Larvae of Epargyreus clarus (Hesperiidae),  the silver-spotted skipper, inhabit leaf-and-silk shelters that they construct on their leguminous host plants.  In the field Polistes spp. (Vespidae) wasps land on the shelters, quickly extracting and killing the larvae within.  In marked contrast, wasps that emerged from field-collected colonies maintained in the laboratory visited and examined leaflets bearing sheltered caterpillars, but only rarely did they extract and kill the sheltered larvae.  However, after killing and processing an unsheltered larva that was visible on an opened leaf shelter, a majority of foragers subsequently extracted and killed larvae from closed shelters.  Wasps that killed and processed a larva away from a leaf shelter generally did not later open shelters, whereas those that did so while larval and shelter odors were blown over the larva subsequently did open shelters.  We conclude that the wasps must learn to associate the taste of the larva with shelter-related olfactory cues, such as induced volatile compounds, in order to be able to prey on sheltered larvae.  In nature, this initial exposure may occur when a wasp encounters a larva or larval remains in or near its shelter.  Learning opportunities for wasps will thus depend on larval density.  Our results demonstrate that invertebrate predators can learn to overcome their prey's defenses, and are therefore able to make use of previously inaccessible prey.