The parasitic wasp Trogus pennator (Ichneumonidae) attacks the larvae of swallowtail butterflies (Papilionidae). Female T. pennator were followed in the field as they searched for larvae of the zebra swallowtail butterfly, Eurytides marcellus (Papilionidae), feeding on Asimina (Axinonaceae) in central Florida, USA. Predictions of host-finding theory and interactions with the host were thus investigated in a natural setting. Wasps seldom flew to plants other than Asimina and apparently recognized the plants by visual cues. Plants were attractive regardless of host presence, as the wasps approached plants lacking E. marcellus feeding damage twice as often as they approached damaged plants. However, wasps approached damaged plants at a rate greater than their frequency in the Asimina population, indicating that they can detect host damage before they approach the plants. After approaching within 0.5 m of plants, wasps tended not to land on plants lacking feeding damage. A comparison of the plants they landed on, however, showed no consistent preferences for damaged plants. The weaker trends in the latter data indicate that the presence of feeding damage was not the sole criterion for landing, although it evidently influenced behaviour. After landing, wasps spent more time searching damaged plants than undamaged plants. Butterflies preferred to oviposit on plants shorter than those searched by T. pennator. Possible adaptive consequences of this phenomenon are discussed. Eurytides marcellus (Papilionidae) : Host Trogus pennator (Ichneumonidae) : Parasite Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Land zones; Nearctic region; North America; USA Eurytides marcellus; Hymenopteran parasites; Host-finding behaviour & evidence for possible countermeasures in hosts; pine forests; Trogus pennator; Florida; Central, Ocala National Forest Trogus pennator; Lepidopteran hosts; Host-finding behaviour & evidence for possible countermeasures in hosts; pine forests; Eurytides marcellus; Florida; Central, Ocala National Forest none