S. G. Karpova. The role of endogenous and exogenous factors in regulation of synchronous emergence of Trichogramma embryophagum Hartig and T. principium Sug. et Sor.(Hymenoptera, Trichogrammatidae). Entomological Review. 2006. Vol. 86, No. 3, pp. 252-263.

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The interaction of exogenous and endogenous factors in their influence on the daily rhythm of adult eclosion was studied in two laboratory strains of egg parasitoids, Trichogramma embryophagum and T. principium. Under constant temperature of 20°C and different photoperiods (photophase of 12, 16 or 20 h) both species exhibited the distinct rhythm, most of adults eclosed within 4 first hours after light-on. The fraction of individuals emerging during this period, depended on photoperiod, the extent was different in two species. Experiments with non-periodic (constant darkness or constant light, 20°C) have showed that the rhythm is based on the endogenous circadian oscillations, which could be synchronised with an external photoperiod. However, the time of emergence was not rigidly "pre-programmed" and could be considerably modified by direct influence of light and temperature. Light-on and/or a temperature step-up (from 20 up to 30°C) induced eclosion in a certain percentage of individuals practically at any time of the 24-h-long cycle. The magnitude of a response, measured as a fraction of adults eclosing within 2 h after the stimulation, changed daily under the control of circadian oscillation.  Thus, the plasticity of eclosion daily dynamics was limited to a certain extent, due to endogenous rhythm of responsiveness to the stimuli. The interspecies differences were revealed in the relative responsiveness to light-on and temperature step-up. When light and temperature stimuli were applied simultaneously, they caused synergistic effect which exceeded the simple sum of their separate impacts. The results suggest a considerable role of exogenous factors in the control of eclosion rhythm in Trichogramma species and can be used in the development of possible methods for regulation and synchronisation of emergence in these parasitoids.