Washington, D.C.—Mosquito-borne diseases kill more than 700,000 people every year, according to the World Health Organization, and the mosquitos that spread the disease are difficult to control. Most species have developed resistance to all major classes of synthetic insecticides, many of which pose both environmental and health risks.
Biopesticides, derived from living organisms, may mitigate chemical insecticide resistance and offer an environmentally friendly way forward. This week in Applied and Environmental Microbiology , researchers report that bacterial isolates collected from the Mediterranean island of Crete act as insecticides against Culex pipiens molestus mosquitoes, which can transmit human pathogens such as West Nile Virus and Rift Valley Fever Virus. In lab tests, extracts containing metabolites produced by 3 of the isolates killed 100% of mosquito larvae within 24 hours of exposure.
Those metabolites might guide the development of biopesticides with minimal ecological side effects, the researchers noted. "They degrade more quickly in the environment and therefore don't accumulate, and they often don't kill such a wide range of different insect species as chemical insecticides," said George Dimopoulos, Ph.D., a molecular entomologist and microbiologist at Johns Hopkins University in Baltimore and at the Institute of Molecular Biology and Biotechnology (IMBB) in Crete. He co-led the new study, conducted in Crete, together with molecular biologist John Vontas, Ph.D., at the IMBB.
Dimopoulos' research focuses on mosquitoes that transmit human pathogens, and over the past 15 years his group has found microbes that produce metabolites that interfere with the pathogens that cause malaria and dengue, and some bacteria that can kill mosquitoes. More recently, they have been investigating mosquito-killing bacteria in the Mediterranean region as part of the MicroBioPest project, funded by the European Union.
For the new work, they collected 186 samples from 65 locations across Crete. The samples included topsoil, soil from around plant roots, plant tissues, water samples and dead insects. They then exposed C. pipiens molestus larvae to water solutions containing some of the most promising isolates found in the samples. More than 100 of the isolates killed all the mosquito larvae within 7 days, and 37 of those killed the larvae within 3 days. Those 37 isolates represented 20 genera, many of which have not previously been identified as potential biopesticides, said Dimopoulos.
Further analyses showed that the rapid-acting bacteria killed the larvae not through infection but through the production of compounds like proteins and metabolites. This is promising, Dimopoulos noted, because it suggests that an insecticide based on these bacteria would not depend on the microbes staying alive. The findings have implications not only for controlling mosquitoes but also as safe biopesticides to use for controlling agricultural pests.
The researchers have now begun studying the chemical nature of those insecticidal molecules more closely and identifying whether they are proteins or metabolites. They're also mapping out the spectrum of pesticidal activity demonstrated by the bacteria, including screening the isolates against other strains of pathogen-bearing mosquitoes and agricultural pest insects.
Biopesticides often degrade quickly and require multiple applications, Dimopoulos said, and finding the right way to formulate and deliver the compounds will be a challenge in the future. The new study represents the discovery phase.
"It's now entering the basic science phase to understand the molecules' chemical structures and modes of action, and then we'll shift to a more applied path, really aiming at prototype product development," he said. "There is a major push toward developing ecologically friendly insecticides."
