Killing mosquitoes with cashews

LOS ANGELES – “In the year that I started my project, my city, Campo Grande, had a large number of Dengue fever cases,” says Gabriel Galdino. The 19-year-old, who has just started college at the University of São Paulo in Brazil, decided he wanted to stop the spread of dengue fever. The disease, which causes fevers, headaches, achy joints and a nasty rash, comes from a virus spread by the dengue mosquito. So if Gabriel was going to stop dengue, he would have to first stop the insects that spread it.

The teen came up with a new solution, in the form of cashew nut shell liquid. The liquid both repels mosquitoes and kills their larvae. Gabriel presented his new mosquito killer and a filter to release it at the 2014 International Science and Engineering Fair, or ISEF. Created by Society for Science & the Public and sponsored by Intel, the 64th annual science competition brings young scientists from around the world together to share their projects. (SSP also publishes Science News for Students and this blog).

Gabriel came up with his new solution as he searched for ways to kill off mosquitoes. “I read many articles about insecticides,” he says, “but these are often toxic to humans and have to be sprayed in the air.”

Then he came across cashew nut shell liquid, a resin found inside the cashew nut shell. Right now it is used as resin and to kill fungi and termites. “I saw some studies which showed that cashew nut shell liquid could be larvicidal,” he says, which means it could kill mosquito larvae, the immature stage before the insect takes up its blood-sucking livelihood. If Gabriel could use the liquid to kill mosquito larvae, he might be able to stop the spread of dengue.

But there was a problem. The shell liquid doesn’t dissolve in water. Because mosquitoes lay their eggs in still bodies of water, any treatment to kill the larvae will have to dissolve in water. To get around this, Gabriel mixed the cashew liquid with castor oil, creating a surfactant, a chemical that allows two liquids to mix. His shell liquid could now dissolve in water.

When he tested it on mosquito larvae, Gabriel found that his solution killed more than 90 percent of mosquito larvae in a single day! Now, the teen needed to figure out how to release his new larvicide over time. He wanted it to leach out slowly into water to kill all the mosquito larvae.

Gabriel designed a filter that could be glued to the inside of a rain barrel or gutter, anywhere where water might collect and make a desirable mosquito breeding ground. The filter is coated with a hardened varnish made of the cashew nut shell liquid. As the filter is exposed to water, the cashew nut shell varnish liquefies and mixes with castor oil, getting released slowly through tiny filter holes into the water and killing any mosquito larvae that might be present. The filter makes the larvicide last much longer than it might if it was just poured into the water.

The teen also found that his formula doesn’t just kill larvae, it stops mosquitoes from laying eggs in the water at all. The mixture serves as a repellant as well as a larvicide.

Brazil is an important producer of cashew nuts, so the cashew nut shell liquid would be easy to obtain. And because the liquid is a by-product, it is relatively cheap to produce. Gabriel hopes that he has found a local solution to a local problem. If his mixture continues to work well, he may be on his way to stopping mosquitoes, and the dengue fever they carry.

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Power Words

larva  (plural: larvae) An immature life stage of an insect, which often has a distinctly different form as an adult.

larvicide  An insecticide that targets the larval stage of an insect. Larvicides are most commonly used against insects such as mosquitoes to stop them from reproducing and spreading diseases.

dengue fever  A potentially lethal infectious disease transmitted by mosquitoes. No vaccine yet exists to prevent infection with the virus responsible for the disease, which causes high fevers, severe headache, joint pain, pain behind the eyes, rash, bone pain and sometimes mild bleeding. A more severe form of the disease, known as dengue hemorrhagic fever can cause uncontrolled bleeding if not treated right away.

cashew nut shell liquid  A resin in the shell of a cashew. It is used in drug development and can also protect against fungi and termites.

insoluble  Incapable of being dissolved into a fluid or gas. Salt and sugar can dissolve in water, for example, but some other substances, including some of those with large molecules such as proteins, do not.

solubility  A measure of the ability of one chemical to dissolve into another, creating a chemical solution.

insecticide  A substance used to kill insects.

resin  A thick and stick secretion from a plant that hardens into a tough finish. Resins are often used as lacquers, but can also be used for anything from incense to increasing friction on a violin. Some resins are irritants that can kill fungi or insects.