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The density of cicadas Cicada orni in
Mediterranean coastal habitats
a b c
Ian J. Patterson , Giovanna Massei & Peter Genov
a
Zoology Department , Aberdeen University , Culterty Field Station, Newburgh,
Grampian, AB41 6AA, United Kindom
b
Institute of Terrestrial Ecology , Hill of Brathens, Banchory, AB31 4BY, United
Kindom
c
BASc, Institute of Zoology , Boul. Osvoboditel 1, Sofia, 1000, Bulgaria
Published online: 28 Jan 2009.
To cite this article: Ian J. Patterson , Giovanna Massei & Peter Genov (1997) The density of cicadas Cicada orni in
Mediterranean coastal habitats, Italian Journal of Zoology, 64:2, 141-146, DOI: 10.1080/11250009709356187
To link to this article: http://dx.doi.org/10.1080/11250009709356187
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www.tandfonline.com/page/terms-and-conditionsItal. J. Zool., 64: 141-146 (1997)
The density of cicadas Cicada orni INTRODUCTION
in Mediterranean coastal habitats Cicadas are among the largest and most numerous in-
sects in the habitats in which they occur and as such they
are important both as components of the ecosystem (An-
IAN J. PATTERSON derson, 1987, 1994) and as prey (Rosenberg et al., 1982;
Zoology Department, Aberdeen University, Steward et al., 1988; Langham, 1990; Krohne et al.,
Culterty Field.Station, 1991). In coastal Mediterranean habitats Cicada orni are
Newburgh, Grampian, AB41 6AA (United Kindom) eaten both as underground larvae, taken by wild boars
Sus serof a (Massei et al., in press), badgers Mêles mêles
GIOVANNA MASSEI (Pigozzi, 1991) and foxes Vulpes vulpes (Ciampalini &
Institute of Terrestrial Ecology, Lovari, 1985) and as adults in summer, taken by bird
Hill of Brathens, Banchory AB31 4BY (United Kindom)
predators such as jays Garrulus glandarius (Patterson et
PETER GENOV al., 1991).
BASc, Institute of Zoology, The abundance of cicadas is difficult to estimate since
Boul. Osvoboditel 1, 1000 Sofia (Bulgaria) the larvae live deep in hard soil while the adults are cryp-
tic, mobile and often high in tree canopies. Methods for
estimating the density of cicadas {Cicada and Magi-
cicada spp.) include counts of emergence holes, counts
of larval skins (exuviae) and emergence traps (Dybas &
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Davis, 1962; Wolda, 1989; Dean & Milton, 1991; Milton
& Dean, 1992; White & Sedeóle, 1993; Anderson, 1994).
All these methods are time and effort consuming and in
some cases have been recognised as poor or biased
estimators of cicada densities (Lane, 1993; Anderson,
1994). In this paper we suggest an index to estimate
cicadas numbers based on sound level, and we compare
it with other measures of cicadas density. Patterson &
Cavallini (1996) found that even crude measurements of
the volume of sound produced by singing male cicadas
showed significant differences between habitats which
corresponded with variation in the density of jays (Patter-
son ei«/., 1991).
The aim of the present study was to assess the use of
more precise measurement of sound volume as an index
of the relative abundance of cicadas in different habitats,
to compare such measurements with indices of larval
density at the same sites and to estimate the density of
singing males.
ABSTRACT
The aim of the study was to assess the use of sound level
measurement as an index of cicada Cicada orni abundance in STUDY AREA AND METHODS
Mediterranean coastal habitats, to compare such measurements with
indices of larval density at the same sites and to estimate the density The study was carried out in the Maremma Natural Park (42° 39'
of singing males. Sound levels were significantly higher in pinewood N, 11° 05' E), Tuscany, Italy, in the same areas used by Patterson &
than in Mediterranean scrub and were intermediate in olive groves. Cavallini (1996) and Patterson et al., (1991). The principal habitats
There was a close correlation between sound level and the number were pinewoods, dominated by Pinus pinea and P. pinaster,
of larval skins per tree, which explained 95% of the variation in Mediterranean maquis scrub and olive groves (Fig. 1). The coastal
sound. Measurements of the sound produced by individual cicadas pinewoods were dominated by Pinus pinaster, 5-10 m high, while
(mean 97.43 dB at 1 cm) allowed an estimate, by computer those further inland were mainly of P. pinea, 10-15 m high with
simulation, of the density of singing males in July. Mean estimates of characteristic broad canopies. Species composition changed
9 861 males per hectare in pinewood and 1 618 in olive grove gradually over a wide transition zone, with no sharp division be-
were obtained. The pinewood values were considerably lower than tween the two types of pinewood. Both had an extensive shrubby
densities of final-instar larvae, measured in late May, but the understory, mainly of the species which occurred in the maquis
discrepancy can be accounted for by larval and adult mortality in the scrub. The habitats are described in detail in Pigozzi (1991).
period between the two sets of measurements. Larval densities were measured by sifting the soil from 28 0.5 x 0.5
m (0.25 m2) quadrats dug to 0.3 m depth under tree canopies, in an
area oí Pinus pinea (Point 2 in Fig. 1) and in 14 quadrats in coastal
KEY WORDS: Density - Cicada - Song - Mediterranean pinewood P. pinaster (between points 4 and 5 in Fig. 1) on 25 and
26 May 1993, before any larvae had emerged. The soil in the other
habitats was too rocky to allow sampling. The animals were divided
(Received 20 September 1996 - Accepted 26 October 1996) into their three clearly-separable year classes but only the largest142 I. J. PATTERSON, G. MASSEI, P. GENOV
1 2 3 4 5 6 7 8 g the canopy and the top of the tree) was estimated at each pinewood
A
^ ^
*>& f 7 . ttmm
y
and olive site by reference to a 2 m pole and the mean spacing be-
tween 50 trees was determined by pacing. No such measurements
were possible in the scrub. These data were used to adjust the sound
B
"m \v
Scrub
volume values to allow for the greater attenuation in habitats with a
larger mean distance from the observer to the surrounding tree
canopies (e.g. tall, well-spaced pines compared to low, densely-
C spaced olives). This was done by a computer programme, which
^Ü simulated for each study site a grid of trees, each of average canopy
height, spaced evenly at the mean spacing for that site. For each tree
D Scrub
in the simulated grid, the straight-line distance from the centre of the
4 3
—-. ¡\^Sk \ canopy to the sound meter, and the sound volume which would be
"^--^ i expected to be registered at the meter from one cicada singing in
E / \
i
that canopy, were calculated (see Results). The total sound volume
expected per single cicada in each tree was then used as an index to
F Pine wood -
M Tower
canopy density and closeness. The study site which gave the highest
expected volume on this basis (a site with a high density of low trees)
was then used as a baseline and the observed sound volumes at the
G other sites were adjusted upwards in proportion to their index
values, relative to that of the baseline site. Since the dB scale is a
H
Sea
ix : \
logarithmic one (10 times the logarithm of the original sound
pressure), all calculations were made on values converted back to
the original scale.
The same simulation was used to estimate the density of singing
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male cicadas at each site, by adding additional animals per tree until
t^m Roads
O
^ 3 1 Canal
Habitat boundaries
Meadows EH Olives
_ _ Tracks
•" " Limestone cliff
|_
Î the calculated total sound volume predicted at the sound meter
reached the value actually recorded at the site. This procedure gave
the estimated total number of cicadas per tree and the estimated total
per hectare (ha) required to produce the observed sound level at
Fig. 1 - The study area. The main habitat divisions and the study sites each site.
(numbered 1-13) are shown, with olive areas shaded.
RESULTS
class, due to emerge in the following two months, was considered in
subsequent analysis. Larval density
At the inland site, larval emergence holes were counted in 0.5 x
0.5 m (0.25 m2) quadrats distributed outward from pine trunks at 0.5 Underground larvae
m intervals to the edge of the canopy on 1 June 1993, to test an im-
pression that most larvae emerged under the pines, especially close A mean density of 3.75 ± 0.59 final-instar larvae per
to the trunk. 0.25 m2 was recorded in the 28 quadrats in the north-
The larval skins from which adults had emerged were counted and
removed from the trunks of 10-24 trees at each of five pinewood and
west part of the pinewood (Fig. 1, point 2) and 1.78 ±
three olive grove sites on 27 June, 13 July and 28 July 1993. Skins 0.48 per 0.25 m2 in 14 quadrats in the coastal pines (Fig.
were also counted and removed from the ground and from ground 1, points 4-5). These values applied only under tree
vegetation within 1 m of the tree trunks and these were included in canopies where the sampling was carried out; densities
the total for each tree. Very few larval skins were seen on the ground were estimated to be 68% lower beyond the canopies
away from trees; this, together with the likelihood that an unknown
proportion of these fragile items may have been destroyed in the (see below, Table I). Since the canopy covered only
two-week periods between counts, made it unrealistic to calculate 67.73% of the area at sites 1-2 but was effectively con-
overall densities of skins per hectare. Three areas in the maquis tinuous at sites 4-5, the estimated overall densities of un-
scrub were also searched but no larval skins were found. derground larvae were 117 000 per ha in the inland pines
Measurements of sound volume in decibels (dB) were made at and 71 200 per ha in the coastal ones.
least five times at each of the above sites, using a Eurisem EP626
sound level meter at its fast response setting on the C loading. After
a 5 min settling period, when the observer remained still against a Larval emergence holes
tree trunk to allow nearby cicadas to resume singing, 10 readings
were taken over a 20-30 sec period and the mean level calculated The density of larval emergence holes was significantly
later. The air temperature was measured during the settling period. higher under pine canopies than in open ground (Table I)
The maximum sound level over a 1 min period was also measured
but was found to be very closely correlated with the mean value and there was a slight (but non-significant) tendency for
(r = 0.970, P < 0.001, n = 83) and was not used in analyses. densities to increase closer to the tree trunk.
At each site, several measurements were made in the morning just
before the cicadas began to sing, to establish the level of background Larval skins
noise. At two pinewood sites (Points 1 and 2 in Fig. 1), measure-
ments were continued throughout the day from dawn todusk, to Of the 1836 larval skins found, 45.8% had emerged by
confirm that singing continued at a similar level throughout the day. 29 June, a further 46.4% by 13 July and only 7.8% be-
At the inland pinewood sites (Fig. 1, sites 1 and 2), where the
canopy was not closed, the proportion of ground overhung by tween then and 28 July. There was, however, con-
canopy was measured on three transects, 180-284 m long. siderable variation in the timing of peak emergence be-
The mean tree canopy height (mid-way between the lower edge of tween the study sites, with some having clear peaks atMEDITERRANEAN CICADA DENSITY 143
TABLE I - The mean density of final-instar cicada larvae. Densities TABLE II - The mean number of cicada larval skins per tree in pine
are expressed as number per 0.25 mr in relation to distance from and olive areas (Fig. 1). T-test between habitats for skins per ha; t =
pine trunks (F72?J = 6.19, P < 0.001: excluding outside canopy; 4.80, P = 0.009: no other significant differences.
F
6.m> = 104, P = 0.402: 50-350 cm under canopy vs outside
canopy; t = 6.76, P < 0.001).
Site Habitat Trees Skins per tree
Mean SE
Distance Mean SE N
(cm) (quadrats)
1 pine 23 32.4 2.8
2 pine 20 22.1 3.2
50 2.45 0.45 33 3 pine 10 9.0 1.6
100 2.06 0.31 33 4 pine 17 5.4 0.7
150 1.42 0.31 33 5 pine 24 7.8 0.9
200 1.97 0.32 30
6 olive 16 14.4 2.3
250 1.59 0.35 27 7 olive 15 1.9 0.6
300 1.56 0.40 18 8 olive 15 1.3 0.5
350 1.80 0.63 10 Pine mean 15.3 5.2
Outside canopy 0.59 0.12 100 Olive mean 5.9 4.3
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The discrepancy persisted when the mean background
either the first or second count. At individual trees, noise level (59.28 ± 1.92 dB) was taken into account,
emergence tended to be concentrated into one of the resulting in a corrected value of 70.60 dB at 0.5 m. Fur-
count periods, especially early in the season; the number thermore, there was no significant correlation between
of skins per tree was significantly correlated between the sound level at 0.5 m and either that at 5 cm (r =
counts 1 and 2 for only one site (point 6, Fig. 1; r = 0.782, P > 0.05, n = 6) or that at 1 cm (r = 0.114, P >
0.589, P < 0.02, n = 16 trees), while between counts 2 0.05, n = 8), in cicadas where pairs of measurements
and 3, the number per tree was correlated only for point were possible.
1 (r = 0.632, P < 0.01, n = 23), point 3 (r = 696, P < 0.05,
n = 10) and point 8 (r = 0.704, P < 0.01, n = 15). Variation in sound level between habitats
There were more than twice as many skins per tree in
There was significant variation in sound level, correct-
pinewoods than in olive groves (Table II) but there was
ed for differences in tree height and spacing, between
considerable and significant variation between sites in
habitats (Table III). The overall mean value in pines
each habitat (pine, F489 = 27.96, P < 0.01: olive, F443 =
(73.58 ± 3.31 dB) was significantly higher than that in
26.85, P < 0.001 and no overall significant difference bet-
scrub (56.43 ± 1.20 dB); the level in olives (67.33 ± 3.24
ween habitats.
Sound produced by individual male cicadas
TABLE III - The mean sound level (dB) of cicada song, the index of
Sixteen male cicadas buzzing while being restrained in tree height and spacing and the corrected sound level for each study
the hand produced a mean sound level of 97.29 ± 0.74 site. ANOVA between habitats on corrected values; F2S = 7.59, P =
dB at 1 cm while six (some of the same individuals) 0.014.
singing naturally produced 85.15 ± 2.15 dB at 5 cm.
These two values are consistent, assuming a decrease of 6
dB for every doubling of distance (White, 1975), together Site Habitat Mean dB Index Corrected
with some inaccuracy in the estimation of distance, since mean dB
the 1 cm value would predict 85 dB at 4 cm and the 5 cm
reading would suggest 97 dB at 1.25 cm. In addition, the
two values were significantly correlated in the four in- 1 pine 70.4 10.90 84.5
dividual cicadas in which both measurements could be 2 pine 71.3 3.48 76.7
made (r = 0.953, r2 = 90.8%, P < 0.05). The two 3 pine 63.4 4.30 69.7
measures taken together suggest that the sound level at 4 pine 65.1 0.00 65.1
5 pine 69.2 1.85 71.9
the tympanal organ exceeded 100 dB. 6 olive 64.6 7.97 73.7
Measurements made at 0.5 m from singing male cicadas 7 olive 61A 1.56 63.6
had a mean of 70.91 ± 0.83 dB, a higher level than would 8 olive 63.0 1.38 64.4
9 scrub 56.5 - (56.5)
be predicted from either the values at 1 cm (which would 10 scrub 58.5 - (58.5)
predict 63 dB) or those at 5 cm (predicting 65 dB), again 11 scrub 54.3 - (54.3)
assuming attenuation of 6 dB per doubling of distance.144 I. J. PATTERSON, G. MASSEI, P. GENOV
dB) was intermediate and not significantly different from
that in either of the other two habitats (Tukey tests). The
level of background noise (mean 41.77 ± 0.56 dB) was
too low to influence the measurements of cicada song.
There was a general decline in sound level through the
study period, significantly so at 5 of 13 sites (Table IV), in
spite of the small number of measurements at most
places. However, since sound level readings were made
over the same spread of dates at all of the sites, the
decline in volume over time did not affect comparisons
between habitats.
Relationship between sound and larval density
10 20
The sound level, corrected for tree height and spacing, Mean larval skin per tree
was significantly higher at sites with larger numbers of
larval skins per tree, particularly when sites in the scrub Fig. 2 - Sound levels (dB), corrected for variations in tree height and
spacing, in relation to the number of cicada larval skins per tree. Study
(where no skins were found) were omitted from the sites in the pinewood are shown by circles, olive groves by triangles
analysis (Fig. 2). This relationship was also present in the and scrub areas by squares. Including scrub; r = 0.934, r2 = 82%,
uncorrected data, although with a poorer fit (r = 0.796, P < 0.001: excluding scrub; r = 0.974, r2 = 95%, P < 0.001.
r2 = 63%, P < 0.01, including scrub sites).
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Estimated adult density
pine and olive areas (Fig. 2), suggested that even in mid to
The calculated density of singing male cicadas was late July the adults were still concentrated in the same
very much higher in the pinewood, especially at sites 1 areas where most larvae had emerged several weeks
and 2 (Fig. 1), than in the olive groves (Table V), although earlier. This confirms that the measurement of sound
the difference was not quite significant at the 0.05 level, level could be used as a quick, convenient and robust
presumably associated with the high variability between technique to determine the relative abundance of cicadas
sites. in different areas or in different years, especially if
corrections are made for tree height and spacing.
The principal discrepancy in the relationship was the
DISCUSSION occurrence of singing male cicadas in maquis scrub,
where no larval skins were found in spite of extensive
The relationship between sound level and larval skins searching. It is possible that such males had dispersed to
The very close relationship between the level of cicada the scrub from high-density pinewood areas, since
song and the number of larval skins per tree, which ex- cicadas are strong fliers. Similar low densities were detect-
plained over 95% of the variation in sound level in the ed in the surrounding agricultural areas.
TABLE IV - Correlations between cicada sound level and date (day TABLE V - The estimated number of singing male cicadas per hec-
of season).
tare in pine and olive habitats. T-test between habitats; t = 2.64,
P = 0.058.
Site Habitat N (days) Coefficient (r) P
Site Habitat Males per ha
1 pine 6 - 0.766 NS
2 pine 7 - 0.357 NS
3 pine 6 - 0.824MEDITERRANEAN CICADA DENSITY 145
Variation in sound level between habitats decline in sound level through the study period at many
The finding that sound levels from singing male of the sites (Table IV) is also consistent with adult mor-
cicadas were significantly higher in the pinewoods than tality. The estimates of adult cicada abundance in late
in the scrub areas, with intermediate values in the olive July may thus be a reasonable first approximation of the
groves, and the tendency for levels to decrease from numbers present.
inland towards the coast in the pinewood, were both The cicada densities recorded at the pinewood sites
consistent with Patterson & Cavallini's (1996) (13 201 and 33 660 per hectare) were higher than those
preliminary observations made in 1990. This suggests previously reported from South Africa; 6100 and 8 800
that the relative abundance of cicadas in different per hectare (Dean & Milton, 1991) and 2 000 - 9 400 per
habitats remains stable from year to year, (even though hectare (Milton & Dean, 1992) and were similar to those
absolute population density in the areas as a whole may found in New Zealand; 22 600 and 56 100 per hectare
vary). (White & Sedeóle, 1993). Densities of periodical cicadas
Magicicada in the United States of America tend,
The factors underlying the local variation in abundan- however, to be considerably higher at 50 000 to 100 000
ce may well be complex, involving both the occurrence per hectare (Anderson, 1987), for example 40 000 and
of suitable roots for larval feeding and variations in soil 118 000 per hectare (Anderson, 1994) and 66 500 per
characteristics. The higher larval densities in the inland hectare (Williams et al., 1993). This confirms that the
pines than in the coastal ones, for example, might be densities derived from measurements of sound volume
related either to the difference in pine species (Pinus may not be unreasonable estimates of the abundance of
pinea and P. pinaster, respectively), or to a difference adult cicadas.
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(e.g. in salinity) between coastal and inland soils. The
light sandy soil in the pinewood may well be more
suitable for burrowing in than the very hard and stony
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