Plants respond to leaf vibrations caused by insect herbivore chewing.

Plant germination and growth can be influenced by sound, but the ecological significance of these responses is unclear. We asked whether acoustic energy generated by the feeding of insect herbivores was detected by plants. We report that the vibrations caused by insect feeding can elicit chemical defenses. Arabidopsis thaliana (L.) rosettes pre-treated with the vibrations caused by caterpillar feeding had higher levels of glucosinolate and anthocyanin defenses when subsequently fed upon by Pieris rapae (L.) caterpillars than did untreated plants. The plants also discriminated between the vibrations caused by chewing and those caused by wind or insect song. Plants thus respond to herbivore-generated vibrations in a selective and ecologically meaningful way. A vibration signaling pathway would complement the known signaling pathways that rely on volatile, electrical, or phloem-borne signals. We suggest that vibration may represent a new long distance signaling mechanism in plant-insect interactions that contributes to systemic induction of chemical defenses.

A bending wave simulator for investigating directional vibration sensing in insects.

Substrate vibrations are important in social and ecological interactions for many insects and other arthropods. Localization cues include time and amplitude differences among an array of vibration detectors. However, for small species these cues are greatly reduced, and localization mechanisms remain unclear. Here we describe a method of simulating the vibrational environment that facilitates investigation of localization mechanisms in small species. Our model species was the treehopper Umbonia crassicornis (Membracidae; length 1 cm), which communicates using bending waves that propagate along plant stems. We designed a simulator consisting of a length of dowel and two actuators. The actuators were driven with two time signals that created the relationship between slope and displacement characteristic of steady-state bending wave motion. Because the surface of the dowel does not bend, as would a natural stem, close approximation of bending wave motion was limited to a region in the center of the dowel. An example of measurements of the dynamic response of an insect on the simulator is provided to illustrate its utility in the study of directional vibration sensing in insects.

Directionality in the mechanical response to substrate vibration in a treehopper (Hemiptera: Membracidae: Umbonia crassicornis).

The use of substrate vibrations in communication and predator-prey interactions is widespread in arthropods. In many contexts, localization of the vibration source plays an important role. For small species on solid substrates, time and amplitude differences between receptors in different legs may be extremely small, and the mechanisms of vibration localization are unclear. Here we ask whether directional information is contained in the mechanical response of an insect's body to substrate vibration. Our study species was a membracid treehopper (Umbonia crassicornis) that communicates using bending waves in plant stems. We used a bending-wave simulator that allows precise control of the frequency, intensity and direction of the vibrational stimulus. With laser-Doppler vibrometry, we measured points on the substrate and on the insect's thorax and middle leg. Transfer functions showing the response of the body relative to the substrate revealed resonance at lower frequencies and attenuation at higher frequencies. There were two modes of vibration along the body's long axis, a translational and a rotational mode. Furthermore, the transfer functions measured on the body differed substantially depending on whether the stimulus originated in front of or behind the insect. Directional information is thus available in the mechanical response of the body of these insects to substrate vibration. These results suggest a vibration localization mechanism that could function at very small spatial scales.