Inter-plant vibrational communication in a leafhopper insect

Abstract

Vibrational communication is one of the least understood channels of communication. Most studies have focused on the role of substrate-borne signals in insect mating behavior, where a male and a female establish a stereotyped duet that enables partner recognition and localization. While the effective communication range of substrate-borne signals may be up to several meters, it is generally accepted that insect vibrational communication is limited to a continuous substrate. Until now, interplant communication in absence of physical contact between plants has never been demonstrated in a vibrational communicating insect. With a laser vibrometer we investigated transmission of natural and played back vibrational signals of a grapevine leafhopper, Scaphoideus titanus, when being transmitted between leaves of different cuttings without physical contact. Partners established a vibrational duet up to 6 cm gap width between leaves. Ablation of the antennae showed that antennal mechanoreceptors are not essential in detection of mating signals. Our results demonstrate for the first time that substrate discontinuity does not impose a limitation on communication range of vibrational signals. We also suggest that the behavioral response may depend on the signal intensity.

Figure 1. Male-female communication in Scaphoideus titanus recorded on leaves without direct contact (Test 1).

Distances between upper and lower leaf were from 0.5 cm to 7 cm. The percentage of females that responded to the male calling signal (total column height) is divided according to the subsequent male behavioral response: mating duet, followed either by female location (black) or by call-fly (gray), and no male reaction (striped). n indicates the number of insect pairs tested.

Figure 2. Signal properties measured on leaves with discontinuous substrate (Test 2).

Mean (±SE) values of maximum substrate vibration velocity (mm/s) (A, logarithmic scale) and frequency (Hz) (B) of pulses from MCS (Male calling signal) are shown. While substrate velocity progressively decreased (Jonckheere test: J₀ = 5.93, P<0.001) with the distance between leaves, the frequency increased (J₀ = 2.29, P = 0.011). Nat: MCS emitted by natural male recorded on the same leaf; LL: MCS emitted by playback recorded on the same leaf; 0.5–11: MCS emitted on the lower leaf and recorded from the upper leaf with a progressive gap width of 0.5–11 cm.

Figure 3. A schematic drawing of experimental setup.

A male and a female were placed on leaves (surface 6×10 cm) of two separate grapevine cuttings. The bottom of the stem was put in a glass vial filled with water to prevent withering. One cutting was put on an anti-vibration table (Astel S.a.s., Ivrea, Italy). The second cutting was attached to a metal arm suspended from above – without any contact with the table - and positioned in parallel over half the surface of the lower leaf (as shown in the inset as viewed from above). The laser beam was focused on the lamina of the lower leaf with the female. To prevent the insects from escaping, recordings were made within a Plexiglas cylinder (50×30 cm), provided of two openings for the laser beam and the metal arm. Not drawn to the scale.