How One of many Oldest Pure Pesticides Retains Mosquitoes Away

DURHAM, NC – With mosquito season, people fill up with repellants to prevent itchy bites. Insect repellants are important because they not only protect against the buzzing, blood-sucking pests, but also the diseases they carry, which kill around 700,000 people worldwide each year.

Surprisingly, despite widespread use, no one understood exactly how most mosquito repellants keep the insects away. Now the researchers are starting to uncover the first pieces of the puzzle.

A new study has identified a scent receptor in mosquitoes that helps them detect and avoid traces of pyrethrum, a plant extract that has been used for centuries to repel biting insects.

One of the oldest known insecticides, pyrethrum, comes from the dried, crushed flowers of certain species of chrysanthemum. Pyrethrum breaks down quickly in sunlight and is not easily absorbed through the skin. As a result, the insecticide has long been considered one of the safer options for children and pets.

What makes pyrethrum toxic to mosquitoes has been known for some time. It binds to tiny pores in the insect’s nerve cells and paralyzes them upon contact. But it has another property, the mode of operation of which is more of a mystery. At lower concentrations, it protects not by killing mosquitoes, but by preventing them from getting close enough to even land and bite.

Led by biology professor Ke Dong, who recently moved to Duke University’s faculty, the team conducted various tests to understand how mosquitoes detect and avoid pyrethrum, and which chemical components of the extract help them do so.

First, they had people put on special rubber gloves and put their hand in a cage with 50 hungry mosquitos. The back of the glove had a window pane made of two layers of loose mesh. The top layer acts as a barrier that mosquitoes cannot penetrate through. Usually mosquitoes find the heat and aroma of human skin blowing through the net irresistible and quickly land and check it out. However, when the lower mesh layer closest to the skin was treated with pyrethrum, they lost interest.

These early experiments confirmed that mosquitoes need not get close enough to taste or touch pyrethrum-treated skin or clothing to stay away. To find out if it was an odor, the researchers attached tiny wire electrodes to the tiny hairs covering the mosquito’s antennae and measured their electrical responses to puffs of air containing chemicals released by pyrethrum and other repellants.

The mosquito’s ability to smell comes from special receptors embedded in nerve cells on the insect’s antennae and mouth parts. As soon as odor molecules blowing through the air stimulate these receptors, the nerve cells send a message to the brain that identifies the odor.

Dong and her colleagues were able to locate a certain ingredient in pyrethrum flower extracts called EBF, which activates an odor receptor in the mosquito antenna called Or31.

They found that EBF worked with other components called pyrethrins to create a particularly repulsive bouquet. Even tiny doses that mosquitoes barely seem to notice when the compounds appear alone – fewer than five odor molecules per million air molecules – can make the insects fly or crawl away when combined.

While the researchers focused on the mosquito species Aedes aegypti, which spreads viruses such as Zika, yellow fever, and dengue fever, they also found Or31 olfactory receptors with strikingly similar protein sequences in six other mosquito species.

There are more than 200 species of mosquitoes in the US alone. About a dozen of them spread germs that can make people sick.

As mosquitoes become more resistant to our best chemical defenses, researchers are constantly looking for new ways to control them. These results, published May 5 in the journal Nature Communications, could help researchers develop new broad spectrum repellants to keep a wide variety of mosquitoes at bay and stop them from biting people and spreading diseases.

Ke Dong’s research program is supported by the US National Institutes of Health (GM115475). Michigan State University’s Board of Trustees has filed a patent for the discovery.

QUOTE: “A molecular double-target mechanism of pyrethrum defense against mosquitoes”, Feng Liu, Qiang Wang, Peng Xu, Felipe Andreazza, Wilson R. Valbon, Elizabeth Bandason, Mengli Chen, Ru Yan, Bo Feng, Leticia Smith, Jeffrey G. Scott, Genki Takamatsu, Makoto Ihara, Kazuhiko Matsuda, James Klimavicz, Joel Coats, Eugenio E. Oliveira, Yuzhe Du, Ke Dong. Nature Communications, May 5, 2021. DOI: 10.1038 / s41467-021-22847-0