From pantry to pest control: Garlic kills the mood — for mosquitoes, too
Garlic is not a substance that most people consider an aphrodisiac. It turns out that mosquitoes agree.
In fact, a new Yale study finds that garlic also functions as a de facto birth control for mosquitoes and other winged insects, an insight that could lead to eco-friendly pest control strategies.
According to research by the lab of Yale’s John Carlson, the presence of garlic blocks mating in mosquitoes and a variety of fly species. It’s not the pungent odor that’s a turnoff for these pests, the researchers found, but the taste. And the reason lies in a receptor inside their teeny taste organs. The findings are published in the journal Cell.
“We study flies, including harmless ones like the fruit fly, to try to discover new ways of controlling species that pose danger to humans either by spreading disease or damaging crops,” said Carlson, the Eugene Higgins Professor of Molecular, Cellular, and Developmental Biology in Yale’s Faculty of Arts and Sciences. “In this study, we started with fruit flies and then moved on to other species. And to our surprise, we found a natural compound in garlic that shuts down the mating process in these flies.”
Their method of finding this compound, which they call a “phytoscreen,” could spur new pest control strategies that are environmentally friendly, widely available, and inexpensive. Phyto is Greek for “plant.”
In a Q&A, Carlson explains the role of an enterprising postdoc in initiating this research, how it started with a “fruit fly buffet,” and why Victorian author Bram Stoker had it right about garlic and bloodthirsty creatures.
The interview was edited for length and clarity.
What was the impetus for this project?
John Carlson: We have a project in our lab that studies receptors and neural circuits that drive reproductive behaviors in insects, and we use fruit flies as a model organism. Shimaa Ebrahim, an associate research scientist in the lab, had the idea that since fruit flies normally mate on fruits, maybe there’s something in fruits or vegetables that acts as an aphrodisiac and stimulates their mating. So, she went to the supermarket and bought 43 different fruits and vegetables. She made purées from each and put them in Petri dishes for the flies to sample. It was a sort of fruit fly buffet.
What did you expect to see?
Carlson: Shimaa was thinking that at least one of these fruits would produce a big elevation in mating — that it would act as a sort of aphrodisiac. But none of them did. The startling result was that garlic abolished mating completely. It blocked egg-laying, too.
Was it the smell or the taste?
Carlson: We wondered that, too. So Shimaa cleverly placed the garlic purée in a way in which flies could smell it but not taste it, or alternatively in a way that flies could both smell and taste it. It turned out that the taste was the turn-off. Shimaa is a very careful scientist, so she double-checked the findings. She went to a different grocery store to get the same 43 fruits and vegetables and then ran the test again. And she had the exact same result with different bulbs of garlic. Garlic caused a 100% inhibition of mating. We then tested other flies, including tsetse flies, and had similar results.
What exactly is it about garlic that the flies object to?
Carlson: We separated the chemical compounds in the garlic purée and found that one chemical, diallyl disulfide, is the culprit. It prohibits both mating and egg-laying. We were happy to discover that diallyl disulfide is already used in all sorts of culinary products. It’s used in food flavorings and as a component in various nutritional and dietary supplements.
What does diallyl disulfide do to the fly?
Carlson: We found that a sensory receptor in the fly’s taste organs called TrpA1 detects the compound and triggers avoidance behaviors in the flies. This makes sense because TrpA1 can act as a kind of taste receptor. We also found that the garlic activates certain bitter-sensing neurons that have TrpA1 and changes the expression of genes, including one that’s linked to feelings of satiety.
Did it block the mating impulse in both sexes?
It mostly impacted the female fly.
What are the potential broader uses that this work reveals?
There are two. First, we found that garlic deterred the mating and egg-laying of two species of mosquitoes that spread horrible diseases like yellow fever and dengue and Zika virus. Interestingly, it didn’t work at all on wasps, which puzzled us until we learned that wasps don’t have TrpA1 receptors.
The second possible use is that this general approach — testing purées of fruits and vegetables that are cheap and eaten by humans — could be used to find compounds that block other behaviors of other harmful creatures. There’s a vast diversity of compounds in agricultural crops, and Shimaa has invented an easy way of identifying some useful ones.
Some commercial gardening products already use certain plant-based pest deterrents. How is this different?
Carlson: You can already buy some products that have garlic in them, consistent with our findings that insects don’t like garlic. But what we’ve discovered is the “why.” And now that we know how to look easily for natural compounds that act as insect deterrents, the door has been opened to all kinds of possibilities.
Are there any other lessons you learned?
Carlson: I learned quite a bit about garlic doing this research. It’s inexpensive and grown all over the world. It’s been cultivated for thousands of years, was found in the tomb of Tutankhamen, and has been used for medical purposes since the time of the Roman Empire.
The notion of using garlic to deter blood-feeding creatures was proposed in 1897 by Bram Stoker in his novel “Dracula.” Maybe he was onto something.
What have Yale researchers discovered about garlic and mosquitoes?
For mosquitoes and other flying insect pests, a compound in garlic acts as a sort of birth control. The presence of garlic blocks the mating and egg-laying response in a variety of fly species. A receptor in the fly’s taste organs detects the garlic compound and triggers this breeding shutdown. Researchers used a “phytoscreen” to test the insects’ reaction to different plants. Phyto is Greek for “plant.”
Haven’t backyard gardeners known for years that garlic can deter insect pests?
They have. The Yale team has figured out “why.” It’s an important step because researchers now know how to easily identify natural compounds that act as insect repellents, possibly opening the door to new products that are natural, inexpensive, and readily accessible.
What other fly species was garlic effective on?
Garlic deterred the mating and egg-laying of two species of mosquitoes that spread diseases including yellow fever, dengue, and Zika virus. It was also effective on tsetse flies and fruit flies.
Journal
Cell
