Mating and laying sites of Uromenus brevicollis insularis, a Cyrno-Sardininian micro-endemic species, are described from observations conducted at night in several Corsican localities. Asphodelus ramosus was found to be a key host species as both mating and oviposition of this insect take place mainly on the erect dry stems of the plant. Some aspects of the meeting of the sexes are assessed: male stridulation does not appear to play an important role. The females lay their eggs, creating vertical lines in the stem by chewing regularly spaced holes containing nearly 3 eggs per hole. One to three laying lines can be observed on a single stem. Ferula communis is frequently used as an alternative laying site when A. ramosus is absent or rare, but in this case, the eggs can be attacked by woodpeckers (Dendrocopos sp.) or parasitized by Hymenoptera.
The reproductive biology of Tettigoniidae (Orthoptera) has been actively studied, with a focus on acoustic communication involved in mate pairing (see for example Heller 1988, Robinson 1990, Bailey 1991 and Gerhardt & Huber 2002, reviewed in Gwynne 2001) and on the spermatophylax, transferred by the male and consumed by the female at the end or after copulation; the spermatophylax is interpreted as a nuptial gift or a factor increasing sperm transfer (Gwynne 1995; Vahed 1997, 1998; reviewed in Gwynne 2001). Choice of site for mating and laying has been less investigated because field observations by night are needed. In these conditions, quantitative approaches are facilitated when numerous individuals can be observed in a short period during the night and in a limited area.
According to Gwynne (2001) the probable ancestral oviposition site for the family Tettigoniidae is in soil. This is certainly true for most Spanish and French Bradyporinae (Vahed, pers. comm.), e.g., in Ephippiger vitium in Pyrénées-Orientales (France) (Peyerimhoff 1908). Thus it is of particular interest to describe the oviposition habit in plant stems by a species belonging to Uromenus, a genus close to Ephippiger.
Peyerimhoff (1908) gave a detailed report of the laying of the Algerian subspecies of U. (Bolivarius) brevicollis (Fisher 1853). He described how the female attacks the bark of Asphodelus aestivus (Asphodelaceae) with her mandibles, before introducing the ovipositor 2 to 3 times in the same hole. He also noticed that the laying begins at the close of the day.
However, to our knowledge there is no information about the laying habit of the subspecies endemic to Corsica and Sardinia, Uromenus (Bolivarius) brevicollis ssp insularis Chopard, 1923. Interestingly, both laying and mating activities of this subspecies take place on erect dry stems of another species of asphodel (Asphodelus ramosus L.). This species, blooming in spring, has a single stem per plant, ramified at the top, and is generally found in prairies, fallow lands and erms (a degradation stage of scrub and garrigue in the west Mediterranean area, see Diaz Lifante 1996). There is no trophic interest in the climbing behavior, as all the parts of the plant summit are dried when the insects are in the reproductive season.
The aim of this paper is to test if the subspecies present in Corsica shows the same behavior as U. b. brevicollis and if the dry stems of other plants could serve as laying sites. In addition, as the matings were also found to occur mainly on asphodel stems, we assessed some factors that determine the meeting of the sexes. Finally, we estimated the risk of damage to the eggs, by bird predation and insect parasitism, when laid in the stems of different plant species.
Material and methods
Mating sites. — Three sites were studied: the beach of San Pellegrino (Penta-Di-Casinca, Haute-Corse, on 19 July 2006, between 9:00 and 9:30 PM), the prairies of Biaggiola (Porto-Vecchio, Corse-du-Sud, on 22 July 2006, between 11:40 and midnight) and the plateau of Maora (Bonifacio, Corse-du-Sud, on 24 July 2006, between 9:50 and 10:25 PM). A particularity of the San Pellegrino station is that asphodels were spaced over tens of meters, in contrast to the other stations, where the mean distance between stems was rather regular and ranged from 0.70 to 2 m. More than 20 asphodel stems were chosen at random on which to count the males and females, climbing up the stems or waiting for mating at the inflorescence summit. Only insects that had left the plant base and begun to climb or reached the top were counted. No confusion was possible with laying females. On the insect's arrival at the inflorescence its head is oriented upward, but it usually then positions itself with the head downward.
Laying sites and fecundity.— Six females found at the summits of asphodel stems were collected from Baggiola at night and dissected the following morning. Brown mature eggs and yellow immature eggs (size smaller or equal to mature ones) were counted.
Asphodel stems were collected at San Pellegrino (Penta-Di-Casinca, Haute-Corse, on 19 July 2006), at Pertusato (Bonifacio, Corse-du-Sud, on 21 July 2006) and at Biaggiola (Porto-Vecchio, Corse-du-Sud, on 22 July 2006). Two stems found at Pertusato were split lengthwise in order to count the number of eggs laid in each hole. We compared the pattern of laying in 30 stems of asphodels (A. ramosus) and the same in ferulas (Ferula communis, Apiaceae) from samples taken at Bocca di Sant'Alparte, near the crossroad to Rovani (Coggia, Corse-du-Sud, on 31 July 2006).
Statistics.— If the reproductive individuals climb at random on the stems, we expect the number of stems bearing i= 0, 1, 2 or 3 individuals, to follow the Poisson probability distribution
Various combinations of the numbers of males and females per stem can also be compared to a random distribution. In this case, the expected number of males and females per stem would be:
A Chi-square goodness-of-fit was used to test these distributions.
The chance of a male meeting a female was calculated by taking into account the observed number of individuals climbing (individual i) and the probability of success of each one (proba success i), as follows:
We compared the numbers of eggs per hole between Asphodel stems and the number of holes between Asphodel versus Ferula stems by ANOVAs with the use of PAST vers. 1.48 (Hammer et al. 2001). The means are given with standard deviation.
Mating.— The male hangs under the female and holds onto her ovipositor with his first two pairs of legs (Fig. 1), as described for U. rugosicollis (Rigalleau 1936, Vahed 1997). This position is also found in other Ensifera, as in many Tettigoniidae (Chopard 1938).
For the San Pellegrino site, 23 stems were considered and the mean numbers per stem were 0.61 males and 1.43 females. Asphodels (A. ramosus) were the only plant species where mating was observed taking place. For the Biaggiola site, among the 33 stems examined, the respective means per stem were 0.54 males and 0.88 females. At this site, mating was observed not only on asphodels but also on several other herbaceous species, characterized by a single erect dry stem: Dipsacus fullonum (Dipsacaceae), Onopordum illyricum, Carthamus lanatus, and Cirsium vulgare (Compositae). At other sites Silybum marianum (Compositae) was also found to be a suitable substratum for mating.
For each site and for each sex, the distribution appeared to be at random: calculated chi-square were less than threshold values (Table 1). However, it should be pointed out that the female distribution follows the Poisson distribution less closely than that of the male, mainly due to many of the observed stems bearing a single female.
Distribution of individual numbers per stem. In the case of the San Pellegrino station, the observed and expected numbers of stems corresponding to 2 and 3 males per stem have been pooled to avoid an expected value less than 1.
It could be hypothesised that females are present having been attracted by the stridulation of a male. In the case of the San Pellegrino population, we compared the co-occurence distributions of males and females with random distributions from all possible combinations (0 to 3 males × 0 to 3 females). It appears that the presence of j females on a stem is independent of the male number as the calculated Chi square is 7.25 (risk > 40 %). In the case of the Biaggiola population, fewer combinations were found (0 to 2 males × 0 to 3 females). The calculated chi-square is 34.83, so the deviation from the null hypothesis is highly significant (risk = 1.6 10−6), mainly by reason of an excess of stems with an insect of only one sex.
It could be objected that an absence of stems bearing no individual could result from a sampling bias. We simulated several situations where 10, 20, or 30 stems bearing no insect were “forgotten”. The chi-square risks obtained were respectively 0.0009, 0.012, and 0.0066, and the deviation from random distribution was then mainly due to an absence of stems with 2 to 3 females and no male.
We can deduce that in the San Pellegrino population, where asphodel stems are rare, the individuals of both sexes climb onto the stems at random, in contrast to the Biaggiola population, where the stem density is high and many individuals climb on a stem and stay alone.
We obtained preliminary observations supporting the hypothesis that females climb slightly later than males: on the Maora plateau (Bonifacio, Corse-du-Sud, on 30 July 2007, at 9:50 PM), a small population showed the following numbers: 3 males and 1 female already at the summit of asphodel stems, and 1 male and 4 females climbing. However, the time available for observations within a night is limited: 10 min later, no individual was seen climbing. The other sites also gave only a short period for workable study.
Fecundity and laying.— The mean number of mature eggs per female was 38.17 ± 19.81 (n=6). The brown mature eggs were approximately 5 mm long × 1.5 mm wide. We also observed between 8 and 13 yellow (immature) eggs per female developing for a future laying.
When a female climbs on a stem, she progresses along a vertical line and behaves in the same way as described for Uromenus rugosicollis (Chopard 1951). While she introduces her ovipositor into a hole she has made in the stem, she bites the cuticle ahead with her mandibles to prepare the next hole. As a result, the holes follow each other very regularly and their distance corresponds uniformly to the space between mandibles and the base of ovipositor (figs 2A, B), i.e., about 1.3 cm. If the bark is too tough, she moves farther forward, but generally along the same vertical path. In this way, one can count the number of females that have laid in an asphodel stem and the number of holes made during a single laying session by a female.
A mean number of 2.91 and 2.50 eggs per hole was found in two stems collected at Pertusato. We verified their homogeneity (ANOVA, Fr1,39 = 1.39, p > 0.20). Taken together, a hole corresponds to a mean of 2.73 ± 1.12 eggs.
Of the 20 stems collected at Pertusato, Casaglione and San Pellegrino, one to three laying lines were observed per stem (a mean of 1.71 ± 0.85). Among 30 laying lines studied, we counted 13.77 ± 6.3 holes per line, resulting in 37.61 ± 17.21 eggs per laying session; this is close to the number recorded in the abdomen of dissected females. The maximum number of holes dug during an oviposition bout was 26, corresponding to about 71 eggs. Again, there is a good match with the maximum observed in the female collected at Ponte-Leccia. Asphodels thus play a major role in the reproduction biology of U. b. insularis since we recorded a mean number of 64 ± 32 eggs per stem at the sites where the insect is present.
However, other plant species can be used as laying sites, whether the asphodel is present or not. At San Pellegrino for example, we found a few egg-bearing stems of the introduced reed Cortaderia selloana (Schultes & Schultes fil.) Ascherson & Graebner, which was abundant at this site. At the Coggia locality, Ferula communis was also a laying site for the insect. Among 30 stems of asphodels and of ferulas chosen at random, there was a mean number of holes per stem of 7.9 and 15.47 respectively, showing a strong preference for ferula (ANOVA, Fr 1,58 = 5.55, p = 0.022). However, ferula bark is tougher, perhaps explaining the more irregularly spaced holes on these hosts.
Predation and parasitism of eggs.— At Coggia, 2 to 3 ferulas of 30 sampled, showed attack traces from woodpeckers (Dendrocopos sp.) as evidenced by claw and beak marks at the level of holes previously dug by U. brevicollis ssp. insularis (Fig. 3). This tall Apiaceae has sufficiently rigid stems to sustain a bird's weight. No observation of this kind was ever made on asphodel stems in all the study sites on Corsica.
At the same Coggia site, minute Scelionidae (Hymenopterans) were observed at the entrance of holes dug by U. b. insularis on ferula stems (Fig. 4), but never on asphodel stems. We hypothesize that the hole made by the bushcricket on the more rigid ferula stem remains more open, which would facilitate the access of the parasitoid. Peyerimhoff (1908) reported the species Catoteleia peyerimhoffi Kieffer, belonging to the subfamily Scelioninae, parasiting the eggs of U. brevicollis brevicollis in Algeria.
A clear preference for certain mating and oviposition sites by U. b. insularis makes quantitative studies very easy in the field, despite their nocturnal activity. In contrast, reproductive biology studies of numerous other nocturnal Tettigoniidae species need to be conducted with lab-reared individuals, as for Meconema (Vahed 1996) or Leptophyes (Vahed & Gilbert 1997).
When a male is climbing, he has little chance of finding a female at the summit of the stem, since the males are likely the first. This complies with their quite random distribution on stems. What, therefore, is the chance that a given male will meet a female on a given night? If the observed numbers are taken into account, there is about a 91% chance of meeting on a given night at the San Pellegrino site and 52 % at the Biaggiola site. If the reproductive season lasts approximately 10 d, a male therefore has a high probability of mating.
In contrast, when a female climbs onto a stem, she has only a 41 % chance of meeting a male during a night at San Pellegrino and 32 % at Biaggiola. These differences are due to a strong deviation from a balanced sex ratio, to the advantage of the females. This deviation could be related to a sexual refractory period, certainly longer in males than in females (Chopard 1938). In Steropleurus stali for example, the refractory period for females is around 4.8 d (Bateman 1997, Vahed 1998).
It is very unlikely that stridulation of males at the summit of the stem plays a role in attracting females, because in the Biaggiola population we observed a high degree of stems bearing one female and no male. Calling songs were heard in bushes surrounding the asphodels, suggesting that an aggregation of males takes place in the vicinity of mating sites. The bushes may represent an environment safer from predation for the production of calling songs (Alexander et al. 1995), but this needs to be tested. As the meeting of the sexes seems to occur at random, there is a temporal and spatial separation between calling and mating, as in some Gryllidae (Zuk & Simmons 1995).
Peyerhimhoff (1908) also mentioned Ferula sp. as a suitable plant for U. b. brevicollis in Algeria, but found in addition another Apiaceae, Thapsia villosa, and a Compositae, Cirsium scabrum. To our knowledge, the plant species used by the subspecies U. b. ibericus Schroeter & Pfau, 1987 (in Spain) and U. b. trimacriae La Greca 1967 (in Sicilia) have not been identified.
Interestingly, U. rugosicollis (Serville, 1838), a species close to U. brevicollis, has a related oviposition behavior, digging regular holes in dry stems (Chopard 1951), but this is uncommon in Bradyporinae which usually lay in soil. In contrast, many Tettigoniinae in Europe lay their eggs in the pith of stems, e.g., Metrioptera roeseli, Pholidoptera griseoaptera, Platycleis albopunctata or P. tessellata (Ingrisch & Köhler 1998, Gwynne 2001). This behavior likely evolved independently in the different genera, but this should be tested with a robust cladogram of the subfamily.
Distribution of males and females per stem. Each column corresponds to the number of stems bearing 0 to 2–3 females; the fist line refers to the cases where no male occurred and the second one when there was at least one male.
The number of plant species suitable for the shelter of eggs is limited, as the bark must not be too tough and the pith must be spongy: dry stems of asphodel perfectly fill both requirements. Moreover, asphodels appear to afford good protection of eggs against predation and parasitism, as long as the stem is erect. It would be interesting to examine the situation in spring, when the young instars leave the holes to reach the ground. The success of hatching in asphodels and ferulas remains to be compared.
The plant A. ramosus appears to play a key role in this insect's reproduction, as mating and laying activities take place mainly on its dry stems during July. We recorded other plant species where matings occurred: all were herbaceous, erect, and nonbushy, with a principal axis higher than 30 cm, e.g., Dipsacus (Dipsacaceae) and different thistles (Compositae). Given the relatively high pasture pressure in the scrub and garrigue formations in Corsica, asphodel, an indicator of the ultimate degradation stage of these habitats (Braun-Blanquet, 1964), is a very widespread species that favors the high incidence of U. b. insularis populations.
We are grateful to the Société d'Histoire Naturelle Alcide-d'Orbigny (Clermont-Ferrand, 63, France), to the DIREN Corse (Ajaccio, 2A, France) and to the Office pour l'Environnement de la Corse (Corte, 2B, France) for their financial support. Many thanks to Dr Karim Vahed for stimulating suggestions and the improvement of English, and to both anonymous reviewers for their constructive remarks.