Host quality is a determinative factor in the life history and fitness of parasitoids. Cornicle secretions are a defense mechanism that can affect the physiological state of secreting individuals. In this study, we examined the effects of cornicle secretion in the second instar by Aphis fabae Scopoli (Hemiptera: Aphididae) on the development and life history of its parasitoid, Lysiphlebus fabarum (Marshall) (Hymenoptera: Braconidae). Cornicle droplet secretions were elicited by gently stroking the anterior portion of the aphid thorax with a fine brush. Mated, two-day-old L. fabarum females were then observed continuously after they were released singly into experimental arenas, each containing a broad bean leaf infested with 25 treated or control aphids. The attacked aphids were then reared under controlled environmental conditions (21 ± 1°C, 65–75% RH, and a 16:8 (L:D) photoperiod) to observe the life histories of developing wasps. The percentage of aphids parasitized, percentage of mummies emerging, and sex ratio were not significantly affected by treatment, whereas both male (F1,99 = 23.04, P < 0.001) and female (F1,113 = 33.44, P < 0.001) wasps developed faster in treated aphids than in control aphids. Furthermore, wasps emerging from secreting aphids had longer hind tibia than wasps emerging from control aphids, indicating increased body size (male: F1,99 = 10.98, P < 0.001; female: F1,113 = 4.42, P < 0.038). Thus, our results indicate that cornicle secretion by immature A. fabae results in improved host suitability for its parasitoid. Because the shortest developing period and the largest hind tibia length of wasps, were obtained in the secretion aphids.