Cornicle secretions are one of the most important methods of aphids defenses against parasitoids and predators, although they may exact physiological costs. In this study examined the effect of cornicle secretion by Aphis fabae Scopoli (Hemiptera: Aphididae) in the second instar and its effect on the size of resulting aphid adults compared to control aphids that did not secrete. Cornicle droplets were elicited by gently stroking the anterior portion of the thorax with a fine brush. A series of the second instar aphids (n = 60) were stimulated to secrete cornicle droplets and then isolated in glass Petri dishes on a broad bean leaf at 21 ± 1°C, 65–75% RH, and a 16:8 (L:D) photoperiod. Once aphids molted to the adult stage, the nymphs produced by each were counted and removed daily. After death, the hind tibia of each adult aphid was measured with a precision of 0.003 mm. The results indicated that cornicle secretion increased the intrinsic rate of natural increase (rm) (F1,48 = 1738.4, P < 0.001; 0.513 ± 0.002 vs. 0.486 ± 001), and the finite rate of increase (ʎ), while reducing growth time (GT) and net reproductive rate (R0). The hind tibia length of secreting adult aphids was significantly shorter than that of non-secreting aphids (F1,46 = 4.828, P < 0.033; 2.19 ± 0.04 vs. 2.33 ± 0.04 mm). Overall, our results revealed that cornicle secretion by second instar A. fabae exacts a physiological costs for the resulting adults, while raising the population growth of progeny.