Biodegradable plastic mulch films are increasingly adopted in agriculture, yet the ecotoxicological risks arising from their degradation products in mixtures with concurrent chemical inputs remain largely unexplored. This study assessed the lethal and sublethal toxicity of PBAT/PLA biodegradation products combined with four soil-applied insecticides (abamectin, cyantraniliprole, sulfoxaflor, and thiamethoxam) on the soil invertebrates Folsomia candida and Stratiolaelaps scimitus across a 12-week degradation period. In F. candida, PBAT/PLA biodegradation products synergized the lethal toxicity of abamectin, sulfoxaflor, and thiamethoxam. The temporal dynamics of these synergistic interactions mirrored the degradation phases observed in physicochemical analyses, suggesting the importance of selecting appropriate degradation time windows for risk assessments. In contrast, no lethal synergism was detected in S. scimitus. However, sublethal assays showed that the mixtures significantly altered predation activity, with synergistic effects observed for sulfoxaflor and thiamethoxam. These findings demonstrate that interaction patterns are species- and endpoint-dependent, and that lethal effects may not predict sublethal functional impairment. Overall, the results indicate that the timing of mulch-film degradation can influence mixture toxicity under controlled laboratory conditions.