But a growing body of research indicates that mosquitoes are adapting their behavior—feeding at different times of the day or switching to outdoor feeding—apparently in direct response to insecticides. At the same time, in places such as Rwanda, new species that are more resistant to current insecticides are moving in to replace dwindling mosquito populations that have been successfully controlled.
While insecticide resistance is the focus of most current public-health research, behavioral changes are “potentially more challenging” to control, according to a recent report in Malaria Journal. “This ‘behavioral resistance’ will become an increasing problem,” Dondorp says. “So, then, new entomological tools become quite important.”
The Rwanda Biomedical Center is the government agency responsible for health-care delivery, education, and research. In Kigali, it operates an entire office devoted to understanding malarial mosquitoes and analyzing specimens from the country’s collection sites.
As Emmanuel Hakizimana, the director of vector control and entomology surveillance at RBC, stands in a hallway between genetic sequencing labs, he gestures at rows of storage lockers full of preserved mosquitoes meticulously categorized by location and month of collection.
“We used to collect Anopheles gambiae sensu stricto,” he says, explaining that only a few years ago, this species made up about 93 percent of the mosquitoes collected. “Now, the dominant species is Anopheles arabiensis.”
The reason, he says, is insecticides.
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Until recently, insecticides were highly effective. Nyagatare District in northern Rwanda, for instance, used to have the highest rate of malaria in the country, with more than 100,000 cases a year. After spraying began in 2009, that number dropped to about 10,000 a year, one of the lowest rates in the country. But while A. gambiae s.s. has been largely eradicated, A. arabiensis, which is much less susceptible to the insecticides Rwanda has relied on, has moved in to fill the gap. It’s less of an evolution, researchers believe, than an ecological shift: As one subspecies declines, another takes its place.
It’s hard to tell the difference between A. gambiae s.s. and A. arabiensis, especially among adult insects. They both feed at night, and their physical appearance is similar; it often takes microscopic analysis or even DNA sequencing to tell them apart. But when it comes to behavior, the differences are quite pronounced.
A. gambiae s.s. is a homebody. She—it’s always a she; only female Anopheles take blood meals—prefers to feed on people sleeping indoors. After taking a nighttime meal, she usually rests on a nearby wall to digest the blood that provides the nourishment she needs to produce eggs.
But A. arabiensis isn’t such a picky eater. Unlike A. gambiae s.s., she feeds on people and animals alike. In fact, 60 percent of the A. arabiensis mosquitoes captured by volunteers have been feeding on livestock, not people. That means the mosquitoes carrying malaria have more food sources and can bide their time before biting humans.