Since summer is rolling around, many people are stocking up on mosquito repellents to protect themselves in the evening and at night. However, despite the popularity of these repellent products, no one really understands how they work exactly. Until now.

In a new study on the scent receptors of mosquitoes, researchers have discovered how these insects manage to detect and avoid pyrethrum, a natural mosquito repellent that is extracted from the dried and crushed flowers of the chrysanthemum. From a scientific standpoint, the repellent chemical pyrethrum breaks down in the sunlight and it is not absorbed by the skin readily. This makes it one of the safest repellents out there, especially when administered to children and pets.

The reason pyrethrum is so toxic to mosquitoes has been known for quite a while. It affects the nerve cells of the insect, paralyzing it on contact. However, it has another property that works in a way that is currently not understood very well. When it is applied in lower concentration, it does not kill the mosquitoes, it simply repels them.

In order to understand this mysterious property of pyrethrum, researchers ran a series of experiments. To start, they had research subjects wear a rubber glove on their hand, and then place their hand in a cage that had around 50 mosquitoes in it. The glove had two layers of loose-fitting mesh on the back which prevented mosquitoes from biting, but which still attracted them by allowing the body heat and the scent of the human skin to pass through. The mosquitoes responded accordingly when the two meshes contained no chemical compounds. However, as soon as pyrethrum was added to the bottom layer of the mesh, the mosquitoes lost all interest.

The researchers then set out to identify whether it was the smell of pyrethrum which repelled the mosquito, and in order to achieve this, they had placed tiny electrodes on the small hairs which cover the mosquitoes antennae. Mosquitoes sense odors through the nerve cells in their antennae, and the experiment has shown that it is indeed the odor molecules of the pyrethrum which deterred the insects. Furthermore, the researchers were able to identify the exact ingredient which repelled the mosquitoes. It was called EBF, which acts on the receptor Or31 in the mosquito’s antennae. This ingredient is so powerful that even at fewer than five molecules per one million molecules of air, it sent mosquitoes flying in the other direction.