Communication with Signals Produced by Abdominal Vibration

BEHAVIOR
Communication with Signals Produced by Abdominal Vibration,
Tremulation, and Percussion in Podisus maculiventris
(Heteroptera: Pentatomidae)
ALENKA ŽUNIČ,1,2 ANDREJ COKL,1 META VIRANT DOBERLET,1
AND
JOCELYN G. MILLAR3
Ann. Entomol. Soc. Am. 101(6): 1169Ð1178 (2008)
KEY WORDS Podisus maculiventris, substrate-borne communication, song repertoire, mating behavior
Communication with substrate-borne signals is widespread in insects (Čokl and Virant-Doberlet 2003, Virant-Doberlet and Čokl 2004, Cocroft and Rodriguez
2005), and over the past few years, our understanding
of the basic phenomena underlying substrate-borne
communication by insects on plant substrates has increased substantially. The consequences of using different types of plant substrates for vibrational communication have been studied previously (Cocroft et
al. 2006), and it has been demonstrated that nondispersive waves are used by insects communicating with
higher frequency signals through larger stems (Casas
et al. 2007). Furthermore, the spectral characteristics
of vibrational signals produced by vibration of the
abdomen by one group of insects, the stink bugs, have
been conÞrmed to be tuned to the resonant properties
of eight species of herbaceous plant substrates on
which they are typically found (Čokl et al. 2005).
Herbaceous plants act as low pass Þlters (Michelsen et
1 Department of Entomology, National Institute of Biology, Večna
pot 111, SI-1000 Ljubljana, Slovenia.
2 Corresponding author, e-mail: [email protected].
3 Department of Entomology, University of California, Riverside,
CA 92521.
al. 1982, Barth 2002, Cocroft et al. 2006, McNett and
Cocroft 2008), and during transmission, the amplitude
and spectral characteristics of higher frequency
stridulatory signals are changed signiÞcantly during
passage through the plant substrate (Čokl et al. 2006).
Thus, frequency characteristics of the signal play only
a minor role in determining species speciÞcity. However, temporal characteristics have been shown to be
more species speciÞc (Hrabar et al. 2004, Miklas et al.
2001).
Our increasing knowledge of insect-plant interactions in the context of substrate-borne communication
follows investigations of the signals and their production mechanisms in different insect species. In true
bugs in the infraorder Pentatomorpha, substrateborne signals are produced by stridulation and/or vibration of the abdomen (Gogala 2006). For these bugs,
the interaction between the insect and the plant substrate during communication has been studied only for
signals of the fundamental frequency, which is typically in the range of 90 Ð150 Hz. Insect-produced signals of frequencies below 50 Hz generally have been
ignored, and plant vibrations in this frequency range
typically have been described only in the context of
0013-8746/08/1169Ð1178$04.00/0 䉷 2008 Entomological Society of America
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ABSTRACT Mating behaviors of the predaceous spined soldier bug, Podisus maculiventris (Say)
(Pentatomidae: Asopinae), were observed, and vibrational signals used in intraspeciÞc communication
were recorded and analyzed. Only males produced vibrational signals, using three different vibrational
modes: vibration of the abdomen, percussion with the front legs, or tremulation of the body. When
recorded on a nonresonant substrate (a loudspeaker membrane), the mean dominant frequency of
signals produced by abdominal vibration varied between 90 and 140 Hz, and of tremulatory signals
between 8 and 22 Hz. Percussion signals were broad-band, with the dominant frequency ⬇97 Hz and
lower amplitude spectral peaks between 1,500 and 3,000 Hz. In the courtship phase of mating behavior,
males emitted pulse trains composed of an abdominal vibration after a tremulatory pulse. Females
stimulated by signals of abdominal vibration and composite signals expressed searching behavior and
orientation toward the source of vibration, whereas no speciÞc reaction to tremulatory signals was
observed. Observations of mating behavior and playback experiments showed that the abdominal
vibration signals had both long range calling and short range recognition functions. Tremulatory signals
were emitted at close range and were essential for copulation to occur. The sequences of fast and
repeated percussion signals were emitted both as individual signals during calling and courting or
between vibratory and tremulatory pulses. However, unlike the other vibrational signals, percussion
did not seem to be emitted in a particular behavioral context, and its possible function in eliciting
responses from females is unclear.
ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA
Male
* Female
20 - 25 cm
Laser
vibrometer
* Playback device
Materials and Methods
Insects and Plants. A laboratory colony of spined
soldier bugs was started from eggs purchased from
Rincon Vitova Insectaries, Ventura, CA. The bugs,
separated by sex and kept individually in plastic containers (15 cm in diameter and 6 cm in height) were
reared on green beans, Phaseolus vulgaris L.; raw sunßower, Helianthus anuus L., seeds; and larvae of either
almond moth, Caudra cautella (Walker); beet armyworm, Spodoptera exigua (Hübner); or navel orangeworm, Amyelois transitella (Walker). The containers
were held in an environmental chamber at 26 ⫾ 1⬚C,
55% RH, and photophase of 16:8 (L:D) h. Experiments
were conducted with adult males and females at least
4 d after their Þnal molt. Each experimental animal was
used only once per day.
Behavioral and playback experiments were conducted on cuttings of plumbago (leadwort) Plumbago
auriculata Lam. (Plumbaginaceae) on which P. maculiventris may be found in the Þeld (J. S. McElfresh,
personal communication). Cuttings of Y-shaped
branching were cut fresh daily and supported in a
13-cm-high glass cylinder (7 cm in diameter) Þlled
with vermiculite, with 20 Ð25 cm of the stem (2Ð 4 mm
in diameter) protruding above the vermiculite surface
(Fig. 1).
Recording of Vibrational Signals and Behavior. All
experiments were made in an enclosed room between
0900 and 1600 hours (3Ð10 h after the start of photophase) under daylight conditions, and relatively constant room temperature (23 ⫾ 1⬚C) and humidity. We
observed no correlation between frequency of recorded vibrational signals and temperature.
Three types of experiments were conducted: 1)
recording of P. maculiventris vibrational signals on the
loudspeaker membrane; 2) behavioral experiments on
the plumbago cuttings; and 3) playback experiments
on the plumbago cuttings.
Fig. 1. Schematic drawing of experimental setup. When
observing mating behavior male and female were placed on
opposite branches of plumbago cutting; in the playback experiments (*) a female was placed on one branch and stimulus was applied to the stem of the plant. A laser vibrometer
was used for recording the emitted signals in both types of
experiment.
Loudspeaker Recordings. To obtain recordings of
emitted vibrations that were minimally inßuenced by
substrate properties, a male and a female bug were
placed on the membrane of a 10-cm-diameter lowmidrange loudspeaker (40 Ð 6,000-Hz frequency response, 8-⍀ impedance; RadioShack, Taipei, Taiwan),
and the resulting signals were ampliÞed by a microphone ampliÞer (Sonifex Redbox ampliÞer, type RBMA, Sonifex Ltd., Irthlingborough, Northamptonshire, United Kingdom), digitized, and stored via a
sound card (24-bit, 96-kHz, 100-dB signal-to-noise ratio; Sound Blaster Extigy, Creative Laboratories Inc.,
Milpitas, CA) on a laptop computer using Cool Edit
Pro version 2.0 software (Adobe Systems Inc., San
Jose, CA).
Behavioral Experiments. Behavior was observed
with 49 pairs of bugs, each placed on a freshly cut
plumbago shoot (Fig. 1). A female and a male were
placed on the leaves of opposite branches at a distance
of 8 Ð12 cm apart, and behavior was observed for 20
min, or until copulation occurred. While observing
males and females on the plant, vibrational signals
were recorded from the plant with a portable digital
laser vibrometer (PDV-100, Polytec GmbH, Waldbronn, Germany), directly digitized, and stored on the
laptop computer. The laser beam was directed perpendicular to the stem or leaf long axis, parallel to the
direction of propagation of the vibrational signals
through the plant. To obtain better reßection of the
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environmental noise caused, for example, by rain or
wind (Casas et al. 1998).
As a member of the predatory bug subfamily Asopinae, Podisus maculiventris (Say) has received extensive attention as a potential biological control agent
and numerous studies have examined predatorÐprey
interactions (De Clercq 2000). However, little is
known about use of vibrational signals in this group,
apart from one study in which P. maculiventris was
shown to use substrate-borne vibrations produced by
chewing caterpillars (a common prey species) as a cue
for prey location (Pfannenstiel et al. 1995). The occurrence of intraspeciÞc vibrational signaling in
Asopinae has to date received virtually no attention in
the literature (Gogala 2006).
Here, we describe the results of a thorough investigation of the vibrational song repertoire of P. maculiventris, focusing particularly on low-frequency substrate vibrations produced by mechanisms other than
stridulation and abdomen vibration. To our knowledge, this is the Þrst detailed study of substrate-borne
vibratory signaling for any predacious bug species.
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Table 1. Frequency and temporal characteristics of songs of male P. maculiventris signals produced by abdominal vibrations
tremulatory signals and composite and male N. viridula calling song and courtship song used in the playback experiments
Song type
Na
Dominant
frequency (Hz)
Tremualtory signal
Vibratory signal
Composite signal
N. viridula male calling song
N. viridula male courtship song
10
25
11
30
12
9⫾2
126 ⫾ 12
142 ⫾ 3
109 ⫾ 4
109 ⫾ 4
Pulse train
duration (ms)
Pulse train
repetition time
(ms)
254 ⫾ 20
449 ⫾ 15
3901 ⫾ 474
15654 ⫾ 3663
Pulse durationb (ms)
Pulse repetition
time (ms)
181 ⫾ 49
224 ⫾ 18
77 ⫾ 10 (T) 177 ⫾ 19 (V)
88 ⫾ 8
299 ⫾ 37
407 ⫾ 37
546 ⫾ 223
Means ⫾ SD are shown.
Number of pulses or pulse trains within a sequence repeated within 15 min.
Durations of the composite pulse trains are shown separately for the tremulatory (T) and vibratory (V) pulses.
a
b
walking had amplitude peaks at 51, 117, and 179 Hz,
with the amplitudes being 19, 28, and 43 dB, respectively, below that of the dominant peak, which was at
13 Hz. Each bug was tested only once per day with
only one type of stimulation program, and a freshly cut
plumbago shoot was used for each bug tested. Each
bug was tested with all types of stimulation programs,
and only once with each type; the order in which the
bugs were tested and the order of the stimulation
programs used in a particular experiment were randomized. We recorded vibrational responses, and analyzed movements of stimulated bugs by measuring
the number of individuals reaching the source of vibration (playback device), the number of individuals
staying at the source for ⬎2 min, and the time bugs
needed to reach the source of vibration when stimulated with signals from abdominal vibration and composite signals.
In control experiments, bugs were tested under the
same conditions and experimental setup but without
any stimuli transduced to the plumbago stem (Fig. 1).
Signal Analysis and Statistics. Pulses were deÞned as
unitary homogenous parcels of vibrations of Þnite
duration (Broughton 1963). Pulses arranged into repeatable and temporally distinct groups were termed
a pulse train. Loudspeaker recordings of vibrational
signals were used for analyzing the frequency, velocity, and temporal characteristics (duration; repetition
time deÞned as time interval between the start of two
sequential signals) (n ⫽ number of analyzed signals;
N ⫽ number of bugs from which the signals were
obtained). Analyses were done using Sound Forge,
version 6.0 (Sonic Foundry Inc., Madison, WI) or
Raven, version 1.2 (Charif et al. 2004) (Fig. 2). Velocity is a vector quantity that speciÞes time rate of
change of displacement. The velocity of vibrations was
measured with the laser vibrometer (at 5 mm/s/V
sensitivity) and determined at the maximum amplitude of the pulse (millimeters per second). Data are
reported as minimal and maximal individual means. In
analyses of playback experiments, we compared the
behavioral responses of controls to the behaviors recorded under each of the different stimulation conditions with two-tailed FischerÕs exact tests. The difference between the time needed to reach the source
of vibration with vibratory and composite signal stimulation was compared with the nonparametric MannÐ
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laser beam, a small piece of reßective tape (⬃1.43 mg,
⬃1 mm2) was attached to the plant surface, or ⬃1 mm2
area was painted with white typewriter correction
ßuid. To investigate possible maleÐmale interactions,
experiments were carried out as described above, except that two males were used instead of a male and
female (20 pairs tested). Recordings from plants were
made to determine whether speciÞc signal was associated with a speciÞc sequence of mating behavior.
Moreover, with recordings on plants, the possible effects of the plant substrate on the spectral properties
of emitted signals could be observed.
Playback Experiments. In playback experiments,
bugs were stimulated by signals of males prerecorded
from the loudspeaker membrane, or by vibrations produced by a female walking on a plant and recorded
with the laser vibrometer (Fig. 1). Vibrational stimuli
were transduced to the plumbago stem (3 cm above
the vermiculite surface) via a playback device that
consisted of nonresonant loudspeaker membrane (10cm-diameter low-midrange, 40 Ð 6,000-Hz frequency
response, 8-⍀ impedance; RadioShack), with a 3.5-cm
plastic rod (8 mm in diameter) connecting the membrane and the plant (Fig. 1). The velocity of stimulation was adjusted to the level of the speciÞc signal
prerecorded from a test plant with a singing male
(0.5Ð1.6 mm/s, at 3-cm distance from the vibrating
male). Noise produced by a female walking on the
plant also was reproduced at the recorded velocity
level (0.1Ð 0.01 mm/s). Six stimulation programs were
synthesized. Each program included only one type of
vibrational signal: 1) P. maculiventris male signal produced by abdominal vibration (vibratory signal) (30
females and 10 males tested); 2) tremulatory signals of
males (30 females and 10 males tested); 3) composite
pulse trains (composed of a tremualtory pulse followed by a pulse train of abdominal vibrations) (30
females tested); 4) the calling song (24 females tested)
and 5) courtship song (20 females tested) of heterospeciÞc male Nezara viridula L. (pentatomid and
potential prey species; De Clercq et al. 2002); and 6)
vibrations produced by a conspeciÞc female walking
on a plant (30 females tested). Each stimulation consisted of a short sequence of one speciÞc natural signal
replayed in a loop for 15 min. Frequency and temporal
characteristics of stimulatory signals are shown in Table 1. The spectrum of vibrations produced during
1172
ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA
MALE
FEMALE
FEMALE
or
Walk
Search
Song
(vibratory,percussion)
Approach Approach
Song (tremulatory)
Song (composite)
Mount
Copulatory
attempt
Walk
Stationary
Walk
Approach
Song
(tremulatory,antenn.)
Song (composite)
Stationary
Mount
Copulatory
attempt
Copulate
Copulate
Whitney U test. A P value ⬍0.05 was considered signiÞcant.
Results
Mating Behaviors. Mating behaviors were observed
with pairs of males and females placed on opposite
branches of a plumbago shoot (Fig. 1). Of 98 tested
bugs (49 pairs), 94 individuals walked around on the
plant, with only two females and two males remaining
motionless. The main steps of successful courtship
behavior are described in Fig. 2. In 57% of the pairs
tested (28/49), the female approached the male. Antennation was observed in 16 pairs. In nine cases the
male antennated the female, and the antennated female walked away in three cases, whereas in six cases
the male antennated the stationary female and then
mounted and copulated with her. In seven cases, the
female antennated the male. Five of seven antennated
males moved away and no copulation was observed,
whereas in one case the female again approached the
male and they copulated. In subsequent experiments
with pairs of males, a similar reaction (antennated
male walking away) was observed after male-male
antennation (see below). In several cases, the approaching female extended and touched the stationary male with her proboscis. In no case did we record
any vibrational signals from females, whereas 80% of
tested males (39/49) did emitted vibrational signals,
produced by abdominal vibrations, body tremulation,
or percussion with legs. Of 39 singing males, 29 (74%)
successfully copulated, whereas copulation did not
occur in the absence of vibrational signals. Before
copulation, males always emitted tremulatory or combined tremulatory and vibratory signals; therefore, we
assumed that signaling is essential for copulation to
occur (Fig. 2).
The behaviors of 20 pairs of males were observed
under the same experimental conditions. In 11 cases,
males remained motionless on the same branch or
walked but did not meet, whereas in nine cases they
encountered each other on the plant. When standing
next to each other, Þve males emitted tremulatory
signals, and two males emitted abdominal vibrations
and/or percussion signals. Signals were always produced only by one male, and we recorded no regular
alternation of signaling, as is typical for the rivalry
songs produced by other pentatomid bugs and by
Auchenoryncha (Kon et al. 1988, Claridge and Morgan
1993, Čokl and Virant 2003, Čokl et al. 2004).
Vibrational Signals Produced during Mating Behaviors. P. maculiventris males produced vibrational signals by dorso-ventral vibration of the abdomen, by
striking one or more legs on the substrate (percussion
signal), or by tremulation, characterized by right and
left movements of the body with the legs kept Þrmly
on the substrate. These different signals were emitted
in different behavioral contexts.
Abdominal vibration signals (Fig. 3) were recorded
as the Þrst song emitted by males in 12 cases (of 39
singing males). In nine cases, it was produced spontaneously when a male was standing alone on the
plant, and in three cases it was produced after a pair
had met, but the female had moved away from the
male. Thus, abdominal vibration seems to have a calling function, triggering female movement toward the
singing male, and triggering searching behavior which
includes stopping at stem junctions, waiting for the
next male signal, and testing possible paths with legs
and antennae. Analogous searching behavior has been
described for males of phytophagous stink bug N.
viridula (Ota and Čokl 1991). Pulses of abdominal
vibrations recorded from the loudspeaker had a mean
duration of 190 ⫾ 30 ms (n ⫽ 1890, N ⫽ 13) and a
repetition time of 449 ⫾ 78 ms (n ⫽ 60, N ⫽ 6), with
the dominant frequency varying among individuals
from 90 ⫾ 13 (n ⫽ 10) and 140 ⫾ 2 (n ⫽ 10) Hz.
Compared with the spectra of abdominal vibration
recorded from the loudspeaker membrane, signals recorded from plants did not exhibit a second higher
frequency component peak that was seen on the loudspeaker recordings (Fig. 3).
When a female approached a male emitting abdominal vibrations, he started to tremulate (Fig. 4). In 57%
(22/39) of cases, the male produced tremulatory signals without prior emission of abdominal vibration
signals, and in 82% (18/22) of these, this reaction was
triggered by the close vicinity (1Ð2 cm) of a female.
The tremulatory song (recorded on the loudspeaker)
was composed of sequences of single pulses of several
seconds duration (Fig. 4), with the mean duration
varying among individuals from 96 ⫾ 15 (n ⫽ 10) to
138 ⫾ 28 (n ⫽ 10) ms. The repetition time varied
between 286 ⫾ 46 (n ⫽ 10) and 347 ⫾ 77 (n ⫽ 10) ms,
and the dominant frequency varied between 8 ⫾ 2
(n ⫽ 10) and 22 ⫾ 12 (n ⫽ 10) Hz.
Before copulation, in the majority of cases (22/39),
males produced regularly repeated pulse trains (Fig.
5) composed of a tremulatory pulse followed by a
pulse of abdominal vibration. Composite pulse trains
recorded on the loudspeaker were 260 ⫾ 32 ms in
duration (n ⫽ 30, N ⫽ 3), with 473 ⫾ 57 repetition time
(n ⫽ 30, N ⫽ 3), and a dominant frequency (deter-
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Fig. 2. Main steps of successful courtship behavior observed in male and female P. maculiventris on a plumbago
cutting.
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ŽUNIČ ET AL.: SUBSTRATE-BORNE COMMUNICATION IN P. maculiventris
1173
mined for the vibratory unit) varying between 107 ⫾
1 (n ⫽ 10) and 152 ⫾ 9 (n ⫽ 10) Hz.
In eight cases, males emitted percussion signals
(Fig. 6) in connection with abdominal vibrations,
either in the pause between vibratory pulses or as a
longer sequence after the abdominal vibrations terminated. Two males also emitted percussion signals
between tremulatory signals, and in Þve cases per-
cussion was recorded as the Þrst emitted signal. The
percussion signal was produced by the insect tapping on the loudspeaker membrane with one or both
of the Þrst legs, producing pulses the duration of
which in three males ranged from 100 ⫾ 23 (n ⫽ 30)
and 134 ⫾ 41 ms (n ⫽ 30), with repetition time
varying from 464.3 to 510.4 ms. The spectra of 10s-long sequences of percussion pulses recorded on
Fig. 4. Male tremulatory signal recorded on the loudspeaker (left) and on the plumbago plant (right). (A) Oscillogram
of several signals. (B) Frequency spectrum of several pulses.
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Fig. 3. Male abdominal vibration signal recorded on the loudspeaker (left) and on a plumbago plant (right). (A)
Oscillogram of six pulses. (B) Frequency spectrum of one pulse.
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ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA
Vol. 101, no. 6
the loudspeaker showed broad-band characteristics, with a narrow dominant frequency peak around
97 Hz, and a broader peak of lower amplitude at
frequencies between 1,500 and 3,000 Hz (Fig. 6). In
contrast, percussion signals recorded from plant
substrate showed only the peak ⬇100 Hz. These
results are consistent with observations of insectproduced vibrational signals on other herbaceous
plants (Čokl et al. 2005), and they indicate that
spectral properties of signals are inßuenced by the
substrate, with plants acting as low-pass Þlters. The
mean velocity of signals recorded from the stem 3
cm from where the male was tapping on the leaf was
0.19 ⫾ 0.10 mm/s (n ⫽ 20). When recorded (from
the plumbago plant) simultaneously at the same
distance from the singing male with vibratory and
tremulatory signals, the velocity of percussion was
9.4 and 18.7 dB lower compared with vibratory
(0.56 ⫾ 0.11 mm/s; n ⫽ 20) and tremulatory (1.63 ⫾
0.46 mm/s; n ⫽ 20) signals respectively.
Fig. 6. Male percussion signal recorded on the loudspeaker (left) and on the plumbago plant (right). (A) Oscillogram
of several pulses. (B) Frequency spectrum of several pulses.
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Fig. 5. Male-produced composite signal (composed of tremulatory and vibratory signal) recorded on the loudspeaker
(left) and on the plumbago plant (right). (A) Oscillogram of nine pulses. (B) Frequency spectrum of one pulse.
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ŽUNIČ ET AL.: SUBSTRATE-BORNE COMMUNICATION IN P. maculiventris
Discussion
The predaceous spined soldier bug, P. maculiventris,
is common in North America, ranging from Mexico,
the Bahamas, and parts of the West Indies in the south,
and into Canada in the north (De Clercq 2000). It is
found in a variety of natural habitats, including woodlands and shrubs, orchards, and Þeld crops, where it
feeds on ⬎90 insect species from eight orders
(McPherson 1980, De Clercq 2000). This species uses
visual, chemical, tactile, and vibrational cues to locate
prey (Coppel and Jones 1962, Mukerji and LeRoux
1965, Tostowaryk 1971, Pfannenstiel et al. 1995), and
these sensory modalities also can be involved in intraspeciÞc communication. The secretions of the dorsal abdominal glands of males function as long-range
pheromones that attract both adults and immatures of
both sexes (for review, see SantÕAna et al. 1999). The
smaller dorsal abdominal glands of females produce
secretions that have been suggested to act as closerange aggregation pheromones (Aldrich et al. 1984). P.
maculiventris reacts to moving prey at distances up to
10 cm (Heimpel and Hough-Goldstein 1994), indicating that visual signals are probably important at close
range.
Vibrational signals had been previously noted in
two species of the pentatomid subfamily Asopinae,
Picromerus bides (L.) and P. maculiventris (Gogala
2006). The results of the current study show that the
predaceous spined soldier bug emits vibrational communication signals by three different mechanisms: vibration of the abdomen, tremulation of the whole
body, and percussion with the front legs. To our
knowledge, this is the Þrst report of vibrational signals
being produced by three independent modes for the
whole suborder Heteroptera. It had been shown previously that many heteropteran species communicate
with signals produced by two mechanisms, stridulation and abdominal vibration (Gogala 1984, Gogala
2006), with the latter signals being tuned to the resonant properties of herbaceous plants and adapted for
long range calling (Čokl et al. 2005). Long-range communication through monocotyledonous plants has
been described in spiders, exchanging vibrational signals at several meter distance (Barth 2002). However,
experiments with the burrower bug species Scaptocoris castanea (Perty) and Scaptocoris carvalhoi
(Becker) (Čokl et al. 2006) demonstrated that stridulatory signals are signiÞcantly modiÞed and attenuated
during transmission through herbaceous plants, and as
such, are effective only for close range interactions
such as courtship or rivalry.
Of the three vibrational signals produced by different mechanisms by P. maculiventris, behavioral observations and playback experiments support the calling
function of the signal produced by abdominal vibration. Considering its behavioral context, the song is
analogous to the female-produced calling songs of N.
viridula (Čokl et al. 2000), Acrosternum hilare (Say)
(Čokl et al. 2001), Thyanta pallidovirens (Stål) and
Thyanta custator accerra (McAtee) (McBrien et al.
2002), and many other pentatomid species (Čokl et al.
1978, Kon et al. 1988, Moraes et al. 2005). However,
unlike the phytophagous pentatomids, to date, P.
maculiventris is the only pentatomid species in which
males rather than females call from one place for an
extended period with a steady signal repetition rate,
and silent females walk toward them with clearly expressed searching behavior at junction points indicating that they are tracking the vibrational signal. Surprisingly, females exhibited searching responses to
both conspeciÞc and heterospeciÞc calling signals, but
in the latter case more than half of them did not remain
on the vibration source. Repetition time constituting
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Playback Experiments. Females responded differently to stimulation with different vibrational signals.
Eighty percent (24/30) and 69% (19/30) of females
stimulated with abdominal vibration signals or composite signals, respectively, expressed typical searching behavior at junction points, reached the source of
stimulation (playback device), and remained there for
⬎2 min. In control experiments, signiÞcantly fewer
females (10%, 3/30; P ⬍ 0.001, two-tailed Fisher exact
test) walked to the silent source by chance, and none
of them remained on the source for ⬎2 min. Females
stimulated with abdominal vibration signals reached
the source of vibration signiÞcantly faster (median ⫽
257 s, min ⫽ 56 s, max ⫽ 508 s) than those stimulated
by composite signals (median ⫽ 392 s, min ⫽ 55 s,
max ⫽ 730 s) (P ⬍ 0.05, two-tailed MannÐWhitney, z ⫽
⫺2.58). Interestingly, responses also were elicited
from females stimulated with N. viridula male calling
song (of characteristic similar to that of conspeciÞc
vibratory signal), with 56% (14/25) of the females
reaching the source, versus 10% (3/30) reaching the
source in the controls (P ⬍ 0.01, two-tailed Fisher
exact test). There is no signiÞcant difference between
the percentage of females that reach the source of
vibration when presented with the two conspeciÞc
versus heterospeciÞc calling song (vibratory versus
hetrospeciÞc calling song P ⫽ 0.07; composite signal
versus heterospeciÞc calling song; P ⫽ 0.59, two-tailed
Fisher exact test). Upon reaching the source of vibration all females, when presented with conspeciÞc signals, remained on the source for ⬎2 min. However,
only six females that were stimulated with the heterospeciÞc calling song remained at the source for ⬎2
min, whereas the remaining eight resumed walking
within ⬍2 min of reaching the source (P ⬍ 0.001,
two-tailed Fisher exact test). Only 20% (6/30) of females reached the source when stimulated with either
tremulatory or walking noise, and only one and two
females, respectively, remained on the source for ⬎2
min (P ⬍ 0.01, two-tailed Fisher exact test). Females
did not respond to heterospeciÞc (N. viridula) male
courtship song (of temporal characteristics very different from those of the conspeciÞc signal) and remain motionless during stimulation with this heterospeciÞc signal.
No vibrational responses were produced by males
stimulated with tremulatory and vibratory signals.
Males remained motionless during the whole stimulation period or ßew off the plant.
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Percussion resulting from contact with the substrate
has been described in many arthropod groups (Ewing
1989). For example, stoneßies tap against the substrate
with the abdomen (Stewart 1997), jumping spiders
produce percussive signals with the forelegs and abdomen (Elias et al. 2003), crabs and Orthoptera tap
with their appendages (Salmon and Horch 1972, Sismondo 1980), and termites and beetles tap with their
heads (Howse 1964, Connétable 1999, Birch and
Keenlyside 1991). The percussion described here for
P. maculiventris does not seem to be connected with
a speciÞc signal or behavioral context, because percussion pulses were produced as the Þrst emitted signal before, after, or between emission of vibratory or
tremulatory signals. When males emitted percussion
signals no speciÞc responses by females were observed: nevertheless the percussion signals may carry
information in the pulse repetition rate, because the
spectral and amplitude pattern characteristics of
broad-band signals change signiÞcantly with increasing distance along a plant substrate (Čokl et al. 2006,
2007). Thus, for example, regularly repeated percussion signals with amplitude different from vibrational
signals might aid in choosing the correct direction to
the source at plant crossings.
Antennation was observed in femaleÐmale and
maleÐmale interactions. Antennation of a male by
either a female or another male triggered a speciÞc
male reaction, expressed as retreat. Cuticular extracts of P. maculiventris males and females did not
show any obvious sex speciÞc differences (J.G.M.,
unpublished data), indicating that antennation is
probably connected with touch as a speciÞc mechanical stimulation rather than the detection of
sex-speciÞc contact cues.
The results of the current study highlight several
differences between the roles of vibrational signaling
in P. maculiventris and our previous knowledge of
vibrational communication within the Pentatomidae.
SpeciÞcally, male P. maculiventris produce vibratory
signals by three different modes: abdominal vibration,
body tremulation, and percussion with the legs. The
signals are emitted only by males, and females search
for the calling male. These major differences between
the behaviors of the predatory P. maculiventris and the
phytophagous Pentatomidae indicate the need for further comparative investigations of vibrational communication within the Asopinae, and other true bug
subfamilies.
Acknowledgments
We are grateful to Mariana Krugner and J. Steven McElfresh (Department of Entomology University of California,
Riverside) for rearing P. maculiventris. We thank anonymous
reviewers for helpful suggestions to improve the original
manuscript. This work was Þnancially supported by the Slovenian Research Agency (Programme P1-0255-0105 and
Project L1-71299-0105), and by Hatch Project CA-R*ENT5181H.
Downloaded from http://aesa.oxfordjournals.org/ by guest on October 14, 2016
the most reliable parameter of plant-transmitted signals is similar to N. viridula calling song (Čokl et al.
2000) and P. maculiventris vibratory and composite
signals. The signiÞcant distortion of both the spectral
properties and the duration of vibrational signal during transmission through the plant (Michelsen et al.
1982, Miklas et al. 2001) may explain the lower species
speciÞcity of female P. maculiventris responding with
searching behavior when stimulated by vibrational
signals from heterospeciÞcs on plant substrates. In
contrast, females were seen to discriminate between
conspeciÞc and heterospeciÞc calling signals when
they reached the source of vibration (playback device), where the signal integrity was preserved in
duration, amplitude pattern, and frequency. These
results are analogous to those from the studies of N.
viridula vibrational signaling (Miklas et al. 2001).
However, none of the tested P. maculiventris females
responded to heterospeciÞc male courtship songs with
temporal characteristics very different from the songs
of their own species, and very few females responded
and remained at the source of vibrations when stimulated by walking vibrations. These results indicate
that P. maculiventris distinguish between vibrational
signals of different origins, and respond selectively to
particular substrate-borne vibration. Details of signal
distortion and transmission through substrate and species recognition system will be discussed in a subsequent article (in preparation).
Tremulation has been described in many insect
groups but not within Heteroptera. For example, tetigoniids vertically oscillate the body without touching the ground (Busnel et al. 1955, Morris 1971, Gwynne 1977), and in katydids, tremulatory signals
produced by males encode information on body size
(De Luca and Morris 1998). Tremulation in P. maculiventris was different: when triggered by the presence
of females, males oscillated the body parallel to the
substrate. Tremulatory signals of P. maculiventris emitted alone or in combination with vibratory pulses
within composed pulse trains were associated with the
later stages of pair formation, as part of short range
courtship. Tremulation also was characteristic of
maleÐmale interactions within rivalry. Playback experiments excluded any calling function of tremulatory signals, but they were essential for copulation to
occur. Thus, we hypothesize that tremulatory signals
play an important part in close range species and sex
recognition, a multimodal process that probably also
includes antennation, visual contact, and chemical signaling. In fact, vigorous movement of the body during
tremulation may induce suprathreshold air particle
movement that could be detected by sensilla sensitive
to air movement. Furthermore, tremulatory signals
may provide information about body size, as shown for
Conocephalus nigropleurum (Bruner) (De Luca and
Morris 1998). Comparative investigations within the
Asopinae and further behavioral experiments are
needed to elucidate the level of species speciÞcity and
the full biological role of tremulation in the mating
behaviors of P. maculiventris and other asopine species.
Vol. 101, no. 6
November 2008
ŽUNIČ ET AL.: SUBSTRATE-BORNE COMMUNICATION IN P. maculiventris
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