1. 1. It has been accepted that aquatic hyphomycetes colonising submerged leaves increase the nutritional value of leaf detritus and suggested that fungal biomass plays a greater role in the growth of shredders than leaf tissue itself. However, it is not clear what proportion of the nutritional needs of shredders is met by fungal biomass.
2. We fed Pycnopsyche gentilis larvae with tulip poplar (Liriodendron tulipifera) leaf discs colonised by the aquatic hyphomycete, Anguillospora filiformis, which had been radiolabelled to quantify the contribution of fungal carbon to the growth of the shredder at different larval developmental stages. Instantaneous growth rates of larvae on this diet were also estimated.
3. When provided with fungal-colonised leaves (14–16% fungal biomass), the third and the fifth instar larvae of P. gentilis grew at the rates of 0.061 and 0.034 day−1, respectively, but on a diet of sterile leaves, both larval instars lost weight. The incorporation rates of fungal carbon were 31.6 μg C mg−1 AFDM day−1, accounting for 100% of the daily growth rate of the third instar larvae and 8.6 μg C mg−1 AFDM day−1, accounting for 50% of the daily growth rate of the fifth instar larvae.
4. These results suggest that leaf material colonised by A. filiformis is a high quality food resource for P. gentilis larvae, and that fungal biomass can contribute significantly to the growth of these larvae. Differences in feeding behaviour and digestive physiology may explain the significantly greater assimilation of fungal biomass by the earlier instar than the final instar. To satisfy their nutritional needs the fifth instar larvae would have to assimilate detrital mass that may have been modified by fungal exoenzymes.