Chemicals that modulate the response to carbon dioxide in fruit flies and mosquitoes could lead to new insect repellents. Fruit flies normally try to avoid CO2 because it’s a major component of the cocktail of volatile compounds emitted by stressed flies. However, CO2 is also emitted by many of the flies’ food sources, such as overripe fruit, yeast, and beer. Stephanie Lynn Turner and Anandasankar Ray of the University of California, Riverside, find that compounds emitted by these food sources, such as 1-hexanol and 2,3-butanedione, inhibit the response of some neurons to CO2, thus shutting down the flies’ CO2-avoidance behavior (Nature, DOI: 10.1038/nature08295). Culex quinquefasciatus mosquitoes, which transmit pathogens such as filarial parasites and West Nile virus, have some of the same CO2 receptors as fruit flies. In the mosquitoes’ case, however, CO2 in human breath acts as an attractive cue that helps lead the insects to targets. The researchers found that the compounds 1-butanal and 1-hexanol inhibit the CO2 response in mosquitoes. Such findings “may provide a valuable resource for the identification of economical, environmentally safe volatile compounds that may reduce mosquito-human contact by blocking responsiveness to CO2,” the researchers write.