The giant salmonfly (Pteronarcys californica) is the dominant stonefly shredder in many streams and rivers of North America. These stoneflies increase nutrient availability for other herbivores by consuming and processing decomposing leaves. However, riparian tree communities are diverse and produce many species of leaves that have very different chemical characteristics. Whether P. californica can choose among leaf species as they fall into the stream remains unknown. The consequences of such choices could determine the fitness of P. californica. During decomposition, leaves are ‘conditioned’, i.e., are colonized by microbial and fungal communities. These colonies, along with leaf chemistry and toughness, create variation in the rate of leaf conditioning and alter the rate of leaf decomposition. Shredders gain nutrients by consuming conditioned leaves. Because of variability in conditioning and nutrient quality, we predicted that P. californica will prefer leaves that decompose faster under short conditioning periods, but will prefer tough leaves under longer conditioning periods. We first measured stonefly preference among five leaf types based on either leaf species or conditioning time. We then assessed the fitness consequences of those choices by measuring stonefly growth when individuals were fed single leaf types. We found that P. californica shifts its preference from faster to slower-decomposing leaves as conditioning time lengthens, but neither leaf toughness nor an advantage in fitness dictates this shift. We show that interactions between riparian tree species and insect growth are complex. Thus, our results indicate that stream restoration efforts should include planting multiple tree species to improve growth and survival of invertebrates and the quality of the stream community.