Selective Feeding by Stream Caddisfly (Trichoptera) Detritivores on Leaves with Fungal-Colonized Patches

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Abstract
A leaf in a stream represents a mosaic of fungal-colonized areas or "patches". Patches may differ in their fungal composition and stage of processing. They, therefore, represent spatially heterogeneous foods of potentially diverse qualities to detritivores. However, little is known about the ability of detritivores to locate and discriminate among these fungal-colonized patches as food sources. To assess this ability, we manipulated fungal colonization patterns on leaves and fed them to caddisfly (Trichoptera) detritivores. In one experiment, disks were cut from leaves which had been incubated with one of three fungal species for different time periods. Disks from different treatments were randomly fastened to quadrants of uncolonized leaves and fed to caddisfly larvae. In a second experiment, fungal patchiness was produced by incubating leaves with disks attached so that fungi would colonize discrete areas on leaves. Disks were removed before leaves with patches were offered to larvae. Feeding patterns, determined by visual ranking of leaf skeletonization, indicated that larvae fed selectively on fungal-colonized patches in both experiments. Larval choice was affected by fungal species and degree of colonization. Larvae were able to detect fungal hyphae associated with patched before physical changes had occurred in the leaf matrix. Observations of foraging larvae and the feeding patterns they exhibited suggest that these caddisflies view individual leaves as coarse-grained resources.