Abstract Rising temperature has been associated with increased occurrence of herbivorous insect outbreaks, explained by several direct and indirect mechanisms. Whereas natural enemies are known key drivers of forest‐defoliating insect cycles, indirect effects of temperature on insect's ability to defend against pathogens and parasitoids (e.g., immunocompetence), as well as the interaction with other mechanisms (e.g., diet), remain less explored. The aim of this study is to evaluate the effect of temperature and diet on the performance and immune response of the model lepidopteran system Ormiscodes amphimone (Saturniidae) and its host plants Nothofagus spp. (Nothofagaceae). Larvae of O. amphimone were reared under two temperature conditions (ambient 18:6°C and warmed, 21:6°C; light: dark, 14:10 h) and on leaves of two of their preferred Nothofagus host plants, which vary in quality (lower N. antarctica–higher N. pumilio). We measured developmental time, female pupal weight as a proxy of fitness, relative growth rate, nutritional indices and melanisation of a monofilament as a proxy of immune response. Results showed that an average rise of 2°C favours larval immunocompetence, potentially decreasing mortality exerted by parasitoids. Moreover, depending on diet, an increase in temperature can either maintain (on more nutritious N. pumilio leaves) or enhance (on less nutritious N. antarctica leaves) larval nutritional efficiency, performance and female pupal weight. Hence, an increase in temperature could enhance O. amphimone population growth, through attenuating differences caused by diet and enhancing immunocompetence, favouring outbreak frequency, severity and area.