Mosquitoes are growing increasingly resistant to pyrethroids, the only insecticides approved by the WHO for use on bednets.
Safe, cheap, effective and long-lasting, pyrethroids have been used to coat bednets and for spraying indoors to great success — mainly in Africa — saving many lives. But massive scale-up of their use has led to mosquitoes developing resistance to them.
"Data are coming in thick and fast indicating increasing levels of resistance, and also of resistance in new places," Jo Lines, head of vector control at the WHO's Global Malaria Programme, in Switzerland, tells Nature.
Janet Hemingway, director of the Liverpool School of Tropical Medicine, United Kingdom, said the international community had been slow to respond to warnings of resistance.
"A number of us had been banging the drums, saying: 'As soon as you scale up you are going to get resistance,'" she says.
Lines says this is because the malaria-control community felt too many lives were at risk to avoid scale-up.
Now, experts are calling for better monitoring of the emerging resistance and more funding to develop new classes of insecticides.
The WHO will launch a global strategy to tackle this rising problem by, among other things, rotating the use of pyrethroids with other insecticides. But such strategies will be more expensive and less effective. And developing new classes of insecticides altogether may take up to seven years, according to Hemingway.
She adds that mosquito control research is "grossly underfunded", which is why there are so few alternatives to pyrethroids.