Influence of Decomposition Dynamics of Plant–Animal Organic Matter on Distribution Patterns of Calyptrate Flies

Understanding how decomposing organic matter attracts calyptrate flies is fundamental to both medical entomology and forensic science, yet the differential effects of substrate type and decomposition stage remain poorly characterized in tropical environments. In this study, we established controlled decomposition gradients for fish viscera (animal tissue) and leafy vegetables (plant tissue) at 37 °C and 65% RH, defining six physicochemical stages per substrate based on color, odor, and ammonia emissions. Over 20 days, in Haikou, Hainan, we deployed 30 spatially replicated traps per stage, collecting 1538 specimens spanning 13 species across four families. Animal substrates attracted 6.8-fold higher fly densities than plant materials (1.48 vs. 0.22 flies/cage/hour; p < 0.01), with Limnophora nudiseta dominating animal-baited traps (68.43%) and Anthomyia illocata prevailing in plant traps (43.94%). Decomposition stage critically modulated attraction: mid–late decay stages (2–5) yielded 3.6-fold more flies on animal tissues than fresh stages, accompanied by a successional shift from A. illocata to L. nudiseta as the primary colonizer. Plant tissues showed incremental but non-significant density increases, and were consistently dominated by A. illocata. These findings demonstrate that substrate identity and decomposition dynamics jointly govern fly abundance, community composition, and niche partitioning. Our mechanistic framework offers practical guidelines for optimizing fly surveillance through species-specific bait selection and decay-stage calibration, with direct applications in vector control, public health monitoring, and forensic post mortem interval estimation.