Mosquitoes rely on smell to choose victims. In a new study published in Current Biology, mosquitoes learned to associate smells with vibrations mimicking human hand movements. After subsequent exposures to the same smell, the arthropods avoided the respective odor. This behavior suggests that the insects learned that certain scents were associated with a near-death experience.
The smell of fear
Mosquitoes, these tiny, annoying vampires, bother everyone from birds to humans. They are not just terribly vexing, but dangerous as well. Even though the word ‘mosquito’ comes from Spanish and means ‘little fly’, the insects are not innocent at all. Mosquitoes are considered the deadliest animals on Earth, causing 725,000 deaths per year, according to a 2014 World Health Organization survey. Malaria, a mosquito-borne infectious disease, killed 445,000 people in 2016, states WHO.
These alarming numbers are the main reason why scientists are now trying to come up with different methods to reduce mosquito bites.
Previously, researchers discovered that each mosquito species shows a proclivity towards a certain type of host animal, even towards distinct individuals within those species. Unfortunately, the exact mechanisms through which this insect chooses its prey are still unknown. For example, generalist mosquito Culex tarsalis primarily torments birds in the summer but feeds on both mammals and birds in the winter.
Researchers at the University of Washington conducted an experiment to see if mosquito preferences could be learned. The team, led by Jeffrey Riffell, employed mosquitoes, rats, chickens and a machine named the “vortexer”. Scientists first presented the insects with an animal smell — a rat, for example. Next, the vortexer was used to inflict small mechanical shocks on mosquitoes.
The following step was to assess if the mosquitoes learned something. Two groups of mosquitoes took part in the study: a control group of untrained mosquitoes and a group of previously trained ones. Researchers discovered that trained mosquitoes did not attack the rats, as the untrained ones did. When scientists repeated the experiment — but this time with chickens — they observed that the Aedes aegypti mosquitoes encountered some difficulty acquiring avian odors. The reason might be that the Aedes aegypti mosquitoes predominantly suck human blood, so they would be inclined to learn mammal smells faster.
“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 Riffell in a statement. “Moreover, mosquitoes remember the trained odors for days.” he added.
Scientists wondered how the small mosquito brain could process such a large amount of information. One answer came to mind: dopamine, a neurotransmitter that is frequently used in learning processes (especially in remembering with the help of good or bad stimuli) by mammals and insects alike.
The team had one more thing to do: to prove their theory right. So, they genetically engineered mosquitoes that lacked dopamine receptors and glued them to a rack in order to monitor their neuron activity when introducing them to different odors. The researchers discovered that neurons were less likely to fire when presented various smells due to their inability to process dopamine.
“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.”
So, if a mosquito is troubling you, feel free to wiggle your hands at it. You might not kill it, but there is a good chance it will leave you alone.