An illustration of the head and mouth parts of Anopheles sp. female and male mosquitoes. The hairs (or fibrillae) on the antenna of the male enable them to hear the buzz of females in a swarm.
Joe Brock/Francis Crick Institute
Joe Brock/Francis Crick Institute
Bzzzz… Do you hear that? That’s the sound of a mosquito nearby, its wings fluttering rapidly, generating the sound waves that reach your ears and send signals to your brain.
When you hear that distinctive buzz you know it’s time to move, swat, spray or else get bit by the nasty bugger. But when another mosquito hears that buzz, it means something totally different – that it’s mating time.
It might be hard to believe, but those tiny mosquitoes have complex and powerful ears. Males can use their ears to tune into the specific frequency of females in a swarm and locate them to mate.
Scientists have known that mosquitoes are attracted to sound for almost 150 years but didn’t fully understand how that worked. Now, new research published in Nature Communications reveals more about how a mosquito’s amazing ears work. And the findings mean that a mosquito’s so-called ears could be targeted by insecticides to keep them from mating.
You’ve likely never seen ears like this before
There’s no denying that ears come in a wide variety of shapes and sizes.
Elephant ears, cat ears and human ears are all distinct, yet very recognizable. But Joerg Albert, a zoologist at the University College of London and the University of Oldenburg, Germany, says that “most people on the street wouldn’t be able to identify ears on the body of the mosquito or know that they have ears in the first place.”
It’s true! Instead of roundish holes surrounded by flesh, mosquitoes “hear with their antennae, which are these long flagellar projections from the head capsule,” Albert says. Those antennae act like “inverted pendulums” which vibrate as sound waves pass through the air.
And just like your hirsute old uncle, male mosquitoes have really hairy ears that make them distinct from females. Those hairs on their antennae are called fibrillae, and they help the males tune into the frequency of the females during mating.
Those structures are so unlike human ears that Albert says there’s some disagreement about whether we should even be using the word “ears” to describe what mosquitoes have, though he says there’s no logic to that argument “unless you have a very mammalian or human-centric definition of a sensory organ.”
Mosquito ears are certainly intriguing for scientists like Albert, but there’s good reason to study them beyond satisfying an intellectual curiosity. Mosquitoes can carry all sorts of nasty diseases, like malaria, dengue fever, West Nile virus and zika virus, just to name a few.
And because the hearing of mosquitoes is so important for mating, “in principle, if you stop them from hearing or somehow you impair their auditory abilities, that would have a big impact on mosquito mating and reproduction,” says Marta Andrés, a vector biologist at the University College of London, who along with Albert helped direct the new research.
Put simply, if you can stop mosquitoes from mating by disrupting their hearing, then they won’t be around to spread disease.
Andrés and Albert think they might have uncovered a way to do that.
Mosquito ears have a ‘hair-raising’ trick
Let’s take a step back to better understand how mosquitoes mate.
Mosquito mating is a highly regulated and time-sensitive event that only happens for about 20 minutes at dusk. At that time the mosquitoes begin to swarm – and that swarm is mostly male mosquitoes, with just a few females flitting around.
Locating those females in the swarm is the first step in mating for the males. But it’s incredibly loud in the swarm, so they need a way to tune into the distinctive slow wingbeat sound of females. This is where their hairy ears come in.
The fibrillae on the male ears, which are soft throughout the day, become erect during swarm time. The erect fibrillae then act like a tuning fork, resonating with the frequency of the female wingbeat.
What Andrés and Albert found is that the process of fibrillae erection is regulated by a neurotransmitter called octopamine — a chemical signal sent from the brain to control the mosquito ears.
“What we think is happening is that octopamine is preparing the organ [mosquito ears] for swarm time,” says Andrés. With that knowledge, the research team thinks they can disrupt that highly time-specific process of fibrillae erection by messing with the octopamine pathway. “It is a clear route for insecticidal action,” says Albert.
One insecticide already has the potential to do this – amitraz. “We could very beautifully show how this insecticide was going to act in the same way as octopamine in the mosquito ear,” Andrés says.
Amitraz is the generic name for an insecticide that used to be used against fleas and ticks but was banned by the EPA due to concerns it was toxic to humans. The research team showed, however, that it can cause the fibrillae in mosquito ears to go erect, perhaps at a time when they aren’t supposed to.
And if you’re a mosquito, ear hairs that go erect at the wrong time can mean you won’t be having any fun in the swarm. “If you’re slightly off or if your system is somehow confused, you might miss out on reproduction for that day,” says Albert. The mosquito mating system is so time dependent that any little changes to throw off its timing can disrupt the whole thing.
That’s an entirely new concept for the development of an insecticide. “Exploiting the system of swarming behavior, mating and hearing as a potential target for [mosquito] control, that’s something that is not done at all currently,” Andrés says.
It’s unlikely that amitraz itself will be used to control mosquito populations, but it demonstrates that there’s potential for insecticides like it to work on the mosquito hearing pathway.
Such insecticides are a long way off but could provide a “new tool in the toolkit”
“The idea that you could use hearing to target different parts of the mosquito’s life as a way of controlling them is a really important area of research,” says Lauren Cator, a mosquito biologist at the Imperial College of London who was not involved in the research.
An insecticide that disrupts mosquito hearing to prevent mating would be a non-lethal option, whereas current insecticides “pretty much only kill host-seeking females,” Cator says. “Taking an approach where you might be able to target males or different subsets of the population is a really worthwhile thing to explore.”
But why develop a non-lethal insecticide when we have several working, lethal insecticides now? Cator says the problem is that “there’s massive resistance to those [insecticides] because they work” and that they may not work forever.
Cator, Andrés and Albert are all in agreement, however, that there is a long road ahead before an insecticide that targets mosquito ears actually gets used for mosquito control.
“It’s interesting as kind of a first step into looking at possibilities to disrupt mosquito hearing,” Andrés says. “We don’t know yet if this is something that could be directly applied to the field.”
Everybody is optimistic though that this research could lead to something greater on the horizon. And Cator thinks it’s important to keep trying new things: “The more things we have in our toolkit, the better off we are.”