State delays release of genetically modified mosquitoes in Visalia

State regulators say the process is months out, missing the June window to have the greatest impact on reducing the number of invasive mosquitoes

VISALIA – The release of billions of genetically engineered, non-biting mosquitoes in the Visalia area has been postponed for at least a year. 

The Delta Mosquito and Vector Control District based in Visalia was selected as part of a pilot project to release lab grown mosquitoes designed to reduce the population of Aedes aegypti, an invasive mosquito that has been plaguing Visalia for the last few years. 

The mosquitoes were engineered by the British biotech firm Oxitec with a self-limiting gene that infects female mosquitoes and prevents them from creating female offspring, leaving only male mosquitoes in the gene pool. Only female mosquitoes bite and are capable of transmitting diseases, which is why they are targeted and the engineered males will only mate with Aedes aegypti females. The altered mosquitoes also have a fluorescent marker gene which enables them to be distinguished from invasive mosquitoes for effective monitoring.  The self-limiting gene cannot establish in the ecosystem and does not impact non-target species, such as bees, butterflies, and other wildlife. 

The U.S. Environmental Protection Agency approved the release of the mosquitoes in Tulare County, Calif. as well as Monroe County, Fla. on March 7, after a 15-month scientific evaluation process, but is still awaiting approval from the California Department of Pesticide Regulation. The department ended a 15-day public comment period on April 19 but confirmed in an email to The Sun-Gazette this week “that the evaluation and review process will take at least several months.” 

Dr. Mustapha Debboun, general manager of the Delta Mosquito and Vector Control District, said if the mosquitoes are not released by June the release would be pushed back a year, the most likely scenario based on comments from DPR. Since first being detected in 2013, Aedes aegypti has rapidly spread to more than 20 counties throughout the state, increasing the risk of transmission of dengue, chikungunya, Zika, yellow fever and other diseases. While chikunga, yellow fever and Zika are typically not associated with severe illnesses, dengue can lead to shock, internal bleeding, and even death, according to the Centers for Disease Control and Prevention.

“It would be nice to get the data sooner rather than later but we are very excited about the potential to control this non-native and disease spreading vector,” Debboun said. “Aedes is difficult to manage because it is pesticide resistant and prefers to live and feed on humans.”

Oxitec isn’t going to stay dormant for the next year. Mustafa said the company is building a lab here in Visalia and plans to do monthly outreach meetings through December to answer questions about its mosquitoes, how the neighborhoods for release will be selected and the process for releasing the mosquitoes. They held the first of these meetings on April 26. 

Oxitec recently announced promising findings from its results from its initial study of the mosquitoes’ effectiveness in the Florida Keys in 2021. Of the 22,000 larvae taken from the areas where the males were released, none of the female larvae lived into adulthood, and the male larvae passed the gene to half of its offspring, reducing the A. aegypti population with each generation.

“We had quite a number of key performance outcomes that we were hoping to hit and we were able to hit all of those in this trial,” said Nathan Rose, Oxitec’s head of regulatory affairs, during a webinar earlier this month. “This is pretty much what we expected. This really confirms the self-limiting nature of the genes, they’re not going to persist long-term in the environment.”

The full efficacy of the mosquitoes will not be known until a longitudinal study of the Florida Keys can be done, which is still pending approval from state regulators there. 

In the meantime, California regulators will spend the next few months evaluating Oxitec’s second study in Visalia. DPR said its review of the research authorization application will entail a rigorous scientific evaluation, consultation with the California Department of Public Health, Tulare County Agricultural Commissioner’s Office, the Delta Mosquito and Vector Control District and consideration of public comments and input. DPR must approve a research authorization application before an unregistered pesticide can be field tested in the state. DPR toxicologists, entomologists, microbiologists, ecotoxicologists, and other department scientists will review the application, research design, scientific studies, and additional information. 

Research authorizations are approved or refused based on an evaluation of whether that research may involve a hazard to handlers or field workers, public health, or the environment. Approved research authorizations limit both the area and the time period that the product may be tested and may include strict requirements or limitations on use, as well as monitoring and reporting requirements.

Oxitec’s research authorization application proposes to release 5,000-30,000 mosquitoes per week at multiple study sites located in and around Visalia. The company has not yet identified specific sites for the release—a requirement for DPR’s review of the research authorization application. The proposed locations of the release sites, number of sites necessary to conduct the requested research and the number of mosquitoes released per site will be evaluated by DPR as part of its review of the application.

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