For many small insects that communicate using plant-borne vibrations, finding the vibration source is often necessary during mate searching or recruitment to a feeding site. However, determining the direction of a substrate-borne wave is challenging for small species, because time and amplitude differences are minimal between vibration receptors in different legs. An alternative solution for small species is to compare sequential samples along an amplitude gradient. We tested the role of amplitude cues in mate searching in the thornbug treehopper, Umbonia crassicornis In this species, mate-searching males home in on the vibrational signals of stationary females by sampling at multiple locations along host plant stems. We used vibrational playback to create two contrasting amplitude 'landscapes' on a 1-m tall potted host plant (Albizia julibrissin). In one treatment, playback amplitude was increased as the male moved closer to the source (by 3 dB every 4 cm). In the other the pattern was reversed, and playback amplitude was increased as the male moved farther from the source; in each case the amplitude experienced by the male at a given location depended on the interaction of the source amplitude and the plant substrate.
Most males (20/21) located the source when amplitude was increased with proximity, and all males failed to locate the source when the pattern was reversed. Future research will investigate the conditions under which natural amplitude gradients occur on plants, and how changes in amplitude interact with other potential cues during vibration localization.