Comparing decomposition rates and detritivore preferences for caddisfly (Trichoptera) cases versus ambient detritus

Abstract

Humans manage microbes in their built environments by lowering humidity, filtering outside air, and applying antimicrobials that together fosters the selective dominance of microorganisms that can tolerate these harsh conditions and selects for antimicrobial resistance. Other species, however, regulate decomposition of their built environments in warm, moist conditions with diverse microbial communities. For example, larval caddisflies that live in streams and ponds use their labial silk glands to weave detritus, pebbles, and other materials into portable cases. Premature decomposition of cases is detrimental to these invertebrates, as cases serve as protection for vulnerable larvae against predation, aid in respiratory efficiency, and ensure reproductive success of adults. Plant-based cases remain in wetland ponds in Colorado’s East River valley for up to nine months after caddisfly pupation and case abandonment, while the same detritus not associated with cases is absent from the pond bottom. Insect silk, which often contains antimicrobial peptides or proteins, may be a contributing factor in potential antimicrobial properties and decreased detritivore consumption of cases. In this study, I first quantified decomposition of caddisfly cases compared to that of the same non-case associated detritus, expecting caddisfly cases to possess slower rates of decomposition. Second, I explored the feeding preferences of a common invertebrate detritivore for the cases of different caddisfly species and the non-case associated detritus. Lastly, I quantified the fungal communities of cases and detritus as well as the feeding preference of detritivores. I hypothesized that cases contain less fungi, correlating to lower food value and a potential mechanism for detritivore avoidance of cases over non-case associated detritus. While A. bimaculata cases and C. aquatilis had similar slow rates of mass loss, L. externus and H. designatus cases rapidly decomposed over the course of four weeks. The latter cases with finer pieces of detritus also lost organic mass fastest, suggesting higher fungal activity and a strong detritivore feeding preference. L. externus and A. bimaculata larvae avoided A. bimaculata cases and preferred L. externus cases due to the potentially aggressive behavior of larger A. bimaculata larvae and physically easier to eat small pieces of detritus in L. externus cases. With the ideal conditions of agar plates, fungi on C. aquatilis cases possessed the highest colony morphotype diversity, but percent of fungal cover was similar across C. aquatilis, L. externus cases, and A. bimaculata cases. The behavior and case construction of caddisfly cases give insight into the diverse strategies these invertebrates employ to maintain their built environments in wetland ponds conducive to vigorous microorganism proliferation and decomposition of organic matter, informing future design of human-built environments.

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