When George RR Martin made obsidian (dragon glass) the weapon with which his heroes could defeat their worst enemies, little did he know its offshoot might overcome one of the world’s worst diseases. Perlite is a form of volcanic glass that forms from the hydration of obsidian. Its current uses are humble, such as building materials and gardening supplies, but success in malaria control trials could open the way to saving millions of lives.

A combination of bed nets and spraying the inside walls of houses with insecticides has driven malaria deaths down from 1.8 million annually to an estimated 400,000. Even this figure means (2020 aside) it competes with AIDS and tuberculosis for each year’s status of the world’s most lethal infectious disease. Worse still, mosquitoes have rapidly developed resistance to every insecticide we have thrown at them, from DDT to pyrethroids, making victory temporary. Moreover, many of the chemicals used for mosquito control have a heavy impact on the health of humans and other animals.

Professor Mike Roe of North Carolina State University sought a different approach. He mixed crushed perlite and water to make a product he calls Imergard WP and sprayed it on the inside walls of huts in the malaria-prone Republic of Benin (West Africa). Control huts were given the same treatment with deltamethrin, a commonly used pyrethroid, or left unsprayed. A fourth set of huts got both Imergard WP and deltamethrin treatment.

In Insects Roe and co-authors announce that the Imergard outperformed deltamethrin, let alone the unsprayed huts. Five months after spraying, 80 percent of mosquitoes who ventured into the Imergard-sprayed huts died from getting tiny pieces of perlite on their feet, almost double the initial mortality rate in pyrethroid-sprayed huts. Perlite’s effectiveness also declined more slowly than chemical insecticides.

"The statically transferred perlite particles essentially dehydrate the mosquito," Roe said in a statement. "Many die within a few hours of contact with the treated surface. Mosquitoes are not repelled from a treated surface because there is no olfactory mechanism to smell rock."

The deaths were too slow to prevent the mosquitoes feeding on the huts' inhabitants, but a mosquito that acquired the malaria parasite and died shortly thereafter would never get to pass it on. Perlite is also considered so safe for humans it is added to toothpaste.

Malaria is far from the only mosquito-borne disease. Dengue, yellow fever, and Zika are all carried by Aedes aegypti, rather than the Anopheles genus that represents the malaria parasite’s vector. Indeed, a second dengue carrier, Aedes albopictus, is now spreading across the United States to areas unused to facing such threats. Global heating and trade in materials that carry mosquito eggs are assisting their expansion.

Roe’s experiment did not test if Aedes mosquitoes are similarly susceptible to perlite, but the chances are high since it is known to be effective against agricultural pest insects. Moreover, there is reason to hope resistance will evolve more slowly and that perlite should not bioaccumulate through the food chain.