Edible Plant Oils Show Potential for Management of Aedes aegypti
By John P. Roche
The yellowfever mosquito (Aedes aegypti) spreads dangerous human arboviruses that include dengue, Zika, and chikungunya. Therefore, control of yellowfever mosquitoes is a critical public health priority. Chemical pesticides are a leading method of control, but pesticides pose economic costs, contribute to the evolution of insecticidal resistance, pose risks to the environment, and present safety concerns to humans and non-target animal species. An alternative type of control mechanism, edible plant oils are inexpensive, easily available, non-toxic to mammals, and readily biodegradable. And, because they act through multiple mechanisms—as repellents, larvicides, and oviposition inhibitors—it is harder for mosquitoes to evolve resistance to them.
To explore plant oils as safe and environmentally friendly alternatives to chemical pesticides, Teresia Njoroge and May Berenbaum, Ph.D., at the University of Illinois at Urbana-Champaign tested the lethal effects of several edible plant oils on Aedes aegypti. The results of their research were published in March in the Journal of Medical Entomology.
The goal of the study, Noroge says, was to “test the concept that edible oils can be an effective, non-toxic tool for the control of container-dwelling Aedes aegypti in the drinking water storage containers of people in settings that lack piped water systems.”
Aedes aegypti has four life stages— egg, larva, pupa, and adult—and the larval stage has a progression of four instars. In their first experiment, Njoroge and Berenbaum tested the lethality of 13 widely available edible plant oils on third-instar larvae of Aedes aegypti by placing larvae in plastic cups containing water and a plant oil. Control groups were placed in cups with deionized water and no oil. The oils they tested were canola, cashew nut, corn, cottonseed, grapeseed, hempseed, olive, peanut, pumpkinseed, sesame, soybean, sunflower, and walnut. They recorded the number of larvae surviving at 24 hours and 48 hours. Njoroge and Berenbaum found that hempseed oil showed the highest lethality, followed by sesame and pumpkin seed oils.
Next, they exposed individuals from all four larval stages to these three most lethal edible plant oils. They found that mortality from the oils was higher for the third and fourth instars than for the first and second instars. They hypothesize that the larger instars might be more vulnerable because they had higher respiratory demands and thus were more vulnerable to respiratory disruption by the oils.
They also tested whether oils with a higher concentration of a polyunsaturated fatty acid called linoleic acid were more lethal than oils with lower linoleic acid concentrations. They found that the higher linoleic-acid-concentration edible plant oils were more lethal. Then, when they placed third-instar Aedes aegypti in cups with 99 percent linoleic acid, the linoleic was much more lethal than hempseed oil, the plant oil with the highest lethality. Linoleic acid is lethal because it reacts with oxygen to form bonds that create a film, and this film is detrimental to respiration in the trachea of the larvae.
In another experiment, Njoroge and Berenbaum tested if selected edible plant oils affected the percentage of larval Aedes aegypti that emerged to adulthood. In separate treatments, they exposed first instar larvae to a high lethality oil (hempseed oil), moderate lethality oils (sunflower and peanut), and a low lethality oil (olive) at two concentrations, 500 parts per million (ppm) and 1,000 ppm. They discovered that all four oils caused fewer larva to emerge compared to controls, and that fewer larva emerged at higher oil concentrations than at lower concentrations.
Finally, they tested if the oviposition (egg laying) behavior of female Aedes aegypti was affected by hempseed, sesame, sunflower, peanut, or olive oils, and they found that all of these oils deterred egg laying. There are several reasons why the oils might interfere with egg laying, including forming a physical barrier on the top of the water, masking odor cues that trigger egg laying, or repelling females.
In terms of future research, Njoroge says, “I would like to test the duration of efficacy of the oils as larvicides and oviposition deterrents against mosquitoes in artificial water containers. I also want to test the insecticidal activity of the oils against other container-dwelling mosquitoes that spread diseases to humans, such as Aedes albopictus and Culex pipiens.”
When asked about potential challenges that will need to be overcome with the use of edible plant oils for mosquito control, Njoroge says that “a possible challenge would be convincing users to add edible plant oils to their drinking water. And some potential users in the lowest income brackets may not be able to afford the luxury of using edible plant oils in storage water containers.”
Njoroge and Berenbaum found strong support for the efficacy of edible plant oils as viable and attractive control measures for Aedes aegypti mosquitoes. “Edible plant oils are readily available in most countries in which this control strategy is ideal,” Njoroge says. And the oils are inexpensive, environmentally safe, and non-toxic to mammals—all factors that could encourage their broad adoption.
Journal of Medical Entomology
John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions™ and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 195 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com.