A scanning electron microscope image shows the leg of a mosquito covered with Imergard, a new insecticide in which the active ingredient is perlite, a derivative of volcanic rock. Perlite is believed to act by causing dehydration in mosquitoes. (Photo courtesy of Jean M. Deguenon, Ph.D.)

By John P. Roche

Mosquitoes spread microorganisms that cause dangerous diseases, including malaria, dengue virus, yellow fever, West Nile virus, and Zika virus. Malaria alone causes 400,000 deaths per year. Chemical insecticides have been widely used to control mosquito populations, but insecticide resistance to pesticides has become widespread in mosquito populations, making insecticides less effective over time. Therefore, there is an urgent need for insecticides with alternative modes of action that can control mosquito populations without causing cross-resistance to chemical insecticides. North Carolina State University’s Jean M. Deguenon, Ph.D., R. Michael Roe, Ph.D., and colleagues tested a material derived from volcanic rock, perlite, as a potential non-chemical insecticide against Anopheles gambiae, one of the primary mosquitoes that spreads malaria in Africa. In their new report published in August in the  Journal of Medical Entomology, they show that perlite has encouraging potential as a mechanical insecticide.

The investigators discovered perlite in the process of conducting screens of industrial minerals to test if they had activity against the African malaria mosquito. The study by Deguenon and colleagues was composed of four components: they tested the effects of perlite on Anopheles gambiae; they tested whether perlite repelled mosquitoes; they visualized perlite on mosquitoes using scanning electron microscopy; and they tested the effect on mosquitoes in the field by spraying perlite onto wood chips. In this study, perlite was tested in a commercial form for mosquito control called Imergard, made by Imerys Filtration Minerals, Inc. Imergard was made by heating aluminosilicate volcanic rock, drying it, and then grinding it into a fine powder. This was the first time that perlite had ever been tested for vector control.

Postdoctoral fellow Jean M. Deguenon (left) and his advisor R. Michael Roe (right, holding a mosquito chamber) and colleagues studied the effects of a commercial preparation of perlite called Imergard. They found that Imergard was an effective insecticide against the African malaria mosquito Anopheles gambiae, making it a promising option for control of malaria and other mosquito-borne diseases. (Photo courtesy of Jean M. Deguenon, Ph.D.)

In the test of the effects of this special form of perlite against Anopheles gambiae, the investigators used glass funnels capped with stoppers, which were turned upside down to rest in Petri dishes. In treatment cones, they added perlite mixed in ethanol to the Petri dishes. In control cones, they added only ethanol to the Petri dishes. Afterward, they allowed the ethanol to evaporate completely, leaving Imergard behind on the Petri dish bottom. They introduced 25 females to each cone and observed the outcome. They found that the time to 50 percent mortality (which is known as the LT50) was 4.96 hours, and 80 percent mortality was seen at seven hours. Perlite is believed to act by causing dehydration in the mosquitoes.

To test whether or not Imergard repelled mosquitoes, the researchers conducted choice tests in which one half of the Petri dish contained perlite and the other half did not. In six replicate tests, they introduced ten adult female African malaria mosquitoes to test cones and recorded their position after 30 minutes. They found no significant difference in the proportion of mosquitoes observed on a surface treated with perlite and the proportion of mosquitoes observed on a control surface without perlite, supporting the hypothesis that perlite did not repel mosquitoes. This is important because in order to be effective, an insecticide must not repel mosquitoes before it has a chance to act.

When visualizing the distribution of perlite on mosquitoes with scanning electron microscopy, the investigators found Imergard on the mosquitoes’ antennae, wings, abdomens, and tarsi, though the highest concentration was found on the legs of the mosquitoes (see Figure 2).

In field tests, Deguenon et al. sprayed this special form of perlite on wood chips under raised houses in New Orleans, Louisiana, and tested the effects on yellow fever mosquitoes, Aedes aegypti; Asian tiger mosquitoes, Aedes albopictus; and southern house mosquitoes, Culex quinquefasciatus. Of the 18 test sites, six received wood chips treated with perlite, six were treated with the chemical insecticide lambda-cyhalothrin, and six control sites were treated only with water.

Perlite is the active ingredient for the mechanical insecticide made from volcanic rock, which can be expanded (when heated to 760–980 degrees Celsius) and then processed (by drying and grinding) to obtain a fine white powder. (Image originally published in Deguenon et al 2020, Journal of Medical Entomology)

For the yellow fever mosquito, mean percent mortality for both perlite and lambda-cyhalothrin was 100 percent, and there was no significant difference in control between these two insecticides. For both Asian tiger mosquitoes and southern house mosquitoes, however, the median percent mortality for mosquitoes exposed to lambda-cyhalothrin was slightly higher than for those exposed to Imergard. In Asian tiger mosquitoes, the mortality from Imergard was 90 percent and the mortality from lambda-cyhalothrin was 100 percent; for southern house mosquitoes, the mortality from Imergard was 93 percent and the mortality from lambda-cyhalothrin was 100 percent.

Michael Roe says, “Our most important finding was that perlite was active against mosquitoes, it rapidly kills mosquitoes, and it could be applied by simply mixing with water and spraying on a surface using spray equipment already being used for mosquito control.”

How close are we on a practical level to implementing Imergard as an insecticide in the field? Coauthor David Stewart of Imerys Filtration Minerals, Inc. says, “Imergard is pending WHO Prequalification as a new active ingredient. The dossier was submitted to WHO PQ in November 2019 and we are hoping for a decision in October 2020. We are planning a commercial launch for late 2020 or early 2021 for use to control malaria in Africa. Imergard is also under EPA consideration as a biopesticide in the U.S. In addition, we have initiated registration in Central America in both the public health and consumer health segments. For Central America and the U.S., we are targeting the yellow fever mosquito to control Dengue and Zika.”

Perlite has a wide range of applications. According to Roe, “We have DOD funding to expand the use to different mosquito species and from mosquitoes to filth flies and sand flies to protect deployed troops.” In a separate study, Deguenon and his colleagues also successfully developed the use of a different industrial mineral for control of insects in U.S. cotton farming. Roe says, “The use of mineral insecticides like we developed in our cotton work should be useful for many different crops and will reduce the need for chemical insecticides.”

When asked about future research, Roe says, “Possible future projects include testing Imergard’s effects on other vectors such as sand flies, testing in Africa when sprayed on walls, and examining if it can control mosquitoes at the level of the whole community. Other key questions are, can it successfully reduce the rate of malaria infections, and how long it is effective relative to chemical insecticides?”

Perlite is an extremely promising agent in helping with mosquito control. It is safe for mammals and for bees. It is not changed by heat, and it has potential for use in a residual spray that would last a long time. Being mechanical, not chemical, it is expected that it would not contribute to chemical insecticide resistance. All of these are pronounced positives for its ability to contribute to the control of dangerous mosquito disease vectors.

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 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com/.