Variation of the Photoperiodic Reaction in Trichogramma

Abstract—A laboratory study of photoperiodic control of diapause in the egg parasitoid Trichogramma embryo-phagum Htg was carried out. Experiments conducted at 20°C and eight photoperiods (day-lengths of 3, 6, 9, 12, 15, 18, 21 and 24 h) revealed the long-day type response based on the maternal influence on prepupal diapause in progeny at 15°C. The maximum rate of diapause in progeny was recorded at day-lengths of 12–15 h, the estimated threshold photophases were between 6–9 h (left threshold) and 16–17 h (right threshold). A comparative analysis of photoperiodic responses in seven laboratory generations reared under constant conditions revealed endogenous variability of the tendency toward diapause. The left threshold day-length was highly variable, whereas the right threshold day-length remained relatively constant. The possible reason is that the right threshold is subject to stabi-lizing selection in the natural conditions, whereas the left threshold zone is a selectively neutral character revealed only in laboratory experiments. Experimental study of the factors that control the seasonal development is one of the most important lines of investigations in modern entomology (Dani