Tiny caterpillars assess threats by the footsteps of their enemies

Insects inhabit complex vibroscapes shaped by substrate-borne vibrations from multiple biotic and abiotic sources. One underappreciated topic is how vibrations function in predator-prey interactions. Tiny warty birch caterpillars (Falcaria bilineata) are known to produce complex vibratory signals to defend leaf-tip territories against conspecifics, raising the question of whether vibratory signalling and sensing also play roles in predator-prey interactions. We staged encounters between resident neonate caterpillars and three natural intruders: conspecifics, ladybird beetle larvae and adult ladybird beetles, while simultaneously recording behaviour and substrate-borne vibrations. Resident caterpillars showed three key responses - vibratory signalling, freezing and dropping - but these responses varied strongly with intruder identity and stage of encounter. Residents signalled vigorously toward conspecifics, with rates escalating as intruders approached their territories. In contrast, encounters with predators evoked predator-specific defensive strategies including freezing and dropping. Adult ladybird beetles, which caused high mortality (43%), elicited rapid escape responses and subsequent territory abandonment, while ladybird beetle larvae, which caused no mortality, triggered slower responses with initial signalling that ceased upon closer approach. Critically, vibrations generated by the 'footsteps' of each approaching intruder type produced distinct vibratory 'signatures', differing in amplitude, spectral and temporal characteristics. Resident caterpillars also initiated defensive responses before physical contact, often when intruders were still centimetres away. Together, these findings demonstrate that these miniature larvae, no larger than ∼1-2 mm, thrive in complex vibroscapes where vibrations not only function to advertise territory ownership against conspecifics but also provide essential early-warning cues enabling sophisticated threat assessment and context-appropriate defensive responses in predator-rich environments.