Callosobruchus maculatus is an important predator of cowpeas. Due to infestation during storage, this insect affects the quality of seed and crop yield. This study aimed to investigate the effects of CrataBL, a multifunction protein isolated from Crataeva tapia bark, on C. maculatus larvae development. The protein, which is stable even in extreme pH conditions, showed toxic activity reducing the larval mass 45% and 70% at concentrations of 0.25% and 1.0% (w/w), respectively. Acting as inhibitor, CrataBL decreased by 39% the activity of cysteine proteinases from larval gut. Conversely, the activity of serine proteinases was increased about 8-fold. The toxic properties of CrataBL may also be attributed to its capacity of binding to glycoproteins or glycosaminoglycans. Such binding interferes with larval metabolism, since CrataBL-FITC was found in the fat body, Malpighian tubules, and in the feces of larvae. These results demonstrate the potential of this protein for controlling larvae development.