Riparian vegetation influences the quality and availability of organic matter inputs, microclimatic conditions and water chemistry in stream ecosystems. Therefore, the replacement of riparian forests to agricultural areas along stream catchments can affect detritivores and detritus-based food webs in this study, we evaluated the effects of agricultural areas on the size structure and density of larvae of Phylloicus sp. (Trichoptera, Calamoceratidae), a typical shredder in Atlantic Forest streams (SE Brazil). Over a year, we monthly sampled leaf patches in three streams surrounded by coffee plantations and three streams located inside native forest fragments. We measured the body length and biomass of all larvae found and analysed temporal variations in population structure. Our results indicate that forest catchments had a greater number of individuals, with larvae showing larger body sizes and higher biomass values compared to coffee catchments, which had a lower abundance and smaller larvae sizes. Multiple regression analyses suggest that, in forest catchments, higher water pH, increased dissolved oxygen, and greater leaf biomass positively influenced larval body size and biomass, indicating better growth conditions in these preserved environments. As expected, higher water temperature in coffee catchments negatively affected larval growth, likely due to increased metabolic demands and reduced availability of suitable resources. The clear differences in the size structure and frequencies of larvae throughout the year highlight the direct and indirect effects of riparian forest removal on detritivores populations. These findings suggest that environmental stressors from agricultural activities can also change the development of larvae of Phylloicus sp., which may affect organic matter processing and stream ecosystem functioning. Our results also highlight the importance of native riparian vegetation in maintaining favourable environmental conditions to insect shredders development.