The floral nectar ofCatalpa speciosa has a chemical mechanism that limits thievery. A bioassay employing sphingid larvae,Ceratomia catalpae, shows that catalpa iridoid glycosides are present in the floral nectar. When potential nectar thieves are fed nectar, a sucrose solution of identical concentration, or a sucrose solution plus 0.4% catalpol and 0.4% catalposide (catalpa iridoids), the thieves drink significantly more of the pure sucrose solution than either of the other two sugar sources. Those thieves that drink either the nectar or the sucrose solution plus catalpa iridoids develop behavioral abnormalities including regurgitation and loss of locomotion. The response of the potential nectar thieves to nectar or the sucrose solution plus catalpa iridoids cannot be distinguished by the amount consumed or by their behavior. The legitimate diurnal pollinators ofC. speciosa are not affected by the iridoid glycosides.