In agriculture, plastic mulching films enhance nutrient efficiency and provide thermal insulation. However, the use of mulches has been debated due to its potential to adversely affect the biodiversity of agroecosystems. The present study aimed to evaluate the behavioural responses of the arthropods Folsomia candida (springtails) and Porcellionides pruinosus (woodlice) to leachates released from LDPE and PP additive-free plastic granules. The leachates of pristine and artificially aged LDPE and PP were tested at three different concentrations, ranging from environmentally relevant levels to a worst-case scenario of soil contamination.  Avoidance behaviour was evaluated to assess its potential impact on soil habitat function, while aggregation status to identify possible functional impairments in the woodlice population. The loss of aggregation behaviour can in fact expose individuals to dehydration and reducing their fitness. The results showed no statistically significant avoidance responses in arthropods to either treatment. In contrast, useful data on the potentially higher hazard posed by LDPE than by PP were obtained by evaluating the disaggregation behaviour of woodlice. Disaggregation assays revealed that plastic leachates may influence social behaviour. Unlike PP, LDPE induced a statistically significant impairment of gregarious behaviour at the highest tested concentration (150 mg/kg d.w.). Furthermore, pristine LDPE exhibited more pronounced disaggregation than aged LDPE, suggesting that weathering may modulate behavioural responses depending on the polymer type and the endpoint. The differing chemical reactivities of LDPE and PP are hypothesised to have contributed to the observed effects, given their distinct product release profiles. In particular, PP produced larger amounts of low-molecular-weight oxygenated compounds, while LDPE showed higher relative amounts of long-chain alkanes. Therefore, it is suggested that high levels of plastic leachates can adversely affect soil arthropods, and that the aggregation behaviour of woodlice might be a more informative and sensitive biological endpoint than avoidance alone.