Introduction

The mating tactics of bumblebees were studied by various authors and classified into several categories. Schremmer (1972) recognised four categories of behaviour displayed by bumblebee males: 1) siting or flying around the nest entrance waiting for virgin females, 2) regular patrolling flights (usually performed in open areas but allowing the males to approach the nest entrance easily), 3) regular patrolling flights away from the nest, and 4) perching behaviour - resting on a perch, darting on various objects passing, no patrolling behaviour exhibited.

Bombus confusus Schenck, 1859 is usually presented as a typical representative of the group of species characterised by visual searching for females from a perch (Schremmer, 1972). This species is reported to be a non-patrolling, entirely optically oriented bumblebee, characterised by its enormous eyes. The labial gland of such a species would be expected to be reduced and non-functional. Contrary to our expectations, the dissected labial glands of all B. confusus males collected by us were normally developed, equally large or even slightly larger than those of the species exhibiting regular patrolling behaviour. This result led us to analyse the chemical composition of the gland content.

Chemical composition of the gland content

Labial glands of males of the species Bombus confusus were dissected and extracted with hexane. The extracts were analysed using a GC-MS technique. The identification of compounds was based mostly on their mass spectra and on the co-chromatography with synthetic or commercially available standards. Seventeen compounds were detected in the extract of the labial gland secretion. Two main components were identified as geranylcitronellol 1, 38 %) and (Z)-9-octadecenyl acetate (6, 34 %). The standard sample of geranylcitronellol was earlier prepared in our laboratory (Valterová et al., 1996).

Four diterpene derivatives were found as medium-abundant components: geranylcitronellyl acetate (2, 1.6 %), geranylcitronellyl hexanoate (3, 2.7 %), geranylcitronellyl octanoate (1, 10.2 %), and geranylgeranyl acetate (5, 1.4 %). The position of the double bond in octadecenyl acetate (6) was determined by mass spectrum analysis of the prepared dimethyl disulphide adduct. The configuration of the double bond was based on the measurement of infrared spectrum in the gas phase. The presence of the 3012 1/cm band (=C-H stretch), and the absence of a band 890 1/cm (E-wag) established the Z-configuration of the double bond in our isolated sample of 9-octadecenyl acetate.

Biological observations

Males chose elevated perches between 10 cm to 80 cm in height, always in an open sunny area. The most favorite perches are dried plants. The distances between perch sites of individual males are usually 7 - 8 m. Before they start perching, the males mark their perches and vegetation in the vicinity with scent originating from their labial glands. Their labial gland is fully developed and functional and it produces a secretion used for marking a site where a male is waiting for a female. A chemical prove of this fact was given by detecting identical compounds in the labial gland secretion and in the male-marked objects (Fig. 1). After marking, males begin with pursuing flights from their perches. The number of flights per minute ranges from 3 to 10, depending on temperature and wind conditions. When a high population density occurred and the locality was "crowded" by perching males, male-male interactions were observed. A critical distance between perches seems to be 3 - 4 m, depending on the character of habitat.

Fig. 1: Chromatograms (recorded on GC-MS) The rectangle indicates the area of components of the labial gland secretion (1, methyl (Z)-9-octadecenoate; 2, geranylcitronellol; 3, (Z)-9-octadecenyl acetate; 4, geranylcitronellyl acetate; 5, tricosane).

Conclusion

The presence of the components of the labial gland secretion on males-marked objects shows that the labial gland secretion of B. confusus males is used by males for marking their territories. The obtained results lead us to a reinterpretation of the premating behaviour of B. confusus males. The male selects a perch on an elevated site and marks it with his pheromone. He spends all day on this elevated perch waiting for a passing female that would be attracted by his pheromone. To increase the probability of a successful meeting with the female, the male flies toward any flying object resembling a female by its size. A characteristic trait of the Bombus confusus males is their enormous eyes. The enlargement of eyes, a very striking example of a morphological adaptation, makes it possible or at least easier to recognise a female at a long distance.

The observations of the premating behavior of B. confusus males show a daily behavioral pattern similar to other perching species. Our results support the idea that premating behavior of perching bumblebee species belongs to the category "lek behavior" of polygyny mating systems (Thornhil and Alcock, 1983).

References

Hovorka O., Urbanová K., Valterová I.:
Premating behavior of Bombus confusus males and analysis of their labial gland secretion.
Journal of Chemical Ecology 24: 183-193 (1998).

Kindl J., Hovorka O., Urbanová K., Valterová I.:
Scent marking in male premating behavior of Bombus confusus.
Journal of Chemical Ecology 25: 1489-1500 (1999).

Schremmer F.:
Beobachtungen zum Paarungsverhalten der Männchen von Bombus confusus Schenk.
Zeitschrift für Tierpsychologie 31: 503-512 (1972).

Thornhill R., Alcock J.:
in book: The evolution of insect mating systems.
Harvard University Press, Cambridge, 1983, p. 547.

Valterová I., Svatoš A., Hovorka O.:
Analysis of the labial gland secretion of the cuckoo-bumblebee (Psithyrus vestalis) males and synthesis of abundant geranylcitronellol.
Collection of Czechoslovak Chemical Communications 61: 1501-1508 (1996).

Research team

Irena Valterová (Head of the Team), Jiří Kindl, Oldřich Hovorka, Klára Urbanová, Aleš Svatoš

Related themes on our web pages
Bumblebees (part II)
Bumblebees (part III)
Bumblebees (part IV)