Development of the polyembryonic parasitoid Copidosoma floridanum in Trichoplusia ni

Trichoplusia ni (Hubner) (Lepidoptera: Noctuidae) parasitized by the polyembryonic egg-larval parasitoid Copidosoma floridanum (Ashmead) (Hymenoptera: Encyrtidae) attained significantly larger final weights and head capsule widths than unparasitized controls. The difference in weight between parasitized and unparasitized hosts was not entirely accounted for by the weight of the C. floridanum brood. The head capsule widths of all parasitized and unparasitized fifth instars used in the study exceeded the critical threshold of 1.66 mm previously established for T. ni metamorphosis. The critical ratios associated with each T. ni instar of: 1) maximum weight within the instar:head capsule width and 2) maximum weight within the instar:weight at the beginning of the instar differed between parasitized and unparasitized larvae. Development of C. floridanum was synchronized with that of its host. Germ band formation and gastrulation of morulae destined to produce reproductive larvae invariably coincided with the host molt to the ultimate, fifth instar. Reproductive larvae had two instars. Eclosion from the egg to the first instar occurred during day 2 of the host's fifth instar, and ecdysis from the first to the second instar was synchronized with host cocoon spinning. Conversely, embryogenesis of morulae destined to produce precocious larvae began during the host first instar, continued through the second and third instar and ceased during the penultimate, fourth instar. Precocious larvae never molted and died when the host was consumed by the reproductive larvae. Resume Synchronisation du developpement du parasitoide polyembryonnaire Copidosoma floridanum et de son hote Trichoplusia ni. T. ni Hubner parasite par le parasitoide ovo-larvaire C. floridanum Ashmead a developpement polyembryonnaire atteint un poids final signficativement plus eleve avec une capsule cephalique plus grosse que les temoins non parasites, sans subir de mues surnumeraires. La difference de poids entre noctuelles parasitees ou non ne correspondait pas entierement au poids des C. floridanum. Les largeurs des capsules cephaliques de tous les T. ni du cinquieme stade depassaient toutes le seuil critique de 1,66 mm lie a la metamorphose, mais les seuils critiques de taille du corps:largeur de la capsule cephalique et/ou taille et corps, taille initiale du corps au debut du stade associe a la mue, differaient chez T. ni parasites ou non. Les developpements de T. ni et de C. floridanum etaient synchrones. La formation de la bande germinative et la gastrulation de la morula produisant la multiplication des larves ont coincide invariablement avec la mue de l'hote donnant le dernier stade. Les larves polyembryonnaires ont presente deux stades. l'eclosion des oeufs s'est produite le deuxieme jour du cinquieme stade de T. ni, et le passage du premier au second etait synchrone de la formation du cocon de l'hote. Reciproquement, l'embryogenese de la morula qui donnait des larves precoces commencait pendant le premier stade de l'hote et se poursuivait a travers les second et troisieme, pour cesser pendant le quatrieme et penultieme stade.