To help prevent diseases such as malaria, Zika, and West Nile, the World Health Organization and other public health groups are calling for new tools to control the mosquitoes that transmit the illnesses. At the ACS national meeting in Boston last week, Joel R. Coats of Iowa State University and colleagues presented some compounds that could help: a new generation of spatial mosquito repellents that act like those in citronella candles (below). People have long lit candles containing citronella oil to release compounds that repel mosquitoes from the surrounding area. But the molecules in the oils tend to dissipate quickly and thus their effects are short lived. Coats’s team has made about 300 derivatives of plant oil compounds, increasing their molecular weight to make them less volatile and more likely to hang around. The researchers started with monoterpenes and sesquiterpenes and added carboxylic acids such as isovaleric acid to the molecules’ alcohol groups. Some of the best-performing compounds (shown above) equaled citronella at repelling mosquitoes in lab tests. But while citronella lost its potency after two hours, some of these compounds kept working after five hours. Coats says more research on the compounds is needed, including testing the compounds in the field, assessing their toxicity, and analyzing their ability to activate receptors in mosquitoes’ antennae.