What’s shaking for caterpillars? Leaf-borne vibratory stimuli and behavioral responses in the fall armyworm, Spodoptera frugiperda

Abstract

Leaf-borne vibrations are predicted to be significant for caterpillar communication and risk assessment, but the caterpillar’s vibratory landscape remains largely unknown. To address this, we used the fall armyworm Spodoptera frugiperda, as a model in our study with two main goals: (1) to characterize the vibratory landscape on a leaf in the presence of abiotic (wind and rain) and biotic (conspecifics and invertebrate predator) stimuli; and (2) to assess whether different larval instars detect and respond to those vibrations. Our findings show that abiotic and biotic vibrations were distinct from background noise, except for those produced by 1st instar larvae. Wind-induced leaf movement produced vibrations with a low-frequency and high-amplitude (< 100 Hz and 2.97 mm s⁻¹), in contrast with raindrops (> 174 Hz; 3.25 mm s⁻¹). The 2nd to 5th instar larvae and predatory stinkbugs moving on leaves produced vibrations with dominant frequencies ranging from 140 to 326 Hz and amplitudes from 1.42 to 2.95 mm s⁻¹. Furthermore, the spatial distribution of vibrations across bean leaves revealed that abiotic vibrations were more widely spread across leaves, unlike the more concentrated biotic vibrations. Regarding the caterpillar response to vibratory stimuli, caterpillars exposed to abiotic stimuli behaved differently from undisturbed caterpillars, regardless of instar. By contrast, caterpillars exposed to biotic stimuli do not respond consistently. Our findings contribute insights into a caterpillar’s vibroscape and support the hypothesis that armyworms can perceive and respond to both abiotic and biotic vibrations, filling a knowledge gap about this economically important pest species' sensory ecology.