The temperature dependence of the developmental rate of ectothermic organisms may change under the influence of abiotic and biotic environmental factors that modify the thermal reaction norms for development. These reaction norms are characterized by the lower temperature threshold, the regression coefficient (i.e., the degree of thermal sensitivity of development), and the sum of degree-days. Such manifestation of phenotypic plasticity of the thermal reaction norms for development has been described in many species of insects. We studied the effect of photoperiodic conditions on the development of the bug Graphosoma lineatum (L.) from St. Petersburg (59.9° N) and Bryansk (53.2° N) populations. Two experiments were performed with the bugs from either population that were collected in the field at different times. Two photoperiodic regimes were used (12 vs. 22 hours of light per day for the St. Petersburg population and 12 vs. 18 hours of light per day for the Bryansk population) and five constant temperatures (20, 22, 24, 26, and 28°C) in each photoperiodic regime. Photoperiodic conditions did not affect egg development in all of the experiments. Under the short-day conditions, there was a disproportionate (i.e., more pronounced at low temperatures) acceleration of nymphal development in bugs from the Bryansk population (in both experiments) and in bugs from the St. Petersburg population (in the experiment with late-season collection of parental adults). Thus, under short-day conditions, thermal sensitivity of development and the sum of degree-days did not change, whereas the temperature threshold became lower. The nymphs from the St. Petersburg population reared under long-day conditions showed different thermal reaction norms for development, depending on the “early” or “late” date of collection of adults in the field. The latter response was presumably explained by different physiological age of the parental individuals. Thus, the thermal reaction norms for nymphal development in the bug G. lineatum exhibit phenotypic plasticity during the summer season, depending on the oviposition date and currently experienced photoperiodic conditions. There were also interpopulation differences in the thermal reaction norms for egg and nymphal development. Bugs from both populations showed a greater body mass after developing at higher temperatures.
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