AbstractTrichoplusia niparasitized byCopidosoma floridanumweighed more in the fourth and fifth instar than unparasitized controls. While parasitized and unparasitized fourth‐instar larvae molted at the same time, parasitized fifth‐instar larvae ceased feeding and initiated wandering 1 day later than unparasitized fifth‐instar larvae. Measurement of hemolymph ecdysteroid, juvenile hormone esterase (JHE), and protein titers suggested that the delay in wandering had an endocrinological basis. Ecdysteroid titers of parasitized and unparasitized fourth‐instar larvae were similar, but the ecdysteroid titer in parasitized fifth‐instar larvae rose about 24 h later than in unparasitized fifth‐instar larvae. Two major peaks of JHE activity occurred in unparasitized fifth‐instar larvae, but only one occurred in parasitized fifth‐instar larvae. JHE activity coincided on the first day of the fifth instar (L5D1) in both parasitized and unparasitized fifth‐instar larvae; however, activity peaked and fell later in parasitized individuals. Few differences were noted in the hemolymph protein levels of parasitized and unparasitized hosts. Ligation of parasitizedT. nibetween the head and prothorax adversely affectedC. floridanumdevelopment. Ligation before or immediately after ecdysis to the fourth instar inhibited differentiation of theC. floridanummorulae.C. floridanumecdysed to the second instar or pupated in hosts ligated prior to wandering in the fifth instar, but the number of progeny that pupated and emerged as adult wasps was reduced relative to unl