Mosquitoes that carry malaria are increasingly becoming resistant to pyrethroid insecticides, which are the only insecticides approved by the WHO to treat bed nets and are the most effective and cost efficient for indoor spraying, Nature News reports.

By the end of the year, the WHO plans “to launch a global strategy to tackle the problem,” including recommending “that control programs rotate insecticides sprayed indoors, using pyrethroids one year and a different class the next. This would be more costly and less effective than relying only on pyrethroids, however, so control programs may be reluctant to adopt this measure,” the news service writes. In the long term, new insecticides will be needed to combat the disease, according to researchers (Butler, 7/5).