Leaf-borne vibrations are potentially important to caterpillars for communication and risk assessment. Yet, little is known about the vibratory environment of caterpillars, or how they detect and discriminate between vibrations from relevant and non-relevant sources. We measured the vibratory ‘landscape’ of the territorial masked birch caterpillar Drepana arcuata (Drepanidae), and assessed its ability to detect and respond to vibrations generated by conspecific and predatory intruders, wind and rain. Residents of leaf shelters were shown to respond to low amplitude vibrations generated by a crawling conspecific intruder, since removal of the vibrations through leaf incision prevented the resident’s response. Residents did not respond to large amplitude, low frequency disturbances caused by wind and rain alone, but did respond to approaching conspecifics under windy conditions, indicating an ability to discriminate between these sources. Residents also responded differently in the presence of vibrations generated by approaching predators (Podisus) and conspecifics. An analysis of vibration characteristics suggests that despite significant overlap between vibrations from different sources, there are differences in frequency and amplitude characteristics that caterpillars may use to discriminate between sources. Caterpillars live in a vibration-rich environment that we argue forms a prominent part of the sensory world of substrate bound holometabolous larvae.
Drepana arcuata Wlk. (Lepidoptera: Drepanidae) were collected as gravid females from May to August between 2004 and 2011 near Ottawa, ON, Canada. Females laid eggs and larvae were reared on cuttings of paper birch (Betula papyfera) maintained in indoor enclosures (25 ± 2°C, 70 ± 5% relative humidity, and 12:12 LD). All caterpillars used in the trials were 4th or 5th instars (residents were 29.75 ± 5.42 mg and intruders were 25.61 ± 3.95 mg). Adults (29.71 ± 5.78 mg) of the generalist predator Podisus maculiventris Say (Heteroptera: Pentatomidae) were field-collected from birch trees in the Ottawa region during August 2007. They were maintained in indoor enclosures and fed caterpillars of D. arcuata.
Experimental set up
Leaf vibrations and insect behaviors were recorded while a resident caterpillar was exposed to an approaching conspecific or predator, an abiotic factor (simulated wind or rain), or a combination of a conspecific and abiotic factor. The set-up for all trials consisted of a resident caterpillar that was placed on a birch leaf attached to a 10 to 15-cm long twig placed in a water-filled plastic vial (Fig. 1). Leaves were pre-selected to be within 5–7 cm wide and 6–9 cm in length. The resident was left undisturbed for at least 6 h on the isolated leaf to allow it to build its shelter. At least 1 h prior to an experiment, reflective tape used for laser recordings, or in some cases an accelerometer, was affixed to the leaf (see below for details on vibration recordings). All trials were videotaped simultaneously using two cameras: one for close-ups of the leaf shelter (Handycam HDV 1081i/MiniDV, Sony), and one for full-trial visualization (Handycam DCR-TRV19/MiniDV, Sony). Video clips were imported to a Power Macintosh (G4) as Imovie files, saved as Quicktime Pro files, and analyzed using Image J (NIH, Bethesda, MD, USA). All trials were carried out inside an acoustic chamber (C-14A MR, Eckel, Morrisburg, ON, Canada).
In trials where a resident was exposed to an intruder, the recordings were initiated 1–2 min prior to introducing the intruder to establish baseline levels. In trials with conspecifics, the intruder was isolated on a birch twig without leaves for 15–20 min before the trial. In predator trials, the intruder was individually held in a plastic vial and food deprived for at least 12 h prior to the trial. Following the baseline recording, the intruder was introduced with a paintbrush to the twig, which it crawled up and onto the leaf. Interactions were recorded for at least 5 min after the onset of the resident caterpillar signaling or until one of the contestants left the leaf, or the predator attacked the resident.
The resident’s response to two simulated environmental factors, wind and rain, was recorded under the same conditions described above except an accelerometer was attached to the underside of the leaf to record the vibrations instead of using the laser. An accelerometer was used during these trials, since wind and rain often caused large movements of the leaf that displaced the laser beam from its target. Leaf vibrations during wind and rain exposure were also recorded with a laser for measuring vibration characteristics (see next section). Artificial ‘gusts’ of wind were generated using a domestic fan (Sears 564-42-28002, Sears Canada Inc.) at a velocity of 1.0 m/s (measured with a Hotwire Anemometer, VWR 21800-024). Wind gusts lasted approximately 4 s and were presented at a rate of about 8 per minute. A moderate rain was simulated using a manual atomizer, delivering multiple and simultaneous water droplets to the leaf surface at a rate of about 1 spritz per second and a volume of ~0.15 ml per spritz. Trials combining the simultaneous exposure of a conspecific intruder and either wind or rain simulation were also carried out following the same methods described above, but the conspecific intruder was added 1 min following the onset of the abiotic stimulus.