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Artress is loath to get into an arms race with mosquitoes. “You hate to drag out all the heavy poisons,” he says, standing in front of the medical clinic he and his wife built in this rural town. But to fend off the voracious insects and their payload of malaria parasites, he knows there are few other choices.

Artress, a physician from California, frowns as he looks out over the tiny earthen houses straggled across the flank of the Ngorongoro Crater. Their screenless windows and doors, open to damp forest and red, puddle-pocked fields, are bullseyes for mosquitoes. Like many communities in sub-Saharan Africa, Karatu is reliant on house nets laced with insecticides called pyrethroids to keep malaria at bay.

But a decade of blanketing Africa with pyrethroids has fueled resistance to this front-line chemical weapon. Now pyrethroid-immune mosquitoes are spreading quickly throughout the continent.

“At some level, to really control the mosquitoes,” Artress says, “they’re going to have to do more.”

What that “more” is, however, is uncertain. Because of a lack of research, no new chemicals for killing malaria-infected mosquitoes have emerged in more than 40 years.