Global warming causes concomitant changes in several environmental factors that often have synergistic effects on populations and ecosystem processes. We examined how increased water temperature and reduced litter quality affected a leaf-shredding detritivore’s performance and its effect on litter breakdown. Detritivores were exposed in microcosms at two temperatures (10 and 15 °C) and four categories of litter quality (based on nitrogen and condensed tannin concentrations). We hypothesized that (1) high-quality litter mixtures would breakdown faster, improving detritivore performance; (2) differences would occur regardless of which plant species in the mixture were preferentially consumed; and (3) litter quality effects on detritivore-mediated breakdown and performance would be intensified at higher temperatures. Unexpectedly, we found faster breakdown at intermediate litter quality and lower temperature. Additionally, we found cases of detritivore selection and rejection of different resources driven by litter traits other than nitrogen and tannin concentrations. Detritivore performance increased with temperature, regardless of litter quality. Our results support non-additive and unpredictable joint effects of temperature and litter quality, suggesting that these concomitant changes may affect stream functioning.