[4] T. pusillus may be distinguished from other British woodlice chiefly by its small size, which reaches no more than 5 millimetres (0.2 in).

[10] Like other woodlice, T. pusillus eats decaying plant matter of various kinds, although only alder litter is capable of sustaining a stable reproducing population.

[10] Predators of T. pusillus include the common shrew Sorex araneus, lycosid and dysderid spiders, centipedes such as Lithobius variegatus and perhaps carabid beetles.

[11] Trichoniscus pusillus is susceptible to infection by isopod iridescence virus, or Iridovirus (Iridoviridae).

This is first apparent as a blue sheen on the unpigmented underside of the animals, but soon spreads to give the entire exoskeleton a bluish iridescence.