Interaction of exogenous factors (light and temperature) in their influence on the daily pattern of adult eclosion in Trichogramma embryophagum (Hymenoptera: Trichogrammatidae)

Abstract

Abstract.A comparative study of the direct influence of single, non-repetitive changes of light and/or temperature on the daily pat tern of adult eclosion was conducted in laboratory conditions with the egg parasitoid, Trichogramma embryophagum (Hartig). In all experiments, the entire development occurred under the same regimen (12L : 12D, 20°C). However, just in the day when records of eclosion were made, the time of light-on was changed (from the total absence of a dark period to the 12-h-long scotophase). In addi tion, in different treatments of each experiment a thermophase (30°C during 2 or 4 hours) was applied in various positions relative to light-on. Results showed that light-on or a temperature step-up could induce eclosion in a certain percentage of individuals practi cally at any time of the 24-h-long cycle (masking effects). The fraction of adults emerging during 2 h after the beginning of the action of an exogenous factor depended not only on the preceding scotophase duration, but also on the stimulus itself (light, tempera ture or both). The temperature stimulus was found to modify the responsiveness to light considerably, depending on the order of application of the stimuli. The simultaneous action of light-on and a temperature step-up exerted a more considerable influence than could be expected based on their separate impacts. On the contrary, a high temperature pulse preceding the onset of the photophase inhibited the response of individuals to light-on, evidently acting by phase delay of a circadian rhythm of light responsiveness. These effects were more pronounced in the first half of the 12-h-long scotophase. The data suggest a complex interaction between the exogenous factors in their influence on the daily pattern of adult eclosion.

Arrhythmic adult ecdysis but rhythmic emergence from the host chorion in Trichogramma embryophagum (Hymenoptera: Trichogrammatidae)

“…The above results strongly suggest that the observed rhythm of Trichogramma adult emergence (Dahiya et al, 1993;Corrigan et al, 1995;Pompanon et al, 1995;Zaslavski et al, 1995Zaslavski et al, , 1999Karpova & Reznik, 2002;Doyon & Boivin, 2005;Karpova, 2006) is based exclusively (or at least mostly) on the rhythmic egression from the host egg chorion, not on the rhythmic pupal-adult ecdysis, although these two steps were not distinguished in the previous studies. Noteworthy, in most of the other studied insect taxa, adult ecdysis shows a robust circadian rhythm, although this rhythmicity could be based not only on the entrained circadian rhythm, but also on exogenous "masking" effects (Saunders, 2002).…”

“…The timing of the next step, i.e., of the emergence from the host chorion is much more important. Synchronous eclosion of Trichogramma during the morning hours apparently minimizes the risk of desiccation and optimizes the reproductive activity of these parasitoids (Zaslavski et al, 1995(Zaslavski et al, , 1999Karpova & Reznik, 2002;Karpova, 2006). Interestingly, a similar distinct rhythm of emergence from the host was also observed in the braconid wasp Cotesia congregata (Say) that parasitizes larvae of the tobacco hornworm, Manduca sexta L. (Beckage et al, 2002).…”

“…Therefore, the method used in the earlier studies (Zaslavski et al, 1995(Zaslavski et al, , 1999Karpova & Reznik, 2002;Karpova, 2006), i.e., periodic counting of emerged individuals, was no longer feasible. Thus, a new method was elaborated to monitor the temporal patterns of the processes under study: at regular intervals, one card was randomly selected from the cohort and immediately placed in a freezer (-18°C).…”

“…The rhythms persisted with a period close to 24 h after transfer to aperiodic conditions (constant darkness and constant temperature), suggesting they were under the control of endogenous circadian oscillators (Dahiya et al, 1993;Karpova, 2006). However, direct influence of a light-on and/or a temperature step-up can induce emergence of a considerable fraction of adults almost at any moment of the circadian time, suggesting a strong exogenous (masking) effect (Zaslavski et al, 1999;Karpova & Reznik, 2002;Karpova, 2006). Similar results were obtained for other egg parasitoids (Fantinou et al, 1998).…”

Maternal thermal effect on diapause inTrichogrammaspecies (Hymenoptera: Trichogrammatidae)

“…; Zaslavski et al. ; Karpova and Reznik ). Thus, the domination of the maternal photoperiodic response also can be a feature acquired by the ancestors of modern Trichogrammatidae that parasitized cryptic hosts (see Reznik for more discussion on these hypotheses).…”