“A few years ago I could not even walk to my mailbox without getting several bites on my legs…This year I have only received one bite all summer… Thank you from a happy Clovis resident.” -B.B., Oct 17, 2019
“A few years ago I could not even walk to my mailbox without getting several bites on my legs…This year I have only received one bite all summer… Thank you from a happy Clovis resident.” -B.B., Oct 17, 2019


Debug Fresno, the largest U.S. release of male mosquitoes carrying Wolbachia, launched in 2017. Using an approach known as the Sterile Insect Technique (SIT), Debug Fresno released male Aedes aegypti mosquitoes, made effectively sterile by carrying Wolbachia, over 222 acres of residential neighborhoods in 2017. Our aim was to suppress the wild population of Aedes aegypti, which is invasive to California and has the potential to spread diseases, such as Zika and dengue. We continued the study during Fresno County’s 2018 and 2019 mosquito seasons, increasing our area almost 8-fold and adjusting our technologies to be even more efficient and effective. This past October we completed our final year of Debug Fresno with three seasons of strong suppression while gaining invaluable experience for scaling mosquito SIT around the globe.

Members of the Debug team in our larval rearing robot.




Nearly half of all humans are at risk of contracting diseases transmitted by the mosquito Aedes aegypti, and in 2019, many regions around the globe experienced a dramatic increase in dengue—reaching epidemic levels in some cases. Traditional approaches to controlling Aedes aegypti have not been sufficient to halt its spread. Mosquito SIT has the potential to dramatically reduce the incidence of disease spread by Aedes aegypti, while precisely targeting just this one species of mosquito.

Researchers have been developing mosquito-targeted SIT protocols and technology for more than a half century. However, most efforts have been limited by their inability to consistently produce healthy male mosquitoes on an industrial scale and efficiently remove biting female mosquitoes prior to release. In 2015, Debug by Verily began to develop technology to address these issues, including a highly controlled mosquito rearing system and precise sex-sorting systems. Field testing under realistic conditions is a critical step in the development of any new technology, so we were fortunate to form our partnership with the Consolidated Mosquito Abatement District (CMAD) and MosquitoMate, who were already studying this approach in the Fresno County area. Together, we made Debug Fresno a reality.

The Debug Fresno program design was fairly simple. With our partners, we chose several neighborhoods in the Fresno/Clovis metropolitan area where invasive Aedes aegypti had become widely established. We reared and released sterile, male Aedes aegypti, which cannot bite, into some of these areas to mate with local, wild female Aedes aegypti. If SIT were to work, we would see a considerable difference in the population of this mosquito between release and non-release neighborhoods. And, this is exactly what happened. At peak mosquito season, Debug Fresno observed decreases of biting, female Aedes aegypti in our release neighborhoods by up to 68% in 2017, 95% in 2018, and 84% in 2019.

While Debug Fresno may be conceptually straightforward, it was a significant operational and technical challenge. We measured impact in the field using a network of mosquito traps deployed at more than 2,000 locations by our partners at CMAD over the three years, who checked these traps 54,648 times, counted 1,569,589 Aedes aegypti, and in the process provided unparalleled insight into wild mosquito population dynamics. In South San Francisco, the Debug lab continued to grow and optimize. We reared a tremendous number of mosquitoes, converting approximately 1,100 pounds of larval diet into adult mosquitoes, while constantly improving the efficiency of our larval rearing and sex sorting technology. In total, we released approximately 48 million sterile male mosquitoes over 431 days spanning 3 seasons in Fresno/Clovis—repeatedly adjusting our release numbers and schedule—to evaluate release strategies and learn how to achieve the strongest suppression.

Members of the CMAD and Verily teams at a Debug Fresno outreach event in 2017.




Each year, our collaboration learned something new that helped our approach evolve. This year, our goal was to maintain strong rates of suppression in our 2018 areas, while continuing to expand into new neighborhoods. This year also turned out to be a record year for Aedes aegypti in California with a significantly earlier increase in density in Fresno County and detection in new cities. Even under these conditions, we maintained strong suppression in our 2018 release areas until late July when migration from untreated areas increased substantially. After adjusting and focusing our release strategies to accommodate the increased 2019 mosquito population, we achieved up to 84% suppression of biting, female Aedes aegypti. Results were stronger in some areas than others, which provided valuable data on minimum effective release rates under a range of scenarios. Our 2019 results also reinforced our perspective that starting as early in the season as possible, when mosquito populations are at their lowest, is the best strategy for achieving the maximum suppression with the lowest number of mosquitoes released.

As Verily’s first, and longest, field study has drawn to a close, we must take the time to express gratitude to our Debug Fresno partners, MosquitoMate and CMAD, and in particular to the technicians at CMAD whose hands-on work was critical to our success. We are also endlessly grateful to the residents of Fresno County for welcoming us and engaging in this study. Globally, interest in Wolbachia-based SIT is growing as scientists and governments look for effective and targeted ways to suppress the Aedes aegypti mosquito and reduce the diseases they spread. The experience and technological gains resulting from Debug Fresno have already been transferred to programs in other locations where this mosquito is both a nuisance and the cause of substantial disease burden. Debug’s progress on fighting the Aedes aegypti mosquito around the world continues, and we look forward to what is to come in 2020 and beyond.





Jacob Crawford, PhD, Senior Scientist, Verily and David Clarke, PhD, Factory Operations Manager, Verily