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The evolution of obligate kleptoparasitism, the theft of food, has led to remarkable innovations, including physical weapons and chemical signals that can evolve into chemical weapons. Stingless bees in the genus Lestrimelitta are excellent examples of this phenomenon because they are obligate kleptoparasites that no longer collect foral resources and instead steal brood resources from other bees. Their ability to raid successfully is thus essential to their ftness even when they fght species that are physically bigger, have larger defense forces, or both. We conducted morphometric analyses, quantifed Lestrimelitta niitkib mandibular gland pheromone (MGP) components, and carried out individual fghting trials between L. niitkib and the stingless bee Scaptotrigona mexicana, a common victim species, to shed light on the detailed reasons for their success at robbing. Measurements showed that L. niitkib mandibles have thicker exoskeleton cuticles and overall greater width, particularly in the medial and proximal sections, than S. mexicana, which is quite similar in body size. In all fghts, L. niitkib bit victims and released MGP, as it does during raids. Scaptotrigona mexicana victims exhibited signifcantly increased uncoordinated behaviors and showed partial or complete paralysis. We analyzed and quantifed the major components of MGP, which consisted of large quantities of geranial (mean of 253 μg) and neral (48 μg) per bee. Microinjections of 1 bee equivalent (BE) of natural or synthetic MGP and ≥ 0.1 BE of geranial signifcantly increased deleterious behaviors and paralysis as compared to control injections. We suggest that the large quantities of MGP used during raiding have led to an unexpected outcome, a semiochemical evolving the additional function of a toxin, and contribute to the ability of Lestrimelitta to rob its victims. Inglés