The relative fitness rule states that parasitoid females should adopt risk-prone reproductive behaviours when expecting low reproductive success. Temperature influences the reproductive success of insects by affecting their basal metabolic rate during development, their egg load at emergence, and their life expectancy as adults. Using an aphid-parasitoid model system, we investigated the influence of developmental and adult temperature on the risk-sensitive decision-making of females. We considered the use of a low-quality host nymphal instar by the aphid parasitoid Aphidius ervi to be a risk-prone behaviour. Immature females were reared at 12, 20 or 28 °C and had access to the four nymphal instars of the potato aphid Macrosiphum euphorbiae for oviposition at one of these temperatures. Host selection behaviour was continuously recorded during exploitation of an aphid patch. We observed that warm-developed females and parasitoids foraging at high temperature attacked low-quality hosts more frequently than females from other treatments. These results support the hypothesis that a decrease in expected parasitoid reproductive success resulted in risk-prone behaviours. To our knowledge, this is the first experimental evidence suggesting that temperature influences host stage selection and risk-sensitive making decision in parasitoids, and the present study is the first to support the relative fitness rule.