the use of DEET and other mosquito repellents on a large scale in a limited area, in order not to make other mosquito species resistant. The mechanism is still to be established, but in my opinion its a interesting finding....

Ref : http://www.pnas.org/content/early/2010/04/26/1001313107

Study provides better understanding of how mosquitoes find a host......

In continuation of my update on on developments in mosquito repellents ......

The potentially deadly yellow-fever-transmitting Aedes aegypti mosquito detects the specific chemical structure of a compound called octenol as one way to find a mammalian host for a blood meal, Agricultural Research Service (ARS) scientists report".......

Ref : http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0007032....

Nonanal (odour produced in humans and birds) - an attractant for the Culex mosquito ?

In continuation of my update on developments in mosquito repellents,  I  found this  interesting info. In  my earlier blog, I mentioned that carbon dioxide   emitted in human  breath  is  the main  attractant  for the    Culex mosquito to find people, aiding the transmission of these deadly diseases. Now scientists from University of California, Davis, have identified the dominant odor naturally produced in humans and birds that attracts the blood-feeding Culex mosquitoes, which transmit West Nile virus and other life-threatening diseases. As per the claim by the researchers, Nonanal (nonanaldehyde or pelargonaldehyde see below structure) is the powerful semiochemical that triggers the mosquitoes' keen sense of smell, directing them toward a blood meal. A semiochemical is a chemical substance or mixture that carries a message.The antennae of the Culex quinquefasciatus are highly developed to detect even extremely low concentrations of nonanal. Mosquitoes detect smells with the olfactory receptor neurons of their antennae.

The UC Davis researchers tested hundreds of naturally occurring compounds emitted by people and birds. They collected chemical odors from 16 adult human subjects, representing multiple races and ethnic groups. More interestingly, Leal and Syed found that nonanal acts synergistically with carbon dioxide, a known mosquito attractant.  Nonanal, in combination with carbon dioxide, increased trap captures by more than 50 percent, compared to traps baited with carbon dioxide alone. Hope this discovery will help those searching for cheaper, environment friendly  repellents.

Ref : http://www.news.ucdavis.edu/search/news_detail.lasso?id=9289