The volatile chemicals produced by four poaceous plant species, blue thatching grass, Hyparrhenia tamba, Napier grass, Pennisetum purpureum, sorghum, Sorghum bicolor, and maize, Zea mays, which are host plants for the lepidopterous stemborers, Chilo partellus and Busseola fusca, were collected by air entrainment and analyzed by gas chromatography. The total quantities of volatiles collected hourly, over a 9-hr period, from P. purpureum and H. tamba showed an approximately hundredfold increase in the first hour of the scotophase. Thereafter, the amount decreased rapidly to levels present during photophase. Although onset of scotophase also triggered an increase in quantities of volatiles collected from two cultivars of S. bicolor and two out of three cultivars of Z. mays, these increases were less dramatic than in the two wild grasses, being only up to 10 times as much as in the last hour of photophase. Analysis showed that up to 95% of the increase in volatiles at the onset of the scotophase was due to just four compounds, the green leaf volatiles hexanal, (E)-2-hexenal, (Z)-3-hexen-1-ol, and (Z)-3-hexen-1-yl acetate, with the latter dominating the volatile profile. Volatiles from P. purpureum were also collected at 10-min intervals for 70 min spanning the transition from light to dark. The vast increase in production of the green leaf volatiles in this species occurs in the first 10 min of the scotophase followed by a rapid decline within the next 20 min. The relevance of these results to the control of stemborers in a "push-pull" strategy is discussed.