The predator Diomus seminulus (Mulsant) has been found frequently on forage grasses feeding of Sipha flava (Forbes), suggesting that it is a natural predator of this aphid and has potential as a biological control agent for this pest in Brazil. The objectives of this study were to evaluate the effect of temperature on biological aspects of D. seminulus and to determine whether odor is used by this predator to locate prey on forage grass infested with S. flava. We performed daily evaluations of the viability and duration of each instar of the larval and pupal stages at 16, 20, 24, and 28°C. The reproductive capacity and longevity of adults were estimated at 24°C. The use of odor by D. seminulus to search for aphids was evaluated using a Y-tube olfactometer. The average duration of the embryonic period, each instar, larval, and pupal stages decreased significantly with increasing temperatures from 16 to 24°C. Eggs were blackened and parched, leading to low viability at 16 and 28°C. However, viability of the instars and larvae were not significantly affected by temperature. Adults of D. seminulus survived for >135 d; however, peak oviposition occurred 20–50 d after emergence, suggesting that this is the ideal duration of adult maintenance in laboratory culture. The predator D. seminulus used the odors from aphid prey or volatile compounds released from attacked plants to locate their prey. These results provide support for the use of D. seminulus as a successful biological control for aphids.