The effect of bibionid larvae feeding on the microbial community of litterand on reconsumed excrements

The effect of larval bibionid feeding on microbial respiration (CO2 product ion and O-2 consumption), fluorescein diacetate activity (FDA), direct coun ts of bacteria and composition of the microbial community were studied. Lea f litter and larval excrement (1-3 days old) were used as food. Two species of bibionid larvae and two types of leaf litter (Bibio marci feeding on Ca rpinus betulus litter and Bibio pomonae on Populus nigra) were used. Gut pa ssage resulted in an increase of microbial O-2 consumption, total microbial metabolic activity, direct counts of bacteria and number of actinomycetes in the fresh excrement. This effect was only short term. Reduction of bacte rial counts and shift in composition of microbial communities were observed during both the first and second gut passages. Passaging induced decrease in the C and N contents of the substrate, the C/N ratio and the amounts of water soluble carbohydrates. Animal - microbial interactions differed betwe en the first and the second gut passage. During the second gut passage high er numbers of bacteria in the gut content were found. This appears to be du e to the higher counts of bacteria in the food and a higher proportion of b acteria which could survive gut passage. The effect of gut passage was depe ndent on the species of bibionid larvae and on the food that was consumed. CO2 production responded differently than O-2 consumption and most of the m icrobial parameters suggesting that CO2 production is influenced not only b y microbial activities but also by other factors (aeration status, pH).