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

Abstract

A comparative study of the direct influence of single, non-repetitive changes of light and/or temperature on the daily pattern 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 addition, 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 practically 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, temperature 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.