Short term and long term effects of bibionid (Diptera: Bibionidae) larvae feeding on microbial respiration and alder litter decomposition

Temporal changes of microbial respiration of leaf litter during gut passage by two species of bibionid flies (Penthetria holosericae and Bibio marci) and immediately after defecation were studied as well as the effect of B. marci feeding on microbial respiration in the remaining consumed leaf litter. Respiration in the guts of both species was lower than respiration of the original leaves. Microbial respiration increased after defecation, but respiration was higher in comparison with the original litter only for B. marci. Later, microbial respiration in both species decreased dramatically over several hours. Respiration of the remains of consumed leaves, from which the excrements were removed, was significantly higher than the respiration of non-consumed leaves. Scanning electron microscopy indicated that plant cells in the unconsumed part of the litter were massively broken-up by feeding activity and potentially exposed to microbial colonisation from the surrounding soil and microflora coming from the excrements deposited on the consumed litter. Long term (11 months) exposure of leaf litter and excrements produced from this litter showed that the decomposition of excrements was significantly slower than that of leaves. These findings indicate that bibionid feeding activity causes a short term increase in microbial mineralization of litter, but slows it down in the long term.