Sound production in Scolytidae: Stridulation by female Dendroctonus beetles

J. Insect Physic& 1973, Vol. 19, pp. 689

to 705. Pergamon Press. Printed in Great Britain

SOUND PRQDUCTION IN SCOLYTIDAE: STRIDULATION BY FEMALE DENDROCTONUS BEETLES J. A. RUDINSKY’ ‘Department of Entomology; Engineering, Oregon

and R. R. MICHAEL2

and 2Department of Electronics State University, Corvallis, Oregon

(Received

14 August

and Electrical 97331

1972)

Abstract-A

new stridulatory apparatus on the last sternite and the eighth segment is described in six female Dendroctonus bark beetles, and elytral files similar to those of males are found on four of these species, which were hitherto believed to be silent. Electronically recorded sounds include an infrequent ‘spontaneous’ click from females alone and rapidly repeated clicks (D. pseudotsugae and D. rufipennis) or a multi-impulse chirp (D. brevicomis and D. ponderosue) from females in close acoustic range of other females. An intraspecific spacing function during gallery site selection is suggested.

INTRODUCTION

IT IS CURRENTLYassumed that only one sex in each species of the worldwide family of bark and timber beetles (Scolytidae) stridulates, or produces sound by friction of bodily surfaces (BARR, 1969; SCH~~NHERR,1970). Stridulation is not known in that sex which selects the host tree and begins the gallery, e.g. female Dendroctonus and male Ips. In the other sex, sounds that are audible to the unaided human ear occur during handling and other stress, and at the entrance of the gallery of the host-selecting sex. BARR (1969) suggested that the latter sounds have a presenceannouncing function, and in Dendroctonus at least they are species-specific chirps evoked by the chemostimulus of the female attractant (MICHAEL and RUDINSKY, 1972; RUDINSKY and MICHAEL, 1972). Dimorphism and behavioural duality are not at all unique (DUMORTIER, 1963a), and the ‘silent’ beetles communicate effectively with their well-known sex and aggregative pheromones, by which they attract other beetles to the selected host. However, there is no obvious reason why they should not have evolved sonic signals also. Development of a repertoire of low-intensity sound signals may be expected wherever high population density gives them survival value (BUSNEL, 1963). The galleries of bark and timber beetles are such a site, which is further enhanced by the excellent sound-transmitting properties of wood. In the early work of ALLEN et al. (1958), one sound recorded from a pair of Douglas-fir beetles, Dmdroctonus pseudotsugae Hopk., inside a log was interpreted as a female sound because it differed from the common sound attributed to the 689

690

J. A. RUDINSKYANDR. R. MICHAEL

male. Direct comparison with our recent study of stridulation by male D. pseudobecause of differences in tsugae (RUDINSKY and MICHAEL, 1972) is impossible equipment used, but this sound was probably the occasional double chirp of the male that may occur after entering the gallery (unpublished data). WILKINSON (1962) and WILKINSON et al. (1967) recorded only female Ips calligraphus (Germar), not the host-selecting male beetle. We report for the first time the production of sounds by host-selecting scolytids, the females of four Dendroctonus species. MORPHOLOGY

OF FEMALE

STRIDULATORY

APPARATUS

The male Dendroctonus stridulatory organ consists of two processes on the median posterior margin of the seventh abdominal tergite which strike or scrape a file of parallel, transverse teeth on the ventral surface of the elytra (HOPKINS, 1909), with species-specific differences in both the apparatus and the resultant sounds (MICHAEL and RUDINSKY, 1972). The female Dendroctonus does not have such processes on the seventh tergite (HOPKINS, 1909). Other possible stridulatory apparatus were not reported. Materials and methods For morphological examination, the same methods were used as with male Dendroctonus previously (MICHAEL and RUDINSKY, 1972). From 60 to 80 female specimens of each of the following species were examined: D. pseudotsugae Hopk. from Pseudotsugae menxiesii (Mirb.), France, Mary’s Peak, Oregon; the mountain pine beetle, D. ponderosae Hopk. from Pinus ponderosa, Laws, LaPine, Oregon; the red turpentine beetle, D. valens Let. from P. ponde-rosa, Sisters, Oregon; the spruce beetle, D. ruJipennis Kirby from Picea sitchensis (Bong.), Carr., Yakutat, Alaska; P. glauca (Moench.), Voss, Yukon, Alaska; P. engelmanni, Parry, Cedar Breaks, Utah; the western pine beetle, D. brevicomis Let. from P. ponderosa, Sisters, Oregon; the southern pine beetle, D. frontalis Zimmer. from Pinus taeda L., Rye, Texas. In addition to the known secondary sexual dimorphism in the male tergal processes (HOPKINS, 1909), we determined the sex also by the striae on the elytral declivity in D. pseudotsugae (JANTZ and JOHNSEY, 1964), and by the processes on the frons in D. brevicomis (TATE and BEDARD, 1967). In the four species which showed a file of teeth on the left elytron (see page 693), 10 specimens were taken at random and the file was measured as with male Dendroctonus (MICHAEL and RUDINSKY, 1972), using a binocular microscope at 90 x magnification except with D. frontalis, where 650 x was used. With the stridulatory file found on the abdomen (see page 699), the length and the number of ridges were measured in 10 individuals taken at random. The eighth abdominal segment as well as the seventh was examined for processes similar to that of the male or possible other forms of a plectrum. The scanning electron micrographs were made on a Cambridge SEM ‘Stereoscan’ Model S4. Results Specialized apparatus for stridulation were found in two places. One of them, similar but not identical to that of male Dendroctonus, was found in only four of the

SOUND PRODUCTION

IN SCOLYTIDAE

691

examined species. The other, which was not previously described as a stridulatory adaptation (DUMORTIER, 1963; BARR, 1969), was present in all six species examined. Both apparatus showed species-specific differences. Pars &dens-elytral file. The species D. pseudotsugae, D. ponderosae, D. valens, and D. rujipennis possess a file of ridges or teeth along the sutural margin and tip of the left elytron which is similar to that of males (MICHAEL and RUDINSKY, 1972) but does not continue on the right elytron (Fig. la and Fig. 2a-b). The LEFT

(b)

FIG. 1. Drawing of female Dendroctonus stridulatory apparatus (a) file F on left elytron, WL wing lock, and (b) file F on the last sternite, plectrum P the eighth segment.

species D. brevicomis and D. frontalis do not have such a file, showing instead a flat area devoid of structures (Fig. 2c-d). The file has a teardrop shape and when the elytra are closed it overlaps the winglock area of the right elytron, as with the males. The distance between teeth is much greater in the lower half than in the narrower cephalad half of the file, as in males, but the number of teeth and the file length (Table 1) are less than in the corresponding male (compare Table 1 in MICHAEL and RUDINSKY, 1972). The teeth are more rounded than those of males and sometimes branch into a narrow Y. Scanning electron micrographs of the teeth of the four species are shown in Fig. 2(e-h). Significant variation between the species was found in the file length, number of teeth, and especially in the ratio of these numbers (Table 1).

(3Z.9)

.

(3.Z.7)

(3.tz.S)

(4.Z)

(mm) 0.66 a (0.59-077) 0.56 b (0.49-0.62) 0.54 bc (0.45-0.56) o-49 c (0.43-0.56)

(mm) *

* Different letters indicate significant Range is given in parentheses.

D. frontalis

D. brevicomis

D. pseudotsugae

D. ponderosae

D. rujpennis

D. valens

Species

File length

~-MORPHOLOGICAL

Elytron length

TABLE

difference

(364-456)

(283-439)

(434-754)

(2s3_341)

Upper half

-

IN THE

at P < 0.01.

(34%)

(202-428)

(283-236)

(2%)

Lower half

(7:%6)

(ir:6)

(5%) 79 b (72-89)

173 d

107 c

141 b

94 a

(mm) 0.138 a (0.123-0.150) 0.113 b (0.106+120) 0.095 b (0~082-0~101) 0.110 b (0.094-0.120) 0.094 b (0~084-0~107) 0.060 c (0.056-0.065)

26 a (24-27) 16 c (14-17) 17.5 bc (17-18) 17bc (15-18) 20 b (19-23) 17 bc (16-18)

File length

Sternal file

SPECIES

No. of teeth on file

OF SIX Dendroctonus

Ratio of number of teeth to length of file

Pars stridens

Total

FEMALE

No. of teeth on file

Elytral file

DIFFERENCES

283 a

212b

154c

188 bc

141 d

188 bc

Ratio of number of teeth to length of file

r

Q g

5:

1

;

z

z W

C s

y + ;j

693

FIG. 2. Scanning electron micrographs of the right (a) and the left (b) elytra of female D. pseudotsugae and the left elytron of (c) D. frontalis and (d) D. brevicomis, and details of the file of teeth on the left elytra of (e) D. pseudotszcgae, (f) D. ponderosae, (a) D. valens, and (h) D. rzljipennis. Vertical lines, 5~.

694

FIG. 3. Scanning electron micrographs of (a) total view of seventh and eighth segments and last sternite of female D. br~oiconzis and details of the sternal files of (b) D. front&, 7 seventh and 8 eighth segments, F file of teeth on last sternite, and the same of (c) D. valens, (d) D. rujipennis, (e) D. ponderosae, (f) D. brevicomis, and (g) D. pseudotsugae.

695

FIG. 4. Oscillograms of typical chirps of (a) female D. brevicomis when alone in a log and (b) with other females $ in. away, 22 April 1972, 28”C, 1.25 msec/division on the abscissa and 12.5 msec/division respectively. (c) Female D. pseudotsugae when alone in a log and (d) with other females, 30 October 1971, 23”C, 1.25 msec/ division. (e and f) Female D. ponderosae when alone and with other females, 11 July 1972,28”C, 100 msec/division. (g and h) Female D. rufipennis when alone and with other females, 11 July 1972, 28”C, 100 msec/division.

microphone over female Dendv‘octonus (b) Spacing of attacks by female (c) Galleries of D. pseud ‘otsugae and Rv DINSKY under the hark, entries marked with white squares, from SCHMITZ in the crown region of a par lderosa (1068 ). (d) Spacing of attacks by /I. hvt~~~icon~is U.S. Forest pine. (e) Galleries of D. pondurosar, entrlcs marked with white circles. Service photograph. F1c:.

5(a)

Recording

equipment

with

beetle‘s in a log. Photograph by R. W. Henderson. II. pscwdotsugae, entries marked with black circles.

697

SOUNDPRODUCTIONIN SCOLYTIDAE

Pars stridens-sternalJile. All 6 Dendroctonus females that we examined possess the sternal file of ridges or teeth. Scanning electron micrographs are shown in Fig. 3(a-g). This file is located on the inside wall of the posterior margin of the last sternite, opposite to the tip of the pygidium and the anal opening (Figs. lb and 3a). The teeth run at right angles to the body axis beginning from the upper edge of the last sternite cephalad toward the membrane which connects to the under side of the pygidium, leaving free the posterior half or tip of the pygidium. As Table 1 shows, there is significant variation especially in the ratio of file length to number of teeth. In D. brevicomis and D. frontalis females the teeth are continuous, whereas in the other four species examined they are frequently interrupted and interlocked. These differences may be related to the fact that these species have no elytral file. The number of teeth in the sternal file is considerably smaller than that of the elytral file (Table l), and apparently only a few teeth are struck during a chirp except in D. brevicomis, which strikes all or almost all of the teeth in one of its characteristic chirps (see below). Plectrum. For both stridulatory files, the plectrum appears to be the posterior In a backward and downhalf of the eighth (last) abdominal segment, or pygidium. ward motion the pygidial tip strikes either the elytral file or the sternal file accordThis kind of plectrum is different from that of the ing to the angle of the motion. Dendroctonus male, in which the seventh abdominal segment has two processes which serve as the plectrum by striking the elytral file. The seventh segment in females is different from that of males, since it is convex and without a sclerotized band (Figs. lb and 3a-b, 7, 8, F). Along the female posterior margin, seen under high magnification (650 x ), are numerous small cone-like structures which are similar to the processes of the male but are not prominent and do not protrude from the margin. In both sexes prominent setae extend along the posterior margin but with the male they are absent in the area of the processes; they probably would prevent engagement of the female spines into the teeth of the file. The eighth segment is better adapted to serve as a plectrum, as the margin is strongly chitinized and does not have setae in the centre, which engages and scrapes the stridulatory file. The stridulatory motion on the elytral file is like that of males, i.e. backward and downward (RUDINSKY and MICHAEL, 1972), but on the sternal file it was observed to go upward sometimes as well as downward and more study is needed to explain this motion. ACOUSTIC

SIGNALS

OF FEMALE

DENDROCTONUS

BEETLES

We recorded the sounds made by females of four of the six species which were found to have stridulatory apparatus. Live D. valens and D. frontalis were not available. With D. pseudotsugae, which has both kinds of stridulatory apparatus, and D. brevicomis, which has only the sternal file, we also performed behavioural tests in an attempt to determine some of the functions of female Dendroctonus stridulation. Preliminary recordings of female D. pseudotsugae showed that an individual alone and undisturbed in a gallery in the bark produced brief chirps (sensu BROUGHTON, 22

J. A. RUDINSKYAND R. R. MICHAEL

698

1963) somewhat irregularly, infrequently, and spontaneously, i.e. without known stimulus. However, the stridulation increased when another beetle, either male or female, was placed nearby. We have not yet pursued the apparent response to males because of technical problems; moreover, the apparent response to other females indicated that in addition to possible premating behaviour, with males, a different function of acoustic communication may occur with other females. Our methodology was designed to explore this possibility by testing for sonic signals under conditions that eliminated other stimuli than sonic. The classical methods to demonstrate positive or negative phonotaxis with barrier cages, etc. do not eliminate the chemical attraction and repellancy that are known to occur in Dendroctonus. In the biology and life cycle of female Dendroctonus, the most likely time for acoustic communication between adult females seemed to be during the selection of each gallery site, i.e. after their flight to the host tree which is directed by host volatiles and/or aggregative pheromones, and before their establishment of separate, regularly spaced galleries under the bark. It is well noted in the literature that the females walk over the bark for some time before boring in. Chemical communication may continue, of course, and thigmotactic stimulus appears important since the galleries are almost invariably begun in crevices in the bark, as was noted early (HOPKINS, 1909), but sonic signals also seemed possible. Therefore we attempted to determine whether acoustic communication actually occurs between females and whether it may function in the selection and spacing of gallery sites. ‘Spontaneous’ chirping was previously reported in other families of Coleoptera (e.g. ALEXANDERet al., 1963; VAN TASSELL, 1965), but no function was attributed to it. Materials

and methods

Our electronic equipment and techniques in recording were as described previously (RUDINSKY and MICHAEL, 1972). A Hewlett-Packard Model 15119 A condenser microphone, a low-noise preamplifier with bandwidth set at 300 Hz to 100 kHz, and an Ampex Model Fr-1300 tape recorder operated in the FM were recorded by first establishing mode at 60 in./sec were used. Oscillograms a cueing track on the tape to initiate a single oscilloscope sweep just prior to onset of the signal. Polaroid photographs of temporally distributed chirps were made frown a Tektronix Type 565 dual beam oscilloscope at a tape speed of 74$in./sec. Live beetles used for acoustic study came from naturally infested Douglas-fir in western Oregon with D. pseudotsugae; from ponderosa pine in eastern Oregon with D. brevicomis and D. ponderosae; and from sitka spruce in the Coast Range in outside, the logs were brought to Oregon with D. rufipennis. After overwintering beetle emergence temperature in cages as needed. At least 3 to 5 healthy individuals were recorded sequentially in each test situation, i.e. alone and stimulated by other In a log 2 ft. long we placed an unfed, unmated female in an females nearby. opening drilled about i in. through the bark to the cambium and approximately the size and shape of a natural gallery entrance. She was covered by a screen and

SOUND PRODUCTION

IN SCOLYTIDAE

699

allowed to ‘settle’ for 2 to 3 min. After observing (with earphones and the oscilloscope) and recording the sound produced by this female when alone, we did the same when 3 to 5 other females of the same species were similarly spaced in holes 4 in. distant from the central female. We do not discount the possibility that stress after handling or from being confined in the screened hole may cause phonokinesis, but preliminary testing showed no difference between the rates of chirping by females newly placed in the holes and either those placed up to 20 hr earlier or those allowed to bore into the log naturally, so long as they were alone. Such confined beetles apparently exhibit normal boring behaviour and were used successfully in studies of pheromone production (e.g. SILVERSTEINet al., 1968; KINZER et al., 1971). For this reason as well as the consistency of the changes in sound production, the observed sounds appear to be a true phonoresponse. With D. pseudotsugae and D. brevicomis we determined the effective distance of the other females’ phonostimulus by successively placing four females in individual holes in circles with decreasing radii of 8, 4, 2, then 4 in., and observing where the number of chirps increased or a new kind of chirp occurred (see below), Three replicates were made with new beetles. Sound production by the surrounding females was ascertained and correlated to that of the central female by monitoring through the microphone and oscilloscope (Fig. 5a). The recorded sounds were counted and their rate/S min was used for analysis of variance. Similar tests were made with females of a different species nearby. This methodology was partly determined by the natural habit of the host-selecting beetle; i.e. a few beetles arrive and bore into the bark before many arrive together. However, in order to eliminate the possibility of chemical communication by the pheromones these females are known to release, all beetles were kept separated and inside the log. Next we tested freely moving beetles ; these tests were in darkness to avoid the possibility of phototropism. In each of 3 replicates 20 females marked by fluorescent dust and illumined by black light were released 10 in. from 3 females that were confined in holes 4 in. apart on a log with the bark planed smooth to remove the crevices. Their movement was observed for 25 min. New beetles were used in each test, and control tests were made on logs with similar but empty holes. Results

Sounds reported here are, for the most part, so distinctly different from the previously known stridulation of scolytids that a clarification of terminology is appropriate. As shown in Fig. 4(a-h), each of the tested females when alone in the log produced a very brief chirp, but when other females of the same species were nearby the number of sound emissions per unit of time increased (D. pseudotsugae and D. rujipennis) and also a different chirp occurred (D. brevicomis and D. ponderosae). When the other females were removed, the earlier form of acoustic behaviour was observed again. The familiar chirp consisting of a rapid succession of sound pressure impulses as observed with male Dendroctonus (MICHAEL and RUDINSKY, 1972; RUDINSKYand MICHAEL, 1972) is the exception with females, being present

700

J. A. RUDINSKYANDR. R. MICHAEL

in only two of the species studied and under conditions of extreme crowding. More typically the sound consisted of a single impulse, for which the term ‘click’, defined as a very short, discrete sound (BROUGHTON, 1963), will be used. Upon bringing two or more females into acoustic range of one another, we observed from the oscilloscope display a group of multiple sound pressure impulses at intervals from the order of 5 msec to in excess of 100 msec. The tested Dendroctonus females emit separate distinct chirps of a single impulse, a few irregularly spaced impulses, or in limited circumstances with two species the more familiar regular succession of impulses. These latter chirps are easily distinguished from the characteristic multi-impulse chirps of male D. ponderosae (MICHAEL and RUDINSKY, 1972) and D. brevicomis (unpublished data). The very low acoustic energy generated by single, or even a few widely spaced sound pressure impulses, is almost inaudible to the untrained human ear. Further, even with amplification the limitations of sound reproducers make positive identification of a single impulse difficult. However, with experience in observation, through earphones the click can be distinguished from the prolonged rough, tearing sound of chewing and from erratic scratching noises during boring and gallery construction. In general, the sound signals from the observed females are not amenable to the elementary but effective analysis used with male signals (MICHAEL and RUDINSKY, 1972). We have made qualitative observations suggesting that meaningful distinctions may possibly be determined from the properties of individual sound pressure impulses. While in all cases observed waveforms are essentially what would be expected to emanate from a piston-like or diaphragm-like mechanism set into motion by abrupt contact of two chitinous surfaces, differences are apparent in ringing frequency and decay rate which show promise in differentiating both behavioural function and species-specificity. Planned studies of sound signals include digital spectral analysis and detailed evaluation of oscillograms. Female D. brevicomis, which has no elytral file (Fig. 2d), stridulates on the sternal file. This was confirmed by filling the sternal teeth of 5 females with ‘Elmer’s Glue’, which when dried prevented the characteristic chirps of the type in Fig. 4(b) that were previously observed in these individuals. D. ponderosae stridulates on the elytral file; this was confirmed when excision of this file as described (RUDINSKY, 1969) prevented chirps of the type in Fig. 4(f) that were previously observed in the treated individuals. It is not known whether the clicks shown in Fig. 4(a,e) are made on the same files. With D. pseudotsugae clicks were produced on both files, as confirmed by the same kind of excision or gluing. The limited number of D. ru$pennis females precluded study of this point. It is very interesting that two stridulatory files have evolved in some species, and further study will be necessary to determine fully their uses. Behavioural tests. Experiments with D. brevicomis showed that a female alone in a log chirped 38 times in 5 min, whereas a female surrounded by other females zi in. away chirped 284.5 times in 5 min (means of seven tests with different females). The kind of chirps also changed as shown in Fig. 4(a-b). Air temperature

SOUND PRODUCTION IN SCOLYTIDAE

701

was 28°C which is within the optimal range of 22 to 29°C for D. brevicomis activity (MILLER and KEEN, 1960). Tests with D. pseudotsugae were also made at 28°C; the mean number of chirps in 5 min for a female alone was 74 and for a female surrounded by other females 4 in. away was 268.4. This response may have been temperature-dependent since optimal temperatures for D. pseudotsugae activity are lower, around 19 to 28°C (RIJDINSKYand VITE, 1956). However, tests at 22°C showed 25.25 chirps in 5 min for a lone female and 79 chirps in 5 min for a stimulated female, which is still a threefold increase. Unlike D. brevicomis, D. pseudotsugae did not change to a dense chirp with many toothstrikes. Tests to determine the distance at which females of the same species evoke phonoresponse in each other are shown in Table 2. It was surprising to us that no response occurred to females of another species. If this should be borne out, it would indicate a highly efficient filtering and perceiving mechanism; such efficiency may be necessary against the level of background noise likely in the galleries in bark and wood. Of course, the later chirp of D. brevicomis is strikingly different from that of D. pseudotsugae. It will be interesting to see whether response occurs to the more similar chirps of D. ponderosae and D. brevicomis. The observed distances at which phonoresponse was evoked (Table 2) correspond to the known typical density of attack by each species. In D. brevicomis successful attacks averaged 11*6/sq. ft. of bark surface in the extensive studies summarized by MILLER and KEEN (1960), i.e. on the average about 3-5 in. apart. In L). pseudotsugae typically less dense attack occurs, 2-3/sq. ft. of bark surface in Pacific coastal areas (SCHMITZ and RUDINSKY,1968), i.e. on the average around 6 in. apart. Though greater density is known to occur under population pressure, it results in greatly reduced brood survival (MILLER and KEEN, 1960; SCHMITZ and RUDINSKY,1968). In the tests with freely moving beetles, 81 per cent of the D. brevicomis females (mean of 3 tests) did not approach the 3 confined chirping females closer than 2 in. ; at this distance they turned away. The other 19 per cent passed closer but did not remain there. With D. pseudotsugae the crucial distance was about 5 in., at which 85 per cent turned away and the rest did not remain nearer to the confined females. In the control tests on logs with empty holes the beetles moved apparently at random over the entire log. A click appears to be an advantageous signal for the function we attribute to it, i.e. simply to reveal the presence of the transmitter to other females near enough to perceive the signal. Though brief it is sufficient. Sound emissions, including modifications of preceding emissions, which are caused by the proximity of another member of the species of the same sex were classified as ‘disturbance songs’ by DUMORTIER(1963b), and their effect is said to be usually the departure of the disturber while the disturbed insect remains, especially in species with territorial behaviour. This appears to fit the female Dendroctonus stridulation. Though visual, tactile, and other stimuli can presumably evoke the disturbance sound in many situations, once the female has gone into the bark only the acoustic signal would be operative. Therefore though the clicks of a lone female are ‘spontaneous’

702

J. A. RUDINSKYANDR. R. MICHAEL

in not having a stimulus, they are not meaningless in ‘warning away’ the disturber.

and could have survival value

TABLE 2-STRIDULATION BY FEMALEDendroctonus BEETLESALONEANDAT VARYING DISTANCES FROM OTHERFEMALES Response *

-

Treatment

D. brevicomis Female alone in log Same female with 2 females 8 in. away Same female with 2 females 4 in. away Same female with 2 females 2 in. away Same female with 2 females $ in. away Female alone in log Same female with 4 females of D. pseudotsugae 2 in. away Same female with 4 females of D. pseudotsugae 3 in. away D. pseudotsugae Female alone in log Same female with females 8 in. away Same female with females 4 in. away Same female with females 2 in. away Same female with females 3 in. away Female alone in log Same female with 4 females of D. brevicomis 2 in. away Same female with 4 females of D. brevicomis 3 in. away

Test No. 3

Test No. 1

Test No. 2

32 32 33 701 254 41 42

38 36 35.l. 125 254 39 43

42 a 46 a 31 a 114t b 316 c 42 a 43 a

40

45

46 a

90 128 180 188 260 84 78

64 94 128 196 268 80 79

76

74

81 102 135 198 272 78 75

a a b c d a a

76 a

Air temperature 28°C. Response is given in number of sounds per 5 min in order to minimize effect of occasional pauses between series of chirps. * Different letters indicate that the mean difference in response at different distances is significant when P
Although these tests exclude many of the factors which influence the spacing of bark beetle attacks in nature and will have to be corroborated in field tests, they show that female acoustic signals can function with a spacing effect. The natural habit of scolytids to utilize efficiently the available food material and brood habitat by their characteristic gallery systems depends first on a minimum

SOUNDPRODUCTIONIN SCOLYTIDAE

703

distance between entries into the bark (Fig. 5b-e). It is striking that with the most destructive Dendroctonus beetles (including those studied here except possibly D. valens), this site selection occurs during the ‘mass attack’, in which several thousand females can initiate galleries on a single large tree in a few hours (Fig. Sd), and which appears as almost a simultaneous attack when observed (e.g. MILLER and KEEN, 1960). Among the systems of animal communication, sound emission seems a pre-eminently suitable signal in these circumstances for its instantaneousness, repeatability, and lack of continuance in the air; it allows transmission of the relevant information in the shortest time with also the possibility of exchange of signals (BUSNEL, 1963). In fact, the only rapid and exact communication likely from insects outside the bark or wood to those inside it, is sonic: chemical signals are probably effective only outward, not inward through the frass-filled gallery. Among insects walking over the bark, chemical signals would vary with air movements over the uneven bark, whereas sonic signals are effective equidistantly. The importance of bark crevices in attack patterns of D. ponderosae was recently demonstrated (SAFRANYIK and VITHAYSAI, 1971), but their presence or absence cannot be the critical limiting factor since many unused crevices can be observed. If phonostimulus and response by attacking females govern or limit to some degree the density of mass attack on a tree, they thereby help to regulate the location and quantity of pheromones released from the galleries. RENWICK and VITE (1970) have proposed that only chemical signals accomplish this regulation, or that olfactory responses to the pheromones of D. brevicomis and D. frontalis ‘control every phase of their population aggregation’ including prevention of overcrowding ‘on a portion of a tree’. With D. ponderosae they proposed that the decrease in the attractive synergistic host volatiles which occurs after some female attack prevents further attack. This would be an oversimplification, according to the theory presented here of the function of female Dendroctonzls stridulation. Moreover on the basis of work done with bees, birds, and porpoises, BUSNEL (1968) has suggested that the present somewhat mechanical idea of separate communication systems among animals may have to be revised. It was earlier shown in D. pseudotsugae and D. ponderosae that the female’s chemostimulus evokes species-specific male stridulation (MICHAEL and RUDINSKY, 1972), and in D. pseudotsugae that the male acoustic signal triggers the release of the female’s antiaggregative pheromone by which she negates her attractant for The other beetles ( RUDINSKY, 1968, 1969; RUDINSKY and MICHAEL, 1972). interaction of acoustic and chemical signals may be typical of Scolytidae. Sonic signals in a female spacing mechanism could be effective (i) before chemical communication occurs since pheromones and bark volatiles are typically released or synergized after attack commences; (ii) near established galleries where the residual effects of released attractants and inhibitors, the close range, the likelihood of olfactory adaptation, the uncertain air movements, etc. may make chemical signals highly variable but do not affect sound signals; and (iii) where chemical signals cannot travel rapidly if at all but acoustic signals are advantaged, i.e. through wood.

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J. A. RUDINSKYANDR. R. MICHAEL

Acknoruledgenzents-We thank R. B. PETERSON,Oregon State University Computer Center, for the statistical analyses. The scanning electron micrographs were made by Electros Analytics, Tigard, Oregon. This research was partially supported by the Oregon State University Research Council and by the National Science Foundation and this is O.S.U. Agricultural Experiment Station Technical Paper No. 3372.

REFERENCES ALEXANDER R. D., MOORET. E., and WOODRUFFR. E. (1963) The evolutionary differentiation of stridulatory signals in beetles (Insecta: Coleoptera). Anim. Behaw. 11, 11 l-l 15. ALLEN D. G., MICHAELR. R., and STONES. A. (1958) Sounds of Douglas fir beetle activity. Oreg. For. Lands Res. Center. Res. Note No. 36. BARRB. A. (1969) Sound production in Scolytidae (Coleoptera) with emphasis on the genus Ips. Can. Em. 101, 636-672. BROUGHTON W. B. (1963) Method in bio-acoustic terminology. In Acoustic Behaoior of Animals (Ed. by BUSNELR. G.), pp. 3-24. Elsevier, New York. BUSNELR. G. (1963) On certain aspects of animal acoustic signals. In Acoustic Behaoior of Animals (Ed. by BUSNELR. G.), pp. 69-111. Elsevier, New York. BUSNELR. G. (1968) Acoustic communication. In Animal Communication (Ed. by SEBEOK T. A.), pp. 127-153. Indiana University Press, Bloomington. DUMORTIERB. (1963a) Morphology of sound emission apparatus in Arthropoda. In Acoustic Behawior of Animals (Ed. by BUSNELR. G.), pp. 277-345. Elsevier, New York. DUMORTIERB. (196313) Ethological and physiological study of sound emissions in Arthropoda. In Acoustic Behavior of Animals (Ed. by BUSNEL R. G.), pp. 583-654. Elsevier, New York. HOPKINSA. D. (1909) Contributions toward a monograph of the scolytid beetles-I. The genus Dendroctonus. Tech. Ser. U.S. Dept. Agric. Bur. Ent. 17, Part I. 164 pp. JANTZ 0. K. and JOHNSEYR. L. (1964) Determination of sex of the Douglas-fir beetle, Dendroctonus pseudotsugue Hopkins (Coleoptera: Scolytidae). Can. Ent. 96, 1327-1329. KINZERG. W., FENTIMANA. W., JR., FOLTZ R. D., and RUDINSKYJ. A. (1971) Bark beetle attractants: 3-methyl-2-cyclohexen-l-one isolated from Dendroctonus pseudotsugue. J. econ. Ent. 64,970-971. MICHAELR. R. and RUDINSKYJ. A. (1972) Sound production in Scolytidae: specificity in male Dendroctonus beetles. J. Insect PhysioZ. 18, 2189-2201. MILLER J. M. and KEEN F. P. (1960) Biology and control of the western pine beetle, Misc. Pub. U.S. Dept. Agric. 800, l-381. RENWICKJ. A. A. and VITE J. P. (1970) Systems of chemical communication in Dendroctonus. Contr. Boyce Thompson Inst. PI. Res. 24, 283-292. RUDINSKYJ. A. (1968) Pheromone-mask by the female Dendroctonuspseudotsugue Hopk., an attractant regulator. Pan-Pucif. Ent. 44, 248-250. RUDINSKYJ. A. (1969) Masking of the aggregation pheromone in Dendroctonus pseudotsugae Hopk. Science, Wash. 166, 884-885. RUDINSKYJ. A. and MICHAELR. R. (1972) S ound production in Scolytidae: chemostimulus of sonic signal by the Douglas-fir beetle. Science, Wash. 175, 1386-1390. RUDINSKYJ. A. and VITE J. P. (1956) Effects of temperature upon the activity and the behavior of the Douglas fir beetle. For. Sci. 2, 258-267. SAFRANYIKL. and VITHAYSAIC. (1971) Some characteristics of the spatial arrangement of attacks by the mountain pine beetle, Dendroctonus ponderosae (Coleoptera: Scolytidae), on lodgepole pine. Can. Ent. 103, 1607-1625. SCHMITZR. F. and RUDINSKYJ. A. (1968) Effect of competition on survival in western Oregon on the Douglas-fir beetle. Ore. State Univ., For. Res. Lab. Res. Paper 8. 42 pp. SCH~~NHERR J. (1970) Stridulation einheimischer Borkenkafer. 2. angew Ent. 65, 309-312.

SOUNDPRODUCTION IN SCOLYTIDAE

705

SILVERSTEIN R. M., BROWNLEE R. G., BELLAST. E., WOOD D. L., and BROWNEL. E. (1968) Brevicomin: principal sex attractant in the frass of the female western pine beetle. Science, Wash. 158, 889-891. TATE N. L. and BEDARDW. D. (1967) Methods of sexing live adult western pine beetles. J. econ. Ent. 60, 1688-1690. VAN TASSELLE. R. (1965) An audiospectrographic study of stridulation as an isolating mechanism in the genus Berosus (Coleoptera: Hydrophilidae). Ann. ent. Sot. Am. 58, 407-413. WILKINSONR. C. (1962) Stridulating organs in three southeastern Ips bark beetles. FZ. Ent. 45,434. WILKINSONR. C., MCCLELLANDW. T., MURILLO R. M., and OSTMARKE. 0. (1967) Stridulation and behavior in two southeastern I& bark beetles (Coleoptera: Scolytidae). Fl. Ent. 50. 185-195.