Influence of temperature on the photoperiodic time measurement and on the induction of Trichogramma telengai progeny diapause: The separation of the two effects

Thermal effects on photoperiodic time measurement and accumulation of inductive photoperiods have been studied in many insect species whereas the influence of temperature on the last step of the photoperiodic response, the induction of diapause, received less attention from researchers. We investigated thermal modification of the maternal photoperiodic response in Trichogramma telengai (Hymenoptera: Trichogrammatidae). Even a single long-night photoperiod experienced by females of this minute egg parasitoid immediately before oviposition causes a substantial increase in larval diapause incidence in the progeny. This feature allows separation of the thermal effects on different steps of the diapause-inducing photoperiodic response. Laboratory experiments showed that the temperature of the last scotophase (when the final decisive photoperiodic time measurement occurs) caused an inverted U-shaped diapause-inducing response similar to that observed in some other long-day insects. The temperature of the last photophase (when progeny diapause is induced) had a positive linear effect that has not been reported for the induction of winter diapause in any long-day insect. Most probably, such a thermal response is not a specific seasonal adaptation but a direct consequence of the influence of temperature on the rate of metabolism.