Mosquitoes that can spread Zika had one weakness – sex – and it was their downfall

A research gamble that relied on insect libido to fight mosquitoes that can spread the Zika and dengue viruses has paid off in Fresno.

The field study, which mated female Aedes aegypti mosquitoes with sterile males, reduced the number of the biting females by an average of 68 percent during the peak of this year’s mosquito season.

“It’s one of the largest reductions that people have seen in these type of studies,” said Jacob Crawford, lead scientist for the project with the apt name of Debug Fresno.

It was the second time in California that male mosquitoes, altered with a bacterium to make them sterile, had been released to mate with females. The first time was a smaller-scale study in a Clovis neighborhood. This year, thousands of nonbiting male mosquitoes were let out in batches every day from a customized van, beginning July 14 in the Fancher Creek area between Clovis and Fowler and Belmont and Tulare avenues in southeast Fresno. The last release of male mosquitoes will be Nov. 15.

So far, results of the field study give hope that innovative strategies can help control the day-biting mosquito, which can transmit the Zika virus that can cause severe birth defects. Since colonizing in Clovis in 2013, the Aedes aegypti mosquito has spread to Fresno, Fowler, Parlier, Reedley, Sanger, Selma and Kingsburg in Fresno County. And it’s established itself in Madera, Merced, Visalia, Hanford and Lemoore, among cities in other central San Joaquin Valley counties.

“It’s important that we try to use every method that we can get to get control of this mosquito,” said Steve Mulligan, manager of the Consolidated Mosquito Abatement District.

The study was a collaboration with Consolidated, which serves Clovis; MosquitoMate, a Kentucky-based biotechnology company; and Debug Fresno, a project with Verily – a sister company to Google.

MosquitoMate developed a mosquito colony infected with a Wolbachia bacterium. Debug Fresno learned how to rear lots of sterile male mosquitoes in its South San Francisco factory and shipped them to Clovis. Consolidated workers strategically released the male mosquitoes to find receptive females.

Debug Fresno has given researchers information that will be applied to an upcoming study in Australia that will begin later this month, Crawford said. “We’ve learned an enormous amount, and that was one of the main goals of doing this study – to be able to scale up and do this research in the field.”

The study improved the efficiency of the mosquito production factory, especially in the sorting of male mosquitoes from females. Only the sterile, non-biting males were shipped to Fresno.

“We have a very elaborate and fantastic sex-sorting process,” Crawford said. “But after you look at quite a few of these, it’s not as hard as it sounds.”

Researchers may have learned something about mosquito modesty in the process, though. Mosquitoes did not like the heat from a bright light that was shined underneath them to determine their gender, Crawford said. “We turned the lights down a little bit, and still got good images.”

Crawford is still analyzing data from Debug Fresno. There’s still a lot to learn from the study, he said. But he’s encouraged by the reduced numbers of the female biting Aedes aegypti in Clovis. “Our hope is this technique can be one of many tools to bring this population down and ultimately control it,” he said.

Consolidated is interested in continuing the study and possibly expanding it next year, Mulligan said. “We’ve learned a lot and we feel we can use that to go forward and get even better results.”

This story was originally published November 7, 2017 at 6:30 AM with the headline "Mosquitoes that can spread Zika had one weakness – sex – and it was their downfall."