The air in California and Florida is abuzz with activity—and it’s not due to the summer mosquitoes, but rather because of news about their control strategy. Recently, Google filed for permission to release millions of sterile mosquitoes in these states to curb disease-causing mosquito populations, as part of their Debug project. The initiative specifically targets Culex quinquefasciatus mosquitoes, which are the primary carriers of the West Nile virus.
To accomplish this, the Debug team infected male mosquitoes with a strain of Wolbachia, a bacterial endosymbiont that is known to manipulate the reproductive capacity of its host, often resulting in sterility.
“It's a good idea. There are decades of science underlying these Wolbachia-based interventions for controlling mosquitoes,” explained Eric Caragata, a medical entomology expert at University of Florida, who is not involved in the Debug project.
Teaming Up with Wolbachia Bacteria to Control Mosquito Populations
Bacteria from the Wolbachia genus naturally occur in many arthropods. These bacteria are master reproductive manipulators and can result in non-viable offspring through a phenomenon known as cytoplasmic incompatibility.1 For successful mating in species that harbor Wolbachia, the males and females must carry the same strain of the bacteria. If a Wolbachia-carrying male mates either with an uninfected female or with one that carries a different type of Wolbachia, the mating is incompatible and the eggs do not hatch.2
For more than 50 years, scientists have extensively studied Wolbachia’s ability to interfere with its host reproduction for its potential to control pathogen-carrying mosquito populations.3
Google is leveraging Wolbachia-driven mosquito sterility at a whole new level. According to the information on Debug’s website, the team developed automated rearing systems to scale up the number of their Wolbachia-infected mosquitoes to reduce the target mosquito population. They have also applied new methods that combine sensors and algorithms to sort males from females to ensure that only male mosquitoes are released. Finally, the Debug team plans to use new software and monitoring tools to ensure that enough mosquitoes are released in the right locations.
Entomologist and disease epidemiologist Jason Rasgon investigates how to develop, assess, and deploy strategies to spread introduced transgenes into natural mosquito populations.
Pennsylvania State University
In 2017, Debug tested a Wolbachia-based intervention in Aedes aegypti mosquitoes, the primary vector of viruses that cause dengue fever, chikungunya disease, and Zika fever. By introducing a strain of the bacterium into male A. aegypti to make them sterile and then releasing the mosquitoes in Fresno County, California, the team reported a decrease in biting mosquitoes—an indicator of population reduction—of up to 95 percent in a three-year-long field experiment.4
This time, Google has requested permission from the US Environmental Protection Agency to release 32 million male Culex quinquefasciatus “good” mosquitoes in Florida and an additional 32 million in California over two years. Culex mosquitoes are the primary carriers of the West Nile virus, the leading cause of mosquito-borne disease in the contiguous US, affecting over 2,000 people each year.
Although C. quinquefasciatus mosquitoes naturally harbor Wolbachia, this strategy would still work, explained Jason Rasgon, an entomologist and disease epidemiologist at Pennsylvania State University, who is not associated with the Debug project. “They can release these [mosquitoes], and even though what they're releasing has Wolbachia and the population they're releasing it into has Wolbachia, the two strains are mutually incompatible with one another,” Rasgon said.
Male Wolbachia-engineered Mosquitoes in the Wild: Should People be Concerned?
Wolbachia-based strategies generally aim to suppress or replace a target wild mosquito population. Population replacement approaches use both male and female mosquitoes, and the release of females, which naturally feed on blood, may be seen negatively by stakeholders.1
In contrast, population suppression interventions that focus on the mass release of Wolbachia-infected males are often seen more favorable, and both Caragata and Rasgon said that these approaches are considered safer. Unlike female mosquitoes, males do not have mouthparts designed to pierce the skin. “When people think about this number that they mention in their release—32 million male mosquitoes—it sounds a little frightening, but actually there is no risk to people in the areas because the mosquitoes will not be able to feed on people,” Caragata said.
University of Florida researcher Eric Caragata studies the interactions between mosquitoes, microorganisms and arboviruses to develop new mosquito control strategies and optimize existing approaches.
James Newman
He also pointed out that male-based interventions should have a minimal environmental impact because they are self-limited, meaning that, once the mosquito releases stop, the target mosquito population may rebound. While the temporary nature of the strategy has a much lower environmental footprint, Rasgon explained it could also be a disadvantage in terms of biocontrol strategy because it requires continuous monitoring and release to keep the target mosquito population in check, which may be challenging to do in large areas. Also, Culex mosquito populations are generally much higher than Aedes, which may require the Debug team to make some adjustments during field work.
While many questions remain about the success of Debug’s new Wolbachia-based intervention, experts agree that testing strategies like this one is important. Insecticide resistance is a growing issue among mosquitoes, and climate change is expanding the geographical range of mosquitoes and other disease-carrying vectors across the globe.5,6
“We have dengue transmission in Florida. We've had large West Nile outbreaks both in Florida and in California,” Rasgon said. “These are problems that are just going to get worse as time goes on. Maybe they're just kind of trying to get the groundwork in place, anticipating for when these things are actually going to be a problem.”
- Caragata EP, et al. Wolbachia as translational science: Controlling mosquito-borne pathogens. Trends Parasitol. 2021;37(12):1050-1067.
- Engelstädter J, Telschow A. Cytoplasmic incompatibility and host population structure. Heredity. 2009;103(3):196-207.
- Laven H. Eradication of Culex pipiens fatigans through cytoplasmic incompatibility. Nature. 1967;216(5113):383-384.
- Crawford JE, et al. Efficient production of male Wolbachia-infected Aedes aegypti mosquitoes enables large-scale suppression of wild populations. Nat Biotechnol. 2020;38(4):482-492.
- Estep AS, et al. Quantification of permethrin resistance and kdr alleles in Florida strains of Aedes aegypti (L.) and Aedes albopictus (Skuse). PLoS Negl Trop Dis. 2018;12(10):e0006544.
- de Souza WM, Weaver SC. Effects of climate change and human activities on vector-borne diseases. Nat Rev Microbiol. 2024;22(8):476-491.
