Changes in the environmental factors of the parasite-host complex affect the developing parasite if its microclimate is disturbed. We have studied the effects of temperature changes (from the 15°–25°C thermoperiod to 20°–30°C) on the energy assimilated and dissipated by the larvae and pupae of the male endoparasiteDiadromus pulchellusWsm. (Hymenoptera, Ichneumonidae) in order to ensure their metabolism in the pupae ofAcrolepiopsis assectella Z. (Lepidoptera, Plutellidae).A 5°C temperature increase in the 8-h dark period and the 16-h light period during the feeding stage leads to greater heat losses by radiation and conduction. The histological alterations of prepupa, the evolution of pupa, imaginal instar, and emergence are accompanied by a heat loss due to radiation and conduction which is identical in the 2 thermoperiods. During the feeding stage and the pre-pupal state, daily energy losses by evaporation are greater in the 20°–30°C thermoperiod. A 5°C temperature increase during the dark and light periods does not change energy losses by evaporation during pupation, imaginal instar, or emergence.After the death of the host, the evolution from larval stage 2 to the free imago stage occurs in 10 days under the 20°–30°C thermoperiod with an energy expenditure of 3.8 calories, of which 63.4% represents heat losses by radiation and conduction. In the 15°–25°C thermoperiod with a 14-day development, the parasite expends 3.7 calories, of which 62% represents heat lost by radiation and conduction.The quantity of food assimilated by the larvae and the pupa is not modified as a function of thermoperiod. An increase in temperature does, however elevate variability during the stage of transformation of assimilates into body substances. In the 15°–25°C thermoperiod, 80.2 % of the energy potentially available to the host is diverted to adult parasite production and 83.9% is diverted in the 20°–30°C regime.