The adaptive function of waste management in a social spider mite

Effective waste management is fundamental to the success of social living, playing a key role in disease prevention and maintaining a habitable environment. However, experimental evidence for its adaptive significance remains limited, primarily due to the difficulties in manipulating defaecation behaviour. The social spider mite Stigmaeopsis longus defaecates cooperatively near nest entrances, guided by tactile and chemical cues. We experimentally altered the number and location of defaecation sites using host plant extracts that mimic the chemical cues and examined the effects on reproduction and survival under different female densities. Centralized and increased defaecation sites reduced egg survival in high-density nests, but not in moderate-density ones. Nevertheless, both densities exhibited increased nest extension, suggesting a compensatory response. When nest extension was physically prevented, egg survival declined even in moderate-density nests, confirming that nest extension mitigates the faecal hazards. These findings demonstrate that spatial control of waste is crucial for supporting group living by reducing exposure to faecal hazards, particularly for immobile life stages such as eggs. This study provides the first empirical evidence for the adaptive function of waste management in spider mites, highlighting its potential role in the evolution of sociality and nest-based living in arthropods.