[Seminar Blog] Hey mosquito, do you smell that? A case of how genetic, chemical and behavioral research determines the true function of mosquito “repellents.”

written by: Amanda ​Brucchieri​

​You’re standing in front of a wall of mosquito repellent. You need to choose the best product, or you are going to be itchy and rather miserable. An ad for some obscure product interrupts the music from the store's speakers and you know you are running out of time to decide. What bottle do you reach for? Dr. Chris Potter, a researcher and professor from Johns Hopkins may have some useful information that will have you choosing the right product for you. 

Figure 1- Mosquito (a) antennae, (b) maxillary palps and (c) labella. Marked with the number of olfactory related neurons present there.

It all starts with the question: “Do you know how mosquitoes smell?” Understanding the answer is crucial as these well-known insects use their olfactory senses for nearly all major life choices. Decisions such as where they can find nectar, mates and for females, what animal to bite for a sample of their blood. They have neurons in their antennae (a), maxillary palps (b) and even in their proboscis and labella (c) to sense important odors in their environment (Figure 1). Mosquitoes use the responses of these neurons to make decisions. It is the goal of researchers to target this olfactory system to stop/disrupt behaviors that lead to a mosquito's decision to bite. We have some tools when it comes to physical guards against these persistent mothers such as sprays, candles and clothing but what about genetically? Well, what makes physical ​tools effective and potential genetic intervention possible could arise from the same vein, pun intended.

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Figure 3 Arm-in-cage experiment one arm is left without repellent and exposed to mosquitoes then removed and the other arm, covered in repellent, is presented in the box. The bites are counted to determine repellent efficacy [2].

Understanding how repellents work is not well understood, even though they have been in use for many years. Dr. Potter explained how we have been using DEET since its discovery between 1942-1947 when 4,137 chemicals were tested using the arm-in-cage experiment (Figure 3), all with the hope of finding ones to stop mosquitoes from biting. About 361 of ​these 4,000+ chemicals provided 3+ hours of protection from biting. The arm-in-cage experiment continues to be the top test for insect repellents, both human-made and many natural, although how they work on the mosquitoes has been unknown, until now.  In fact, this test continues to be the main driver of the bug sprays you buy today. But how do these “repellents” work? Are they repelling mosquitoes or is there something else that prevents their decision to bite?

Dr. Potter brought to the UMD Entomology Department news of some strides his lab has made to answer these questions. Mosquitoes have many different odorant receptors, each of which detects different odors. The odorant receptor co-receptor (ORCO) is necessary for the function of most of these odorant receptors, so mosquitoes lacking ORCO can’t effectively detect odor. Dr. Potter and his lab developed a genetic system to label and visualize olfactory receptor neurons (Figure 4). With access to the vital olfactory neurons, they could determine how repellents affect a mosquito's sense of smell. They investigated both human-made and natural “repellents” on the market today and how each worked. Human-made includes DEET (N, N-diethyl-meta-toluamide), IR3535 (Merck) and Picaridin (Bayer) while natural products include oil of lemon eucalyptus (active ingredient p-Menthane-3,8-diol/PMD) and lemongrass oil. 

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Figure 4. The head of an Anopheles female mosquito whose olfactory neurons are genetically labelled with GFP. Photo from C. Potter.

Dr. Potter brought to the UMD Entomology Department news of some strides his lab has made to answer these questions. Mosquitoes have many different odorant receptors, each of which detects different odors. The odorant receptor co-receptor (ORCO) is necessary for the function of most of these odorant receptors, so mosquitoes lacking ORCO can’t effectively detect odor. Dr. Potter and his lab developed a genetic system to label and visualize olfactory receptor neurons (Figure 4). With access to the vital olfactory neurons, they could determine how repellents affect a mosquito's sense of smell. They investigated both human-made and natural “repellents” on the market today and how each worked. Human-made includes DEET (N, N-diethyl-meta-toluamide), IR3535 (Merck) and Picaridin (Bayer) while natural products include oil of lemon eucalyptus (active ingredient p-Menthane-3,8-diol/PMD) and lemongrass oil.

Dr. Potter and his team identified 2-5 specific neurons that were activated when mosquitoes were exposed to the natural repellents: lemongrass oil and oil of lemon eucalyptus. This demonstrates that natural repellents activate discrete olfactory neurons. When they tested human-made repellents, they found there was no significant activity in the olfactory neurons. In fact, these human-made repellents prevented other odors from activating olfactory neurons. The results of these tests suggested that the human-made repellents (DEET, IR3535 and Picaridin) can act as masking agents, blocking the signal from humans which would otherwise attract mosquitoes [1].

This research team did not stop there. They decided to dig deeper. By puffing air through odor pads onto a mosquito’s antennae to record neural responses, these researchers found that at least a 30% concentration is needed to hide the attractive odor of human skin from a mosquito. From this the team hypothesized that these human-made chemicals did not bind to the ORs but rather, bound to the odors coming off the skin. This would mean if you missed a spot when applying repellent, the mosquitoes can still find it even if they get a good dose of DEET first. With a smile and a properly placed last name Dr. Potter compared these chemicals to the invisibility cloak from Harry Potter.

So, these human-made products hide our odors from mosquitoes, but do they also repel mosquitoes? What about the natural products that the mosquitoes can smell? Well, Dr. Potter and his team asked the mosquitoes. When a female mosquitoes came to rest on the wall of their enclosure they were introduced to a cone of odor. This test allowed the mosquitoes within a cage to move away from an odor that they found repelling. For this study three genera were used: Anopheles, Aedes and Culex. It was important to test all three mosquito genera as they are genetically variable and have significant impacts on human health; Anopheles are carriers of malaria and Aedes and Culex can transmit dengue, yellow fever, chikungunya, west-nile, zika, and others. Anopheles mosquitoes were unperturbed by the human-made chemicals, meaning they stayed at rest after exposure, but when presented with a cone of the natural chemicals (PMD and lemongrass oil) they were repelled. Aedes mosquitoes were not repelled by IR3535 and Picaridin but were repelled by DEET and the naturals. For Culex, DEET was not a great odor repellent like the natural chemicals, but it was more efficient than the other human-made products that had no effect. There is also evidence that DEET, while mainly functioning by masking odors, can also act as a contact repellent via neurons in the mosquitoes’ legs so even if a mosquito, unable to smell, lands on you it may not stick around long enough to bite.

Altering a mosquito's decision to bite us is important for our comfort but more importantly for human health. As disease continues to be spread by these ambitious insects, the need to stop them grows more urgent. These organisms are complex and designed to survive so, while we are far from finding a way to knock out their ability to smell us, our knowledge is growing. Dr. Potter and scientists all over the world are proving crucial to the effort and for the time being have us reaching for the best product on the shelf.
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Citations:
[1] Afify, A., Potter, C.J. Insect repellents mediate species-specific olfactory behaviors in mosquitoes. Malar J 19, 127 (2020). https://doi.org/10.1186/s12936-020-03206-8
[2] Donchai, Worn & Aldred, Arunee & Junkum, Anuluck & Chansang, Arpa. Controlled release of DEET and Picaridin mosquito repellents from microcapsules prepared by complex coacervation using gum Arabic and chitosan. Pharmaceutical Sciences Asia (2022) 506-517. 10.29090/psa.2022.05.22.110. 

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