High ambient temperatures can adversely affect insects through high evaporative water loss (EWL) and reduction of metabolic activity through enzyme denaturation. Establishing the relationship between the temperature at which these processes become detrimental and regulatory behaviour is critical in resolving the mechanisms by which insects cope with physiologically stressful environments. Here, we compare levels of metabolic rate and EWL measured by flow-through respirometry with field activity in the ichneumonid wasp Lissopimpla excelsa. Metabolic rate increased to a maximum of 10.8 ± 0.4 mLCO2.g−1.h−1 at 35°C before decreasing to 8.4 ± 0.4 mLCO2.g−1.h−1 at Ta = 40°C. EWL showed an exponential pattern of increase, with a significant increase in EWL from Ta = 12°C to Ta = 35 and 40°C. Male wasps were active in the field from Ta = 20.1 to 36.8°C (peak activity Ta = 26.5°C and relative humidity = 44.4%), though activity levels were most strongly correlated with time of day. Being active in the mornings may be advantageous in that temperatures are warm enough to maintain activity but avoid excess energy expenditure and EWL. Furthermore, food or calling females may be most abundant during this period. Based on a consensus allometric scaling relationship derived for insect resting metabolic rates, the metabolic rate of L. excelsa at Ta = 25°C was higher than predicted, as was EWL. Since the present study is the first describing the metabolic physiology of an ichneumonid wasp, it remains unclear whether this pattern is characteristic of ichneumonids in general of L. excelsa in particular.