Release of male Aedes aegypti mosquitoes infected with Wolbachia, when repeated and in large enough numbers, can suppress a local population of biting Ae. aegypti females. The Wolbachia bacteria interferes with the mosquitoes’ reproductive process, leading to egg-hatch failure. (Photo credit: Consolidated Mosquito Control District)

By Jimmy Mains, Ph.D., and Stephen Dobson, Ph.D.

Following its rapid spread through South and Central America, Zika made its way to the United States in 2016, with the first cases of locally acquired Zika transmission reported in Miami, Florida, in July of that year. As with many insect-borne diseases, there are no approved preventative vaccines or therapeutic drugs for Zika. Therefore, the only available health response was focused on controlling the mosquito vectors, and an urgent Aedes aegypti control effort was initiated.

Stephen Dobson, Ph.D.

Jimmy Mains, Ph.D.

However, traditional mosquito control techniques against Ae. aegypti have been challenged with problems that include insecticide resistance and public concern with chemical broadcasting and potential environmental impacts. Therefore, additional effort was directed to examine emerging mosquito-suppression technologies, under the guidance of the Florida Department of Health and the Florida Department of Agriculture and Consumer Services (FDACS).

Starting in February 2018, the City of South Miami, Miami Dade County, Clarke Mosquito Control Services, and MosquitoMate, began a project in which Wolbachia-infected Ae. aegypti males were released into an approximately 150-acre area in South Miami. The “WB1” strain of Wolbachia-infected mosquitoes was developed at the University of Kentucky Department of Entomology and has been brought it to market via a spinoff biotech company named MosquitoMate, Inc. MosquitoMate has recently received permits from the U.S. Environmental Protection Agency for pilot field work that has included open releases in Fresno, California, and Stock Island, Florida. The WB1 males cause cytoplasmic incompatibility (CI) when they mate with wild type Ae. aegypti females, which leads to egg-hatch failure. Therefore, the repeated, inundative release of WB1 males can act as a species-specific, self-delivering pesticide.

Throughout the project, field Ae. aegypti populations were monitored within both the WB1-treated release area and an equivalent control area that did not receive WB1 male releases. With a peak weekly release of approximately 375,000 males, the Miami work was one of the largest applications of this type of technology to date. The Miami project was also unusual in that it was not an island, and the treated area was subject to immigration of Ae. aegypti from outside the treatment area.

Employees of MosquitoMate, Inc., worked with the City of South Miami, Miami Dade County, and Clarke Mosquito Control Services on a test program in 2018 that released as many as 375,000 male Aedes aegypti mosquitoes infected with Wolbachia weekly for six months, aimed at suppressing the local population of biting Ae. aegypti females. (Photo credit: MosquitoMate, Inc.)

For the purposes of large-scale releases of male Aedes aegypti mosquitoes infected with Wolbachia, MosquitoMate, Inc., rears the mosquitoes in a lab at its Lexington, Kentucky, headquarters. Here, a technician examines Wolbachia infected Ae. aegypti eggs (WB1) that will be used to produce males used for the project. (Photo credit: MosquitoMate, Inc.)

For the purposes of large-scale releases of male Aedes aegypti mosquitoes infected with Wolbachia, MosquitoMate, Inc., rears the mosquitoes in a lab at its Lexington, Kentucky, headquarters. Here, a technician performs quality control on larval pans as part of the male production process. (Photo credit: MosquitoMate, Inc.)

The results, reported last week in the Journal of Medical Entomology, show a significant decline in the Ae. aegypti population in the area treated with WB1 males. Specifically, the results comparing the treated and untreated areas show a reduction of Ae. aegypti females of more than 75 percent. Also of note: There was good public support for the work. As anticipated, the immigration of Ae. aegypti females from the surrounding, untreated area prevented a stronger impact, though we are encouraged that a more broad-scale application—i.e., broader than the typical flight range of Ae. aegypti—could result in stronger reduction.

In the report, we also emphasize the potential for improved results by combining WB1 males as part of an integrated pest management approach. For example, applications of larvicides would be fully compatible with the release of WB1 males. Specifically, the mode of action for many larvicides will not impact the released adult WB1 males. This type of synergism between control tools could reduce overall costs by both reducing the number of WB1 males needed and allowing for reduced larvicide applications.

Jimmy Mains, Ph.D., is chief science officer of MosquitoMate, Inc., and team leader for its Wolbachia biopesticide program. Email: jmains@mosquitomate.com. Stephen Dobson, Ph.D., is CEO and founder of MosquitoMate, Inc., a professor in the Department of Entomology at the University of Kentucky. Email: sdobson@mosquitomate.com.