Mosquito-borne diseases, such as dengue, malaria and Zika, . Mosquitoes are increasingly becoming resistant to current insecticides, leading to a pressing need for new methods to prevent mosquito bites — and the potential transmission of disease.

New research by an international team, including researchers at the 91̽��, provides insight into how an organic compound common in plant-based mosquito repellents affects mosquitoes. The study, , reveals that Aedes aegypti mosquitoes use a specific sensory receptor to detect and avoid borneol (pronounced “bor-nee-ohl”), an organic compound found in several aromatic plants, including camphor trees, rosemary and other aromatic herbs.

“We were surprised by how sensitive the mosquitoes were to this repellent,” said co-author , a 91̽��professor of biology. “By identifying the odorant receptor, we can now develop and test repellents that are even more effective than borneol, in that they last longer and are more repellent.”

The researchers discovered that Aedes aegypti mosquitoes, which are the major carrier of dengue and yellow fever viruses, have a single odor receptor, called OR49, that is highly tuned to detect borneol.

When a mosquito encounters this compound, OR49 activates a specific nerve cell in a mosquito’s maxillary palp, one of its primary organs for detecting odors and locating human hosts. That signal then travels from the nerve cell to a distinct region of the mosquito’s brain, triggering avoidance behavior.

To test how critical this receptor is, the researchers disabled the Or49 gene. Without OR49, the repellent signal essentially disappeared. The mosquitoes’ neurons no longer responded to borneol and the insects were far less likely to avoid it.

Researchers at the 91̽��were instrumental in collecting neural recordings from the mosquito brains to identify how the mosquito olfactory system processes borneol and other similar compounds and repellents.

“Because the repellency through the OR49 receptor is so strong, we might be able to identify other volatile odors that activate the same receptor to ‘push’ mosquitoes away from people,” said co-senior author , associate professor of biology at Baylor University. “The new compounds might be easier and cheaper to produce, or safer and more acceptable to the human nose than existing repellent formulations.”

This research bridges basic neuroscience and public health, offering fresh insight into how tiny sensory signals can have life-saving implications. That is central to the premise of the team’s larger research goal: understanding the genetic basis for how Aedes aegypti is attracted to sources of nectar. The team hopes to create a new generation of mosquito attractants that can be used in traps for enhancing mosquito surveillance and control.

“The knowledge gained in these studies will inform similar studies in mosquitoes that transmit malaria, plus other biting insects that continue to exert negative impacts on human flourishing on a global scale,” Pitts said.

, a 91̽��postdoctoral scholar in the biology department, is a co-author on this paper. A full list of co-authors is included . This research was funded by the Israel Science Foundation; the National Institutes of Health; the National Science Foundation; the Bill and Melinda Gates Foundation; the Science and Technology Development Plan Project of Jilin Province, China; and the Ministry of Science & Technology, Israel.

For more information, contact Riffell at jriffell@uw.edu.

Adapted from .

An international team led by researchers at the 91̽�� has uncovered surprising details about mosquito mating, which could lead to improved malaria control techniques and even help develop precision drone flight. In a published Aug. 30 in the journal Current Biology, the team revealed that when a male Anopheles coluzzii mosquito hears the sound of female-specific wingbeats, his vision becomes active.

Many mosquito species have relatively poor vision, and Anopheles coluzzii — a major spreader of malaria in Africa — is no exception. But the team found that when a male hears the telltale buzz of female flight, his eyes “activate” and he visually scans the immediate vicinity for a potential mate. Even in a busy, crowded swarm of amorous mosquitoes, which is how Anopheles coluzzii mates, the researchers found that the male can visually lock on to his target. He then speeds up and zooms deftly through the swarm — and avoids colliding with others.

“We have discovered this incredibly strong association in male mosquitoes when they are seeking out a mate: They hear the sound of wingbeats at a specific frequency — the kind that females make — and that stimulus engages the visual system,” said lead author Saumya Gupta, a 91̽��postdoctoral researcher in biology. “It shows the complex interplay at work between different mosquito sensory systems.”

This strong link between males hearing the female-like buzz and moving toward an object in their field of vision may open up a new route for mosquito control: a new generation of traps specific to the Anopheles mosquitoes that spread malaria.

“This sound is so attractive to males that it causes them to steer toward what they think might be the source, be it an actual female or, perhaps, a mosquito trap,” said senior author , a 91̽��professor of biology.

Like most Anopheles species, Anopheles coluzzii mate in large swarms at sunset. The bulk of the bugs in these swarms are males, with only a few females. To human eyes, the swarms may appear chaotic. Mosquitoes of both sexes rapidly zip past each other. Males must use their senses to both avoid collision and find a rare female.

Gupta, Riffell and their colleagues — including scientists from Wageningen University in the Netherlands, the Health Sciences Research Institute in Burkina Faso, and the University of Montpelier in France — wanted to understand the interplay between mosquitoes’ senses and how they work together in these swarms. To test the flight behavior of individual male mosquitoes, they built a miniature arena that uses a curved, pixelated screen to mimic the visual chaos of a swarm. The arena is essentially a mosquito flight simulator. In it, the mosquito test subject, which is tethered and cannot freely move, can still see, smell and hear, and also beat its wings as if it is in flight.

In arena tests with dozens of male Anopheles coluzzii mosquitoes, the researchers discovered that males responded differently to an object in their field of vision based on what sound the researchers broadcast into the arena. If they played to a tone at 450 hertz — the frequency at which female mosquito wings beat in these swarms — males steered toward the object. But males did not try to turn toward the object if the researchers played a tone at 700 hertz, which is closer to the frequency at which their fellow males beat their wings.

The mosquito’s perceived distance to the object also mattered. If the simulated object appeared more than three body lengths away, he would not turn toward it, even in the presence of female-like flight tones.

“The resolving power of the mosquito eye is about 1,000-fold less than the resolving power of the human eye,” said Riffell. “Mosquitoes tend to use vision for more passive behaviors, like avoiding other objects and controlling their position.”

In addition to their dramatic response to objects when hearing female flight tones, arena experiments revealed that males made a different set of subtle flight adjustments to other objects. They modified their wingbeat amplitude and frequency in response to an object in their field of view, even with no wingbeat sounds piped in through the speaker. The team hypothesized that these visually driven responses may be preparatory maneuvers to avoid an object. To learn more, they filmed male-only swarms in the laboratory. Analyses of those movements showed that males accelerated away when they neared another male.

“We believe our results indicate that males use close-range visual cues for collision avoidance within swarms,” said Gupta. “However, hearing female flight tones appears to dramatically alter their behavior, suggesting the importance of integrating sound and visual information.”

This research may demonstrate a new method for mosquito control by targeting how mosquitoes integrate auditory and visual cues. The males’ strong and consistent attraction to visual cues when they hear the female buzz may be a vulnerability that researchers can utilize while designing the next generation of mosquito traps —particularly traps for the Anopheles species, which are a major spreader of malaria pathogens.

“Mosquito swarms are a popular target for mosquito control efforts, because it really leads to a strong reduction in biting overall,” said Riffell. “But today’s measures, like insecticides, are increasingly less effective as mosquitoes evolve resistance. We need new approaches, like lures or traps, which will draw in mosquitoes with high fidelity.”

Co-authors are Antoine Cribellier, Serge Poda and Florian Muijres of Wageningen University of Wageningen University in the Netherlands and Olivier Roux of the University of Montpelier in France. Roux and Poda are also with the Health Sciences Research Institute in Burkina Faso. The research was funded by the Human Frontiers Science Program, the National Institutes of Health, the Air Force Office of Scientific Research and the French National Research Agency.

For more information, contact Riffell at jriffell@uw.edu and Gupta at saumyag@uw.edu.

91̽�� researchers are hoping those itchy bumps could soon become a thing of the past.

Jeffrey Riffell, a 91̽��professor of biology, studies mosquito sensory systems, particularly their sense of smell. He and his team want to understand how mosquitoes find food, whether it be males — who drink nectar — or females, who drink blood when they are trying to produce eggs.

Riffell’s research has shown that hungry female mosquitoes find us by following a trail of scent cues, including chemicals exuded by our skin and sweat, as well as the carbon dioxide gas we exhale with each breath. Mosquitoes also like colors, at least certain ones. His team is investigating how the visual and olfactory senses work together to help a mosquito zero in for the final strike and get her blood meal.

In the United States, climate change is opening new habitats for mosquitos. Washington currently boasts 20 species, including ones that can transmit West Nile virus.

Knowing what attracts mosquitoes — males to flowers, females to people — can help develop better control and containment efforts against these insects, whose bites can also transmit malaria, Zika, dengue, yellow fever and other diseases. Traps that kill or poison mosquitoes, for example, would be more effective if they released a mosquito-attracting scent. Mosquito-borne illnesses kill hundreds of thousands of people each year. Riffell and his team hope their efforts can help take a bite out of those numbers.

For more information, contact Riffell at jriffell@uw.edu.��

A team led by researchers at the 91̽�� has discovered a major cause for a drop in nighttime pollinator activity — and people are largely to blame.

The researchers found that nitrate radicals (NO3) in the air degrade the scent chemicals released by a common wildflower, drastically reducing the scent-based cues that nighttime pollinators rely on to locate the flower. In the atmosphere, NO3 is produced by chemical reactions among other nitrogen oxides, which are themselves released by the combustion of gas and coal from cars, power plants and other sources. The findings, Feb. 9 in the journal Science, are the first to show how nighttime pollution creates a chain of chemical reactions that degrades scent cues, leaving flowers undetectable by smell. The researchers also determined that pollution likely has worldwide impacts on pollination.

The team — co-led by , a 91̽��professor of biology, and , a 91̽��professor of atmospheric sciences — studied the (Oenothera pallida). This wildflower grows in arid environments across the western U.S. They chose this species because its white flowers emit a scent that attracts a diverse group of pollinators, including nocturnal moths, which are one of its most important pollinators.

At field sites in eastern Washington, the researchers collected scent samples from pale evening primrose flowers. Back in the laboratory, they used chemical analysis techniques to identify the dozens of individual chemicals that make up the wildflower’s scent.

“When you smell a rose, you’re smelling a diverse bouquet composed of different types of chemicals,” said Riffell. “The same is true for almost any flower. Each has its own scent made up of a specific chemical recipe.”

Once they had identified the individual chemicals that make up the wildflower’s scent, the team used a more advanced technique called mass spectrometry to observe how each chemical within the scent reacted to NO3. They found that reacting with NO3 nearly eliminated certain scent chemicals. In particular, the pollutant decimated levels of monoterpene scent compounds, which in separate experiments moths found most attractive.

Moths, which smell through their antennae, have a scent-detection ability that is roughly equivalent to dogs — and several thousand times more sensitive than the human sense of smell. Research suggests that several moth species can detect scents from miles away, according to Riffell.

Using a wind tunnel and computer-controlled odor-stimulus system, the team investigated how well two moth species — the (Hyles lineata) and the (Manduca sexta) — could locate and fly toward scents. When the researchers introduced the pale evening primrose’s normal scent, both species would readily fly toward the scent source. But when the researchers introduced the scent and NO3 at levels typical for a nighttime urban setting, Manduca’s accuracy dropped by 50% and Hyles — one of the chief nocturnal pollinators of this flower — could not locate the source at all.

Experiments in a natural setting backed up these findings. In field experiments, the team showed that moths visited a fake flower emitting unaltered scent as often as they visited a real one. But, if they treated the scent first with NO3, moth visitation levels dropped by as much as 70%.

“The NO3 is really reducing a flower’s ‘reach’ — how far its scent can travel and attract a pollinator before it gets broken down and is undetectable,” said Riffell.

The team also compared how daytime and nighttime pollution conditions impacted the wildflower’s scent chemicals. Nighttime pollution had a much more destructive effect on the scent’s chemical makeup than daytime pollution. The researchers believe this is largely due to sunlight degrading NO3.

The team used a computer model that simulates both global weather patterns and atmospheric chemistry to locate areas most likely to have significant problems with plant-pollinator communication. The areas identified include western North America, much of Europe, the Middle East, Central and South Asia, and southern Africa.

“Outside of human activity, some regions accumulate more NO3 because of natural sources, geography and atmospheric circulation,” said Thornton, who added that natural sources of NO3 include wildfires and lightning. “But human activity is producing more NO3 everywhere. We wanted to understand how those two sources — natural and human — combine and where levels could be so high that they could interfere with the ability of pollinators to find flowers.”

The researchers hope their study is just the first of many to help uncover the full scope of pollinator failure.

“Our approach could serve as a roadmap for others to investigate how pollutants impact plant-pollinator interactions, and to really get at the underlying mechanisms,” said Thornton. “You need this kind of holistic approach, especially if you want to understand how widespread the breakdown in plant-pollinator interactions is and what the consequences will be.”

The study highlights the dangers of human-fueled pollution and its implications for all pollinators as well as the future of agriculture.

“Pollution from human activity is altering the chemical composition of critical scent cues, and altering it to such an extent that the pollinators can no longer recognize it and respond to it,” said Riffell.

Approximately three-quarters of the more than 240,000 species of flowering plants rely on pollinators, Riffell said. And more than��70 species of pollinators are endangered or threatened.

Lead author on the paper is Jeremy Chan, a postdoctoral researcher at the University of Copenhagen who conducted this study as a 91̽��doctoral student in biology. Co-authors are Sriram Parasurama in the 91̽��Department of Biology; Rachel Atlas, a postdoctoral researcher at the Pierre Simon Laplace Institute in France who participated in this study as a 91̽��doctoral student in atmospheric sciences; , a 91̽��doctoral students in atmospheric sciences; Ruochong Xu, a doctoral student at Tsinghua University in China; , a 91̽��professor of atmospheric sciences; and , a professor of chemistry at Seattle University. The research was funded by the Air Force Office of Scientific Research, the National Science Foundation, the National Institutes of Health, the Human Frontiers in Science Program, and the 91̽��.

For more information, contact Riffell at 206-348-0789 or jriffell@uw.edu and Thornton at 206-543-4010 or joelt@uw.edu.

Beating the bite of mosquitoes this spring and summer could hinge on your attire and your skin. New research led by scientists at the 91̽�� indicates that a common mosquito species — after detecting a telltale gas that we exhale — flies toward specific colors, including red, orange, black and cyan. The mosquitoes ignore other colors, such as green, purple, blue and white. The researchers believe these findings help explain how mosquitoes find hosts, since human skin, regardless of overall pigmentation, emits a strong red-orange “signal” to their eyes.

“Mosquitoes appear to use odors to help them distinguish what is nearby, like a host to bite,” said , a 91̽��professor of biology. “When they smell specific compounds, like CO2 from our breath, that scent stimulates the eyes to scan for specific colors and other visual patterns, which are associated with a potential host, and head to them.”

The results, Feb. 4 in Nature Communications, reveal how the mosquito sense of smell — known as olfaction — influences how the mosquito responds to visual cues. Knowing which colors attract hungry mosquitoes, and which ones do not, can help design better repellants, traps and other methods to keep mosquitoes at bay.

“One of the most common questions I’m asked is ‘What can I do to stop mosquitoes from biting me?’” said Riffell, who is senior author on the paper. “I used to say there are three major cues that attract mosquitoes: your breath, your sweat and the temperature of your skin. In this study, we found a fourth cue: the color red, which can not only be found on your clothes, but is also found in everyone’s skin. The shade of your skin doesn’t matter, we are all giving off a strong red signature. Filtering out those attractive colors in our skin, or wearing clothes that avoid those colors, could be another way to prevent a mosquito biting.”

In their experiments, the team tracked behavior of female yellow fever mosquitoes, , when presented with different types of visual and scent cues. Like all mosquito species, only females drink blood, and bites from A. aegypti can transmit dengue, yellow fever, chikungunya and Zika. The researchers tracked individual mosquitoes in miniature test chambers, into which they sprayed specific odors and presented different types of visual patterns — such as a colored dot or a tasty human hand.

Without any odor stimulus, mosquitoes largely ignored a dot at the bottom of the chamber, regardless of color. After a spritz of CO2 into the chamber, mosquitos continued to ignore the dot if it was green, blue or purple in color. But if the dot was red, orange, black or cyan, mosquitoes would fly toward it.

Humans can’t smell CO2, which is the gas we and other animals exhale with each breath. Mosquitoes can. Past research by Riffell’s team and other groups showed that smelling CO2 boosts female mosquitoes’ activity level — searching the space around them, presumably for a host. The colored-dot experiments revealed that after smelling CO2, these mosquitoes’ eyes prefer certain wavelengths in the visual spectrum.

It’s similar to what might happen when humans smell something good.

“Imagine you’re on a sidewalk and you smell pie crust and cinnamon,” said Riffell. “That’s probably a sign that there’s a bakery nearby, and you might start looking around for it. Here, we started to learn what visual elements that mosquitoes are looking for after smelling their own version of a bakery.”

Most humans have “true color” vision: We see different wavelengths of light as distinct colors: 650 nanometers shows up as red, while 450 nanometer wavelengths look blue, for example. The researchers do not know whether mosquitoes perceive colors the same way that our eyes do. But most of the colors the mosquitoes prefer after smelling CO2 — orange, red and black — correspond to longer wavelengths of light. Human skin, regardless of pigmentation, also gives off a long-wavelength signal in the red-orange range.

When Riffell’s team repeated the chamber experiments with human skintone pigmentation cards — or a researcher’s bare hand — mosquitoes again flew toward the visual stimulus only after CO2 was sprayed into the chamber. If the researchers used filters to remove long-wavelength signals, or had the researcher wear a green-colored glove, then CO2-primed mosquitoes no longer flew toward the stimulus.

Genes determine the preference of these females for red-orange colors. Mosquitoes with a mutant copy of a gene needed to smell CO2 no longer showed a color preference in the test chamber. Another strain of mutant mosquitoes, with a change related to vision so they could no longer “see” long wavelengths of light, were more color-blind in the presence of CO2.

“These experiments lay out the first steps mosquitoes use to find hosts,” said Riffell.

More research is needed to determine how other visual and odor cues — such as skin secretions — help mosquitoes target potential hosts at close range. Other mosquito species may also have different color preferences, based on their preferred host species. But these new findings add a new layer to mosquito control: color.

Co-lead authors on the paper are Diego Alonso San Alberto, a researcher and lecturer in the 91̽��Department of Biology, and Claire Rusch, a 91̽��doctoral alum in biology. Co-authors are Yinpeng Zhan and Craig Montell at the University of California, Santa Barbara, and Andrew Straw at the University of Freiburg in Germany. The research was funded by the National Institutes of Health, the Air Force Office of Scientific Research, the 91̽��and the U.S. Army Research Office.

For more information, contact Riffell at jriffell@uw.edu.

A published Aug. 11 in the Proceedings of the Royal Society B by researchers at the 91̽�� and Stony Brook University reports on how bats and pepper plants in Central America have coevolved to help each other survive.

The team — led by , a 91̽��professor of biology and curator of mammals at the Burke Museum of Natural History and Culture — focused on the complex mixture of volatile organic compounds, or VOCs, produced by fruits on pepper plants in the genus Piper at prime ripeness. The study showed how these VOCs may have evolved to attract scent-oriented, short-tailed fruit bats from the genus Carollia, who then eat the fruits and excrete the seeds into the landscape.

Plant–animal interactions have captured the attention of biologists for centuries, and are key to maintaining the biodiversity of tropical ecosystems. The dispersal syndrome hypothesis — an explanation of how mutually beneficial relationships between plants and fruit-eating animals may lead to coevolution — proposes that, when animals are effective seed dispersers, they may select for fruit traits, including size, color and odor, that match their sensory abilities, such as vision and olfaction. But few studies have tested this hypothesis for complex traits like fruit scents. This research provides one of the first tests of bat-driven, fruit scent evolution.

The study is based on data collected during fieldwork at La Selva Biological Station in Costa Rica. There, Piper is highly diverse, with more than 50 recognized species. It is also a location where three Carollia species — C. castanea, C. sowellii and C. perspicillata — are some of the most abundant bats year-round and coexist with approximately 62 other bat species.

The team spent hundreds of hours searching and collecting ripe fruits from Piper to extract and quantify the VOCs that make up their fragrant scent. They also collected fecal samples from live bats and then released them back into the wild to determine which Piper species the bats were eating and how much. In addition, the researchers conducted behavioral experiments with wild bats where they offered options of unripe fruits enhanced with the most common VOCs found in local Piper plants. Video cameras and microphones recorded the bats’ feeding behaviors and echolocation calls.

The team found Piper fruit scent bouquets were complex and diverse. The authors identified and quantified 249 VOCs in ripe fruit scents across 22 Piper species. Some compounds were found in the fruit scent of most species — like alpha-caryophyllene, which has a spicy scent like cinnamon or cloves. Others, like 2-heptanol, were only found in a few Piper species. The diet experiments showed that, while the three Carollia fruit bat species varied in their reliance on Piper as a food source, all consumed a lot of a few Piper species, and a little of many others. Surprisingly, this was not related to how abundant the Piper species are at La Selva, so the bats must choose Piper fruits based on other characteristics and not just how well represented they are across the landscape. The team’s behavioral experiments provided some clues to what might be happening: Bats preferred samples spiked with 2-heptanol, a VOC found in the fruit scents of the Piper species they eat the most.

“These findings suggest bats use specific chemicals in the fruit scent bouquet not only to select ripe fruits, but to find the specific Piper species that make up the bulk of their diet,” said Santana, who is co-lead author on the study. “By helping them communicate with the bats, these chemical signals are likely a component of a dispersal syndrome in these plants.”

Through statistical and evolutionary analyses of the data on fruit scent chemistry and bat diet, the team further demonstrated that the evolutionary patterns of chemical diversity and the presence of specific compounds in Piper fruit scents is associated with greater bat consumption and scent preferences. This highlights the potential effect of bat fruit consumption on the evolution of fruit chemistry, a relationship that contributes to the extreme diversity of tropical fruiting plants worldwide.

“Flying in the dark means bats cannot find ripe fruit by sight, but rely on olfaction instead,” said co-author , a professor at Stony Brook University. “Olfaction is the bridge between the plant signal and bat fruit consumption, and finding the specific VOCs bats respond to opens the door to matching olfactory receptor genes to important VOCs, which has been impossible until now.”

Understanding the relationship between bats and pepper plants not only contributes to knowledge about coevolution of these species, but also has benefits for rainforest habitat conservation. Piper are some of the first plants to grow in forest gaps and edges, and Carollia ― as key dispersers of Piper seeds ― can help restore plant life in logged areas.

“Our current and future work is identifying the odorant receptors that allow the bats to detect the fruit scents. This will allow us to link the ecology and evolution of these relationships with the physiological mechanisms,” said co-author , a 91̽��professor of biology.

Co-lead author on the paper is former 91̽��postdoctoral researcher Zofia Kaliszewska. Other co-authors are 91̽��doctoral alum Leith Leiser-Miller, M. Elise Lauterbur at the University of Arizona and Jessica Arbour at Middle Tennessee State University. The research was funded by the National Science Foundation.

For high-resolution images, video and interviews, contact burkepr@uw.edu.

Malaria mosquitoes, the world’s deadliest creatures, cause 400,000 deaths a year. One of two awarded to 91̽��biology professors by the or HFSP, will enable a research team including to study malaria mosquito swarming and mating dynamics to help control swarms and curb disease. The project also aims to explore “novel bio-inspiration” for flight control strategies in aerial robotics research.

The research project is titled “How do malaria mosquitoes swarm and mate? The functional biology of mating swarms.” The three-year HFSP grant is for $1,340,000, evenly divided among four institutions, and lead investigator is Florian Muijres of Wageningen University in the Netherlands.

Biology professor and chair is part of a research team that will combine aerospace engineering and neuroscience approaches to study how the sensing properties of a bird wings and feathers that allow them to “‘feel’ their way through the air, coping with challenging gusty wind conditions, to perform aerial maneuvers.

That research project is titled “Feathers as structures and sensors: Understanding mechanosensing in bird flight.” The three-year HFSP grant is for $1,050,000 split among the three institutions, and lead investigator is Shane Windsor of the University of Bristol, in the U.K.

The Human Frontier Science Program, formerly called Young Investigators, is supported by 13 countries and the European Union. The HFSP funds international research teams involving at least two countries, with preference given for intercontinental collaborations.

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Without their keen sense of smell, mosquitoes wouldn’t get very far. They rely on this sense to find a host to bite and spots to lay eggs.

And without that sense of smell, mosquitoes could not locate their dominant source of food: nectar from flowers.

“Nectar is an important source of food for all mosquitoes,” said , a professor of biology at the 91̽��. “For male mosquitoes, nectar is their only food source, and female mosquitoes feed on nectar for all but a few days of their lives.”

Yet scientists know little about the scents that draw mosquitoes toward certain flowers, or repel them from others. This information could help develop less toxic and better repellents, more effective traps and understand how the mosquito brain responds to sensory information — including the cues that, on occasion, lead a female mosquito to bite one of us.

Riffell’s team, which includes researchers at the UW, Virginia Tech and UC San Diego, has discovered the chemical cues that lead mosquitoes to pollinate a particularly irresistible species of orchid. As they report in a published in the Proceedings of the National Academy of Sciences, the orchid produces a finely balanced bouquet of chemical compounds that stimulate mosquitoes’ sense of smell. On their own, some of these chemicals have either attractive or repressive effects on the mosquito brain. When combined in the same ratio as they’re found in the orchid, they draw in mosquitoes as effectively as a real flower. Riffell’s team also showed that one of the scent chemicals that repels mosquitoes lights up the same region of the mosquito brain as , a common and controversial mosquito repellant.

Their findings show how environmental cues from flowers can stimulate the mosquito brain as much as a warm-blooded host — and can draw the mosquito toward a target or send it flying the other direction, said Riffell, who is the senior author of the study.

The blunt-leaf orchid, or Platanthera obtusata, grows in cool, high-latitude climates across the Northern Hemisphere. From field stations in the Okanogan-Wenatchee National Forest in Washington state, Riffell’s team verified past research showing that local mosquitoes pollinate this species, but not its close relatives that grow in the same habitat. When researchers covered the flowers with bags — depriving the mosquitoes of a visual cue for the flower — the mosquitoes would still land on the bagged flowers and attempt to feed through the canvas.

Orchid scent obviously attracted the mosquitoes. To find out why, Riffell’s team turned to the individual chemicals that make up the blunt-leaf orchid’s scent.

“We often describe ‘scent’ as if it’s one thing — like the scent of a flower, or the scent of a person,” said Riffell. “Scent is actually a complex combination of chemicals — the scent of a rose consists of more than 300 — and mosquitoes can detect the individual types of chemicals that make up a scent.”

Riffell describes the blunt-leaf orchid’s scent as a grassy or musky odor, while its close relatives have a sweeter fragrance. The team used gas chromatography and mass spectroscopy to identify dozens of chemicals in the scents of the Platanthera species. Compared to its relatives, the blunt-leaf orchid’s scent contained high amounts of a compound called , and smaller amounts of another chemical, lilac aldehyde.

Riffell’s team also recorded the electrical activity in mosquito antennae, which detect scents. Both nonanal and lilac aldehyde stimulated antennae of mosquitoes that are native to the blunt-leaf orchid’s habitat. But these compounds also stimulated the antennae of mosquitoes from other regions, including Anopheles stephensi, which spreads malaria, and Aedes aegypti, which spreads dengue, yellow fever, Zika and other diseases.

Experiments of mosquito behavior showed that both native and non-native mosquitoes preferred a solution of nonanal and lilac aldehyde mixed in the same ratio as found in blunt-leaf flowers. If the researchers omitted lilac aldehyde from the recipe, mosquitoes lost interest. If they added more lilac aldehyde — at levels found in the blunt-leaf orchid’s close relatives — mosquitoes were indifferent or repelled by the scent.

Using techniques developed in Riffell’s lab, they also peered directly into the brains of Aedes increpitus mosquitoes, which overlap with blunt-leaf orchids, and a genetically modified strain of Aedes aegypti by Riffell and co-author , an associate professor at UC San Diego. They imaged calcium ions — signatures of actively firing neurons — in the antenna lobe, the region of the mosquito brain that processes signals from the antennae.

These brain imaging experiments revealed that nonanal and lilac aldehyde stimulate different parts of the antenna lobe — and even compete with one another when stimulated: The region that responds to nonanal can suppress activity in the region that responds to lilac aldehyde, and vice versa. Whether this “cross talk” makes a flower attractive or repelling to the mosquito likely depends on the amounts of nonanal and lilac aldehyde in the original scent. Blunt-leaf orchids have a ratio that attracts mosquitoes, while closely related species do not, according to Riffell.

“Mosquitoes are processing the ratio of chemicals, not just the presence or absence of them,” said Riffell. “This isn’t just important for flower discrimination — it’s also important for how mosquitoes discern between you and I. Human scent is very complex, and what is probably important for attracting or repelling mosquitoes is the ratio of particular chemicals. We know that some people get bit more than others, and maybe a difference in ratio explains why.”

The team also discovered that lilac aldehyde stimulates the same region of the antenna lobe as DEET. That region may process “repressive” scents, though further research would need to verify this, said Riffell. It’s too soon to tell if lilac aldehyde may someday be an effective mosquito repellant. But if it is, there is an added bonus.

“It smells wonderful,” said Riffell.

Lead author is , who conducted the research as a 91̽��postdoctoral fellow and is now a research assistant professor at Virginia Tech. Additional co-authors are , a former 91̽��postdoctoral researcher and current assistant professor at Virginia Tech; 91̽��biology graduate students and ; and 91̽��postdoctoral researcher . The research was funded by the National Institutes of Health, the Air Force Office of Scientific Research and the 91̽��.

For more information, contact Riffell at 206-685-2573 or jriffell@uw.edu.

Through behavioral experiments and real-time recording of the female mosquito brain, a team of scientists, led by researchers at the 91̽��, has discovered how the mosquito brain integrates signals from two of its sensory systems — visual and olfactory — to identify, track and hone in on a potential host for her next blood meal.

Their findings, July 18 in the journal , indicate that, when the mosquito’s olfactory system detects certain chemical cues, they trigger changes in the mosquito brain that initiate a behavioral response: The mosquito begins to use her visual system to scan her surroundings for specific types of shapes and fly toward them, presumably associating those shapes with potential hosts.

Only female mosquitoes feed on blood, and these results give scientists a much-needed glimpse of the sensory-integration process that the mosquito brain uses to locate a host. Scientists can use these findings to help develop new methods for mosquito control and reduce the spread of mosquito-borne diseases.

This study focused on the olfactory cue that triggers the hunt for a host: carbon dioxide, or CO2. For mosquitoes, smelling CO2 is a telltale sign that a potential meal is nearby.

“Our breath is just loaded with CO2,” said corresponding author , a 91̽��professor of biology. “It’s a long-range attractant, which mosquitoes use to locate a potential host that could be more than 100 feet away.”

That potential host could be a person or another warm-blooded animal. Prior research by Riffell and his collaborators has shown that smelling CO2 can “prime” the mosquito’s visual system to hunt for a host. In this new research, they measure how CO2 triggers precise changes in mosquito flight behavior and visualize how the mosquito brain responds to combinations of olfactory and visual cues.

The team collected data from approximately 250 individual mosquitoes during behavioral trials conducted in a small circular arena, about 7 inches in diameter. A 360-degree LED display framed the arena and a tungsten wire tether in the middle held each mosquito. An optical sensor below the insect collected data about mosquito wingbeats, an air inlet and vacuum line streamed odors into the arena, and the LED display showed different types of visual stimuli.

The team tested how tethered mosquitoes responded to visual stimuli as well as puffs of CO2-rich air. They found that, in the arena, one-second puffs of air containing 5% CO2 — just above the 4.5% CO2 air emitted by humans — prompted the mosquitoes to beat their wings faster. Some visual elements like a fast-moving starfield had little effect on mosquito behavior. But if the arena showed a horizontally moving bar, mosquitoes beat their wings faster and attempted to steer in the same direction. This response was more pronounced if researchers introduced a puff of CO2 before showing the bar.

To get a clear picture of how smelling CO2 first affected flight behavior, they analyzed their data using a mathematical model of housefly flight behavior.

“We found that CO2 influences the mosquito’s ability to turn toward an object that isn’t directly in their flight path,” said Riffell. “When they smell the CO2, they essentially turn toward the object in their visual field faster and more readily than they would without CO2.”

The researchers repeated the arena experiments with created by Riffell and co-author , an assistant professor at the University of California, San Diego. Cells in these mosquitoes glow fluorescent green if they contain high levels of calcium ions — including neurons of the central nervous system when they are actively firing. In the arena, the researchers removed a small portion of the mosquito skull and used a microscope to view neuronal activity in sections of the brain in real time.

The team focused on 59 “regions of interest” that showed especially high levels of calcium ion levels in the lobula, a part of the mosquito brain’s optic lobe. If the mosquito was shown a horizontal bar, two-thirds of those regions lit up, indicating increased neuronal firing in response to the visual stimulus. When the researchers introduced a puff of CO2 first and then showed the horizontal bar, 23% of the regions had even higher activity than before — indicating that the CO2 odor prompted a larger-magnitude response in these areas of the brain that control vision.

The researchers tried the reverse experiment — seeing if a horizontal bar triggered increased firing in the parts of the mosquito brain that control smell — but saw no response.

“Smell triggers vision, but vision does not trigger the sense of smell,” said Riffell.

Their findings align with the general picture of mosquito senses. The mosquito sense of smell operates at long distances, picking up scents more than 100 feet away. But their eyesight is most effective for objects 15 to 20 feet away, according to Riffell.

“Olfaction is a long-range sense for mosquitoes, while vision is for intermediate-range tracking,” said Riffell. “So, it makes sense that we see an odor — in this case CO2 — affecting parts of the mosquito brain that control vision, and not the reverse.”

In the future, Riffell wants to test whether other shapes affect mosquito behavior and activity in the optic lobe. Those results may further illuminate the hierarchical nature of mosquito host-hunting behaviors: smell first, then see. It may also provide new knowledge for mosquito control.

Co-lead authors on the paper are former 91̽��postdoctoral researchers , now an assistant professor at Virginia Tech, and , now an assistant professor at the University of Nevada-Reno. Additional co-authors include 91̽��alumna Lauren Locke; 91̽��undergraduate student Kennedy Tobin; , a professor at Caltech; and , a 91̽��professor of physiology and biophysics. The research was funded by the Air Force Office of Scientific Research, the National Institutes of Health and the 91̽��.

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For more information, contact Riffell at jriffell@uw.edu or van Breugel at fvanbreugel@unr.edu.

Grant numbers: FA9550-14-1-0398, FA9550-16-1-0167, 1RO1DCO13693, 1R21AI137947, 5K22AI113060, 1R21AI123937.

Most of us surely don’t think of mosquitoes as being especially adept at learning. But that may not be the case.

In a published Jan. 25 in , 91̽�� researchers report that mosquitoes can in fact learn to associate a particular odor with an unpleasant mechanical shock akin to being swatted. As a result, they’ll avoid that scent the next time.

“Once mosquitoes learned odors in an aversive manner, those odors caused aversive responses on the same order as responses to DEET, which is one of the most effective mosquito repellents,” said senior author , a 91̽��professor of biology. “Moreover, mosquitoes remember the trained odors for days.”

Researchers already knew that mosquitoes don’t decide whom to bite at random. They show obvious preferences for some people over others. They are also known to alternate hosts seasonally, feeding on birds in the summer and mammals and birds during other parts of the year, for instance. Riffell and his colleagues wanted to find out more about how learning might influence mosquitoes’ biting preferences.

As a first step, they trained mosquitoes by pairing the odor of a particular person or animal species — a rat versus a chicken, for example — with a mechanical shock. For the mechanical shock, they used a vortex mixer to simulate the vibrations and accelerations a mosquito might experience when a person tried to swat them. The insects quickly learned the association between the host odor and the mechanical shock and used that information in deciding which direction to fly — though interestingly, the mosquitoes could never learn to avoid the smell of a chicken.

Learning in many animals, from honeybees to humans, depends on dopamine in the brain. Additional experiments by Riffell and his team showed that dopamine also is essential in mosquito learning. Genetically modified mosquitoes lacking dopamine receptors lost the ability to learn.

The researchers also glued mosquitoes to a custom, 3-D-printed miniature “arena” in which the insects could fly in place, while researchers recorded the activity of neurons in the olfactory center of their brains. Those experiments showed that without dopamine, those neurons were less likely to fire. As a result, mosquitoes became less able to process and learn from odor information.

These findings may have important implications for mosquito control and the transmission of mosquito-borne diseases, according to the researchers.

“By understanding how mosquitoes are making decisions on whom to bite, and how learning influences those behaviors, we can better understand the genes and neuronal bases of the behaviors,” said Riffell. “This could lead to more effective tools for mosquito control.”

With this new understanding of how mosquitoes learn to avoid certain hosts, the researchers say they are now exploring mosquitoes’ ability to learn and remember favored hosts.

“In both cases, we think dopamine is a critical component,” said Riffell.

Co-lead authors on the paper are former 91̽��postdoctoral researchers and . Vinauger is now an assistant professor of biochemistry at Virginia Tech, and Lahondère is a research assistant professor of biochemistry at Virginia Tech. 91̽��co-authors are postdoctoral researcher Gabriella Wolff, undergraduate alumna Lauren Locke, undergraduate Jessica Liaw and associate professor of biology . Additional co-authors are assistant professor of the University of California, Riverside and professor of the California Institute of Technology.

The research was funded by the Air Force Office of Sponsored Research; the National Institutes of Health; the National Science Foundation; the University of California, Riverside; MaxMind; a 91̽��Endowed Professorship for Excellence in Biology; the 91̽��; and the .

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For more information, contact Riffell at jriffell@uw.edu or Vinauger at vinauger@vt.edu.

Adapted from a by Current Biology.

Grant numbers: FA9550-14-1-0398, FA9550-16-1-0167, NIH1RO1DCO13693-0, IOS-1354159, HFSP-RGP0022.