Abstract

The onset of an overwintering strategy to overcome cold temperatures of a species of ectotherms can include remaining active or entering diapause. This in turn will depend on the relative costs of each strategy and therefore, could differ among populations along a latitudinal gradient. Thus, expecting higher levels of diapause in the coldest conditions and a higher incidence of individuals remaining active in the warmest conditions. We assessed the insect responses to photoperiod and temperature, in five Chilean populations of the aphid parasitoid Aphidius platensis. We analysed the variation in winter temperature along the latitudinal gradient and, under controlled conditions, examining the potential effects of three constant conditions of photoperiod/temperature: 8:16LD (Light: Dark, hours) at 10 degrees C, 10:14LD at 14 degrees C, and 16:8LD at 20 degrees C, on diapause levels. Finally, we measured cold tolerance (CTMin), developmental time, fresh body mass and size, fat and water content, and egg load for the emerging parasitoids. Our results showed no clear latitudinal gradient in temperature but differences among sites were clear. None of the five populations of A. platensis expressed diapause at any tested condition, suggesting that the environmental thresholds for diapause induction are perhaps not reached in this species under the studied latitudes. Insects from the coldest point in the gradient (Pinto) showed the lowest CTMin suggesting local adaptation. Moreover, physiological and life-history traits seem to adjust rapidly through developmental thermal acclimation, showing that plasticity is involved in the parasitoid's responses to the temperature differences found among localities. Consequently, both local adaptation and phenotypic plasticity contribute to this species remaining active during the whole winter, being an effective strategy to diapause in relatively mild and stable thermal environments.