With mosquito season upon us, people are stocking up on repellents to avoid itchy bites. Insect repellents are important because they don’t just protect against small, buzzing, blood-sucking pests – they also protect against the diseases they carry, which kill an estimated 700,000 people worldwide each year.
Surprisingly, despite widespread use, no one has understood exactly how most mosquito repellents repel bugs. Researchers are now starting to uncover the first pieces of the puzzle.
A new study has identified an odor receptor in mosquitoes that helps them sniff 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 and crushed flowers of certain species of chrysanthemums. Pyrethrum breaks down quickly in the sun and is not easily absorbed through the skin, so the insecticide has long been considered one of the safest options for use around children and pets.
What makes pyrethrum toxic to mosquitoes has been known for some time. It works by attaching itself to the tiny pores of insect nerve cells and paralyzing them on contact. But it has another property, the mode of action of which is more mysterious. At lower concentrations, it protects not by killing mosquitoes, but by keeping them from getting close enough to land and bite in the first place.
Led by biology professor Ke Dong, who recently joined the faculty of Duke University, the team performed various tests to understand how mosquitoes detect and avoid pyrethrum, and what chemical components in the extract help them do it.
First, they asked people to put on a special rubber glove and reach into a cage containing 50 hungry mosquitoes. The glove had a window screen on the back made of two layers of loose mesh. The top layer acts as a barrier that mosquitoes cannot bite. Normally, mosquitoes find the warmth and aroma of human skin wafting through the mesh irresistible, and are quick to land and check. But when the bottom mesh layer closest to the skin was treated with pyrethrum, they lost interest.
These early experiments confirmed that mosquitoes do not need to get close enough to taste or touch pyrethrum-treated skin or clothing to stay away. To find out if a scent was involved, the researchers attached tiny metal 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 repellents.
A mosquito’s ability to smell comes from special receptors built into the nerve cells of the insect’s antennae and mouthparts. Once the odor molecules floating in the air stimulate these receptors, the nerve cells send a message to the brain, which identifies the odor.
Dong and his colleagues were able to identify a specific ingredient in extracts from pyrethrum flowers, called EBF, that activates an odor receptor in the mosquito’s antenna called Or31.
They found that EBF works with other compounds called pyrethrins to create a particularly off-putting bouquet. Even small doses that mosquitoes barely seem to notice when compounds occur on their own – less than five odor molecules per million air molecules – can send insects flying or crawling when they occur in combination.
While the researchers focused on the Aedes aegypti mosquito species – which spreads viruses such as Zika, yellow fever, and dengue – they also found Or31 odor receptors with surprisingly similar protein sequences in six other mosquito species.
Over 200 types of mosquitoes live in the United States alone; about a dozen of them spread germs that can make people sick.
As mosquitoes become increasingly resistant to our best chemical defenses, researchers are constantly looking for new ways to combat them.
These results, published on May 5 in the journal Communications of nature, could help researchers develop new, broad-spectrum repellents to ward off a variety of mosquitoes and, by extension, prevent them from biting people and spreading disease.
Ke Dong’s research program is supported by the United States National Institutes of Health (GM115475). The Michigan State University board has filed for a patent for the discovery.
Source of the story:
Material provided by duke university. Original written by Robin A. Smith. Note: Content can be changed for style and length.