Natural enemies locate their herbivorous host and prey through kairomones emitted by host plants and herbivores. These kairomones could be exploited to attract and retain natural enemies in crop fields for insect pest control. The parasitoid Encarsia formosa preferentially parasitises its whitefly host, Trialeurodes vaporariorum, a major pest of tomato Solanum lycopersicum, thus offering an effective way to improve whitefly control. However, little is known about the chemical interactions that occur in E. formosa-T. vaporariorum-S. lycopersicum tritrophic system. Using behavioural assays and chemical analyses, we investigated the kairomones mediating attraction of the parasitoid to host-infested tomato plants. In Y-tube olfactometer bioassays, unlike volatiles of healthy tomato plants, those of T. vaporariorum-infested tomato plants attracted E. formosa, and this response varied with host infestation density. Coupled gas chromatography/mass spectrometric analyses revealed that host infestation densities induced varying qualitative and quantitative differences in volatile compositions between healthy and T. vaporariorum adult-infested tomato plants. Bioassays using synthetic chemicals revealed the attractiveness of 3-carene, ß-ocimene, ß-myrcene and a-phellandrene to the parasitoid, and the blend of the four compounds elicited the greatest attraction. Our results suggest that these terpenes could be used as an attractant lure to recruit the parasitoid E. formosa for the control of whiteflies in tomato crop fields.