Nav: Home

Mosquitoes are drawn to flowers as much as people -- and now scientists know why

January 21, 2020

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 Jeffrey Riffell, a professor of biology at the University of Washington. "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 paper published online Dec. 23 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 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 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 nonanal, 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 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. 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.
-end-
Lead author is Chloé Lahondère, who conducted the research as a UW postdoctoral fellow and is now a research assistant professor at Virginia Tech. Additional co-authors are Clément Vinauger, a former UW postdoctoral researcher and current assistant professor at Virginia Tech; UW biology graduate students Ryo Okubo and Jeremy Chan; and UW postdoctoral researcher Gabriella Wolff. The research was funded by the National Institutes of Health, the Air Force Office of Scientific Research and the University of Washington.

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

Grant numbers: RO1-DC013693, R21-AI137947, FA9550-14-1-0398, FA9550-16-1-0167

Link to full release with images: https://www.washington.edu/news/2020/01/21/mosquitoes-flowers/ James Urton
University of Washington
206-543-2580
jurton@uw.edu

University of Washington

Related Mosquitoes Articles:

Mosquitoes are drawn to flowers as much as people -- and now scientists know why
Despite their reputation as blood-suckers, mosquitoes actually spent most of their time drinking nectar from flowers.
Mosquitoes engineered to repel dengue virus
An international team of scientists has synthetically engineered mosquitoes that halt the transmission of the dengue virus.
Engineered mosquitoes cannot be infected with or transmit any dengue virus
Genetically engineered mosquitoes are resistant to multiple types of dengue virus (DENV), according to a study published Jan.
Researchers identify that mosquitoes can sense toxins through their legs
Researchers at LSTM have identified a completely new mechanism by which mosquitoes that carry malaria are becoming resistant to insecticide.
Mated female mosquitoes are more likely to transmit malaria parasites
Female mosquitoes that have mated are more likely to transmit malaria parasites than virgin females, according to a study published Nov.
In Baltimore, lower income neighborhoods have bigger mosquitoes
Low-income urban neighborhoods not only have more mosquitoes, but they are larger-bodied, indicating that they could be more efficient at transmitting diseases.
Mosquitoes more likely to lay eggs in closely spaced habitats
Patches of standing water that are close together are more likely to be used by mosquitoes to lay eggs in than patches that are farther apart.
Why do mosquitoes choose us? Lindy McBride is on the case
Most of the 3,000+ mosquito species are opportunistic, but Princeton's Lindy McBride is most interested in the mosquitoes that scientists call 'disease vectors' -- carriers of diseases that plague humans -- some of which have evolved to bite humans almost exclusively.
Biting backfire: Some mosquitoes actually benefit from pesticide application
The common perception that pesticides reduce or eliminate target insect species may not always hold.
What makes mosquitoes avoid DEET? An answer in their legs
Many of us slather ourselves in DEET each summer in hopes of avoiding mosquito bites, and it generally works rather well.
More Mosquitoes News and Mosquitoes Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Processing The Pandemic
Between the pandemic and America's reckoning with racism and police brutality, many of us are anxious, angry, and depressed. This hour, TED Fellow and writer Laurel Braitman helps us process it all.
Now Playing: Science for the People

#568 Poker Face Psychology
Anyone who's seen pop culture depictions of poker might think statistics and math is the only way to get ahead. But no, there's psychology too. Author Maria Konnikova took her Ph.D. in psychology to the poker table, and turned out to be good. So good, she went pro in poker, and learned all about her own biases on the way. We're talking about her new book "The Biggest Bluff: How I Learned to Pay Attention, Master Myself, and Win".
Now Playing: Radiolab

Invisible Allies
As scientists have been scrambling to find new and better ways to treat covid-19, they've come across some unexpected allies. Invisible and primordial, these protectors have been with us all along. And they just might help us to better weather this viral storm. To kick things off, we travel through time from a homeless shelter to a military hospital, pondering the pandemic-fighting power of the sun. And then, we dive deep into the periodic table to look at how a simple element might actually be a microbe's biggest foe. This episode was reported by Simon Adler and Molly Webster, and produced by Annie McEwen and Pat Walters. Support Radiolab today at Radiolab.org/donate.