The role of endogenous and exogenous factors in regulation of synchronous emergence of Trichogramma embryophagum Hartig and T. principium Sug. et Sor. (Hymenoptera, Trichogrammatidae)

Abstract

The interaction of exogenous and endogenous factors affecting the daily rhythm of emergence of adult parasitoids Trichogramma embryophagum and T. princupium from laboratory strains was studied. Under a constant temperature of 20°C and different photoperiods (photophase of 12, 16, or 20 h), both species demonstrated a distinct rhythm; the majority of adults emerged during 4 first hours after light-on. The fraction of individuals emerged during this period depended on the photoperiod and was different in both the species studied. Experiments with non-periodic regimes (constant light or constant darkness, 20°C) demonstrated that the rhythm is based on the endogenous circadian oscillations that are synchronized with the external photoperiod. The time of emergence however, was not rigidly “programmed” and could be modified by the direct influence of the light and temperature Light-on and/or temperature increase (from 20° up to 30° C) induced the emergence of a certain percentage of individuals virtually at any time of the 24-h-long cycle. The magnitude of the response, estimated as the fraction of adults emerged during 2 h after the stimulation, changed daily under the control of the circadian oscillation. Thus, the plasticity of the daily dynamics of emergence was controlled (to a certain extent) by the endogenous rhythm of sensitivity to the stimuli. The interspecific differences in the relative sensitivity to light-on and temperature increase were revealed. When light and temperature stimuli were applied simultaneously, they caused a synergistic effect, exceeding a simple sum of separate impacts. These results demonstrate an important role of exogenous factors in control of the emergence rhythm in Trichogramma and can also be used in elaboration of methods of regulation and synchronization of emergence in these parasitoids.