image: A mosquito on the Platanthera orchid. Note the two yellow pollen masses that are stuck on its head. Photo courtesy of Chloé Lahondère for Virginia Tech.
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.
"Interestingly, only females need blood. They use it to produce their eggs. However, males rely 100 percent on nectar to survive. Females also feed on nectar which increases they longevity and survival," said Chloé Lahondère, a research assistant professor from the Department of Biochemistry in the Virginia Tech College of Agriculture and Life Sciences.
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 lead to an understanding of how the mosquito brain responds to sensory information -- including the cues that, on occasion, lead a female mosquito to bite one of us.
Lahondère and the rest of the research team, which includes assistant professor Clément Vinauger from the Department of Biochemistry, as well as researchers from the University of Washington and UC San Diego, have discovered the chemical cues that lead mosquitoes to pollinate a particularly irresistible species of orchid -- the blunt-leaf orchid, also known as Platanthera obtusata.
As they report in a paper 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.
"We found that this orchid emits chemicals that attract different mosquito species, including Aedes aegypti, an invasive disease vector species that is not present in the native area of the orchid. And interestingly, all these mosquitoes respond to the same volatiles that the orchid emits," said Lahondère, who is the lead researcher for this study and an affiliated faculty member of the Global Change Center, an arm of the Fralin Life Sciences Institute at Virginia Tech. "This means that we can use this knowledge to develop new baits based on the flower scent and target a large diversity of mosquito species."
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. The research team also found that one of the scent chemicals that repels mosquitoes lights up the same region of the mosquito brain as DEET, 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 Jeffrey Riffell, a professor of biology at the University of Washington and senior author of the study.
The blunt-leaf orchid grows in cool, high-latitude climates across the Northern Hemisphere. From field stations in the Okanogan-Wenatchee National Forest in Washington state, the research 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, the 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."
The blunt-leaf orchid is described to have a scent that has 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 nonanal, and smaller amounts of another chemical, lilac aldehyde.
Researchers 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.
"Beyond the notion that mosquitoes can actually be useful in that they pollinate endangered orchids, this study also provides new knowledge on the neural circuits that regulate mosquito olfaction. Down the line, these discoveries could lead to the identification of even more targets to prevent mosquitoes from finding us," said Vinauger, who is an affiliated faculty member of the Fralin Life Sciences Institute and the BIOTRANS program.
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, the team peered directly into the brains of Aedes increpitus mosquitoes, which overlap with blunt-leaf orchids, and a genetically modified strain of Aedes aegypti previously developed by Riffell and co-author Omar Akbari, 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.
As far as future projects are concerned, Lahondère and Vinauger hope to use their recent findings from Washington to learn more about a relationship that exists between ornamental plants and invasive mosquitoes in Virginia.
"Sugar feeding is one of our main projects right now. Based on our results, we hope to develop new efficient baits targeting important disease vectors, such as Ae. aegypti and Ae. albopictus, which are vectors of dengue, yellow fever, and Zika," said Lahondère, who is also an affiliated faculty member of the BIOTRANS program.