They bite, they feed, and they never actually leave, but researchers in Central Texas continue to study mosquitoes, tracking their local impact and studying ways to control them by hacking their genes.
The Waco Tribune-Herald reports Baylor University faculty have operated a state-funded mosquito surveillance program with the city of Waco for the last 10 years, while also doing lab research.
At first their biggest local concern was the Asian tiger mosquito, or Aedes albopictus, which can carry chikungunya virus. But last year something changed.
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Cheolho Sim, an associate biology professor at Baylor, said Aedes aegypti, a species that can spread dengue and Zika virus, began showing up in roughly equal numbers to Aedes albopictus last year. Sim said Aedes albopictus tend to dominate an area, and he's still not sure what caused the sudden shift in the local mosquito population.
"The Aedes aegypti have somehow fought back," Sim said. "They said, `Wait a minute, this is my home."'
Sim said they're still not sure what exactly is causing the shift, but intense humidity last year might have contributed.
Patricia Kamanda is a biology graduate student at Baylor. Kamanda has been doing field research with the lab. Kamanda, who moved to the U.S. from Liberia with her family at age 6, said she chose to study mosquitoes because malaria is such a constant health concern in West Africa.
"It's really common, it's almost like the flu," Kamanda said. "Some forms are more aggressive than others. I still have family in Liberia, and it's obviously a constant problem."
Kamanda sets up mosquito traps throughout McLennan County. Each trap contains a fan, a net and a funnel that traps mosquitoes throughout the night. She said her work often requires her to politely ask homeowners and businesses if she can set up the traps on their property.
"Fortunately, people are really nice in Waco," Kamanda said. "We go to a lot of residential areas, and churches give us permission to set up the traps. We also go to Cameron Park."
She said Culex quinquefasciatus, also called the southern house mosquito, is one of the most common in Texas and used to be the most common in the Waco area. But that's changing.
"We're seeing less and less Culex mosquitoes over the years," Kamanda said. "It's been the Aedes."
The researchers recently found another surprise. Species from the genus Psorophora began appearing in the traps as well. In some cases, Psorphora have tested positive for West Nile virus, but they're mostly known for carrying equine diseases.
"It's not just diversifying, it's new species expanding their habitat," Sim said. "Psorophora is more likely in Louisiana, Alabama and Florida, and is expanding west."
Kamanda said Psorophora mosquitoes come with their own host of problems. They're aggressive biters, and they lay their eggs in dirt as opposed to stagnant water, making pest control more complicated.
However, the mosquito labs at Baylor are concerned with pest control on a much more grand scale. In Sim's lab, eggs and larvae sit in plastic containers and mosquito colonies in clear plastic cages line the walls. The National Institutes of Health fund their research, which involves injecting mosquitoes with RNA, DNA or enzymes to alter mosquitoes' genes.
Mosquitoes survive winter by going into a dormant state called diapause, a process Sim is trying to pin down with his research.
"Diapause is a genetic program, and when mosquitoes start diapause, they can survive five times longer than their normal life cycle," Sim said. "This is a key point to controlling mosquito populations, so we're trying to find a way. What is the mechanism? What gene is actually controlling this diapause development program?"
When mosquitoes go dormant, so do any viruses they carry. However, by interrupting diapause, mosquitoes would die off in winter, dramatically cutting down their life span. If Sim succeeded, the Centers for Disease Control would mass produce the successfully genetically modified mosquito and release them in an effort to spread the diapause-deadened genes and put a significant dent in mosquito populations.
Until then, McLennan County will have to rely on more conventional methods of controlling mosquitoes. The CDC recorded no mosquito-borne diseases in McLennan County in 2018, but David Litke, Environmental Health Program Administrator for the Waco-McLennan County Public Health District, remembers when West Nile first came to Waco.
"West Nile first made it to the United States in 1999, and then it started spreading," Litke said. "Everybody in public health was watching. Is it isolated? Is it spreading? Can we contain it? Nothing stopped West Nile Virus. Not our geography, not our location on the globe. Nothing. In five years, it made it from east coast to west coast."
West Nile first arrived in McLennan County in 2002, though there were no reported human cases of the disease. The city began working with Baylor's researchers to monitor the disease shortly after, but when the disease did make the jump to humans it took a dramatic toll.
In 2012, McLennan County reported 43 West Nile cases and two deaths. He said until West Nile Virus came to the county, mosquitoes were mostly just considered a nuisance.
The area had been fortunate in avoiding mosquito-borne illnesses like malaria or dengue.
Instead of pesticide trucks or other methods of large-scale pest control, the health district focuses on education measures and distributing mosquito repellent, while city employees in other departments, such as streets, parks and recreation, code enforcement or animal control, do their best to prevent mosquito-friendly environments from forming.
"It's always kind of talked about, but there hasn't been a huge push for something more," Litke said. "For that to happen would take a city council action."
Litke said today, most citizens have reverted back to considering mosquitoes a simple nuisance, though people often ask him when he will "do something" about them.
"There was less talk last year," Litke said. "In years' past, we've had West Nile, Zika and chikungunya hit the news."
For now, mosquito mitigation is mostly in individuals' hands.
Standing water due to rain and saturation provide a perfect habitat for breeds of mosquitoes called "container species," that breed in contained, stagnant water. Backyard pools that aren't properly maintained or become abandoned can become a breeding ground.
"Measures should be year-round, but nobody thinks about it until the weather starts warming up," Litke said.