Functional responses by Lysiphlebus testaceipes (Cresson), a common parasitoid of small grain aphids, on greenbug, Schizaphis graminum (Rondani), were measured at seven temperatures (14, 12, 10, 8, 6, 4, and 2 degrees C) during a 24-h period (12-h light: 12-h dark). Oviposition by L. testaceipes ceased at temperatures <4 degrees C. At all experimental temperatures, a type I, rather than a type II or type III, functional response was determined to be the best fit based on coefficient of determination (r2) values. L. testaceipes was observed to oviposit in greenbugs at temperatures below the developmental temperature of both the greenbug host (5.8 degrees C) and the parasitoid itself (6.6 degrees C). This ability to oviposit at subdevelopmental temperatures enables the parasitoid to increase the percentage of greenbugs that are parasitized while the greenbugs are unable to reproduce. The implications of these findings regarding population suppression of greenbugs are discussed.