Grandmaternal temperature effect on diapause induction in Trichogramma telengai (Hymenoptera: Trichogrammatidae)

Adaptive grandmaternal thermal effect, wherein the grandmaternal thermal environment affects the induction of progeny diapause two generations later, has not been studied in any insect system. We have studied this effect in the parthenogenetic egg parasitoid Trichogramma telengai Sor. (Hymenoptera: Trichogrammatidae) under laboratory conditions. The grandmaternal generation developed at temperatures from 18 to 30 °C combined with short (L:D = 12:12) or long (L:D = 18:6) photoperiod. The maternal generation developed at the same two photoperiods combined with low (20 °C) or high (30 °C) temperature. The progeny generation developed at 14 °C and L:D = 12:12. The grandmaternal temperature response was consistent with an adaptive response (low temperature induces facultative prepupal winter diapause two generations later) and rather strong (ranged up to 20-25%). The experiments suggested that both grandmaternal and maternal thermal effects are based on the influence of temperature on photoperiodic induction of diapause. However, experiments also revealed substantial differences between the grandmaternal and maternal thermal responses. In particular, the grandmaternal thermal response was observed only at short-day grandmaternal photoperiod, whereas the maternal thermal response was independent both of maternal and grandmaternal photoperiods. Although under natural conditions the adaptive value of the grandmaternal thermal response in T. telengai is most probably low, this effect should be considered in physiological models of diapause induction and can be important for mass rearing of Trichogramma species.