Abiotic and biotic factors influence diapause induction in sexual and asexual strains of Trichogramma brassicae (Hym: Trichogrammatidae)

Gradually Increasing the Temperature Reduces the Diapause Termination Time of Trichogramma dendrolimi While Increasing Parasitoid Performance

Trichogramma dendrolimi Matsumura is widely used as a biological control agent of many lepidopteran pests. Diapause has been used as an effective method to preserve the Trichogramma products during mass rearing production. However, it currently takes at least 70 days to break diapause, and we tested whether gradually increasing the temperature instead of using constant temperature could reduce the time of diapause termination and offer a higher flexibility to Trichogramma producers. The diapause termination rates of individuals kept at different conditions were measured, and five groups for which diapause termination rate reached the 95% were selected to test five biological parameters, including the number of eggs parasitized, the parasitism and emergence rates, the female sex ratio, the wing deformation rate, and the parasitoid longevity. Compared to the currently used procedure (70 days at 3 °C), treatments with at least two different temperatures resulted in higher parasitism and emergence rates while keeping the other parameters constant. The treatment that consisted of at least two different temperatures preceded by only 55 days of induction period had the highest population trend index, meaning that the population under these conditions grows more rapidly. Our results demonstrate that gradually increasing temperature allows T. dendrolimi to complete diapause earlier than at present while increasing its potential pest control capacity and providing additional flexibility in mass production of T. dendrolimi.

Genetic Variability, Population Differentiation, and Correlations for Thermal Tolerance Indices in the Minute Wasp, Trichogramma cacoeciae

Simple Summary

Augmentative biological control relies on the more or less frequent/abundant releases of biological control agents (BCAs) that have to be adapted to their short-term local environment including (micro-)climatic conditions. Thermal biology of BCAs is thus a key component for their success. The extent to which thermal tolerance indices may be relevant predictors of the field efficiency is however still poorly documented. Within this frame, we investigated the intraspecific variability for the ability to move at low temperatures in the minute wasp, Trichogramma cacoeciae. We collected, molecularly characterized, and compared for their thermal tolerance indices numerous strains originating from three contrasting geographic areas. Our findings evidenced both a geographic differentiation between strains for one of the thermal tolerance indices and a positive correlation between two of them, demonstrating the existence of an intraspecific variability.

Abstract

Temperature is a main driver of the ecology and evolution of ectotherms. In particular, the ability to move at sub-lethal low temperatures can be described through three thermal tolerance indices—critical thermal minimum (CTmin), chill coma temperature (CCT), and activity recovery (AR). Although these indices have proven relevant for inter-specific comparisons, little is known about their intraspecific variability as well as possible genetic correlations between them. We thus investigated these two topics (intraspecific variability and genetic correlations between thermal tolerance indices) using the minute wasp, Trichogramma cacoeciae. Strains from T. cacoeciae were sampled across three geographic regions in France—two bioclimatic zones along a sharp altitudinal cline in a Mediterranean context (meso-Mediterranean at low elevations and supra-Mediterranean at higher elevations) and a more northwestern area characterized by continental or mountainous climates. Our results evidenced a significant effect of both the longitude and the severity of the cold during winter months on CCT. Results were however counter-intuitive since the strains from the two bioclimatic zones characterized by more severe winters (northwestern area and supra-Mediterranean) exhibited opposite patterns. In addition, a strong positive correlation was observed between CCT and CTmin. Neither strain differentiation nor the covariations between traits seem to be linked with the molecular diversity observed on the part of the mitochondrial marker COI.

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.