Peter BERGMAN*, Gunnar BERGSTRÖM
Department of Chemical Ecology,
Botanical Institute, Göteborg University, Carl Skottsbergs Gata 22, S-413 19, Göteborg,
Sweden.
Males of many bumblebee species (genera Bombus and Psithyms, Apidae, Hymenoptera) exhibit a conspicuous premating behaviour with two distinct behavioural components: scent-marking and patrol-flying. The present consensus of premating behaviour in these bumblebee species is that the males start first thing in the morning to scent mark objects in their surroundings. Different bumblebee species scent mark different objects (such as small twigs, leaf edges, grass, tree trunks) at different heights from ground-level up to.the tree tops (Bringer, 1973). After repeated scent-marking for two to three hours an individual male-specific flight route has been established. This route will then be patrolled during the rest of the day; the male flies in a regular manner and in the same direction along the route and visits, now without alighting, the earlier repeatedly scent-marked sites. The scent-markings are thought to attract conspecific unmated, young queens and are thus important for species recognition and to avoid interspecific mating (Svensson, 1980). Furthermore, the scent has been shown to be important guideposts for the male itself early in the day before the flight path has been memorised visually (Awram, 1970).
Two different glands have in the past been proposed to be the origin of the compounds used by the bumblebee males for scent-marking: the mandibular gland (Haas, 1952) and the cephalic portion of the labial gland (Kullenberg et a/., 1973; Agren et al., 1979). The labial gland secretions consist of species-specific blends in all the 37 species and two forms of Scandinavian bumblebees analysed (Bergstrom and Svensson, 1973; Bergstrom et al., 1973; Svensson and Bergstrom, 1977, 1979; Cederberg et al., 1984; Appelgren et al., 1991; Bergman et a/., manuscript) as well as in three North American species (Bergstrom et al., 1996).
The marking behaviour and scent-marking compounds in males of the two Scandinavian bumblebee species Bombus pratorum and Bombus lapidarius were studied. Males of B. pratorum scent marked small areas of the edges of birch leaves in a quick and unpredictable manner depositing farnesol, geranylgeranyl acetate, farnesyl acetate, (Z)-11-octadecanol, hexadecanol and a pentacosadiene. In B. Iapidanus scent-marking on birch leaves was much slower and the entire edge of the leaf was marked with (Z)-9-hexadecenol and hexadecanol. These compounds were also detected in the cephalic portion of the labial gland in males. The total amount of scent compounds per mg labial glands was 0.2 pg in B. pratorum and 40 pg in B. Iapidarius. Farnesol and farnesyl acetate were detected via headspace collection inthe air around leaves marked by B pratorum and (Z)-9-hexadecenol was detected around leaves marked by B. Iapidarius.
Minor amounts of compounds originating from a gland other than the labial gland may have been present in the samples with the scent-marked birch leaves, but were not noticed because they were masked by background compounds from the birch leaves themselves (about 150 compounds). In order to reveal if this is the case or not, B. Iapidarius males were allowed to scent-mark artificial leaves made of glass. These samples only contained compounds present in the cephalic portion of the labial glands; (Z)-9-hexadecenol and hexadecanol. Furthermore, (Z)- 9-hexadecenol and hexadecanol in the labial gland show a diurnal variation in consistence with the marking behaviour. Largest amounts are found in the morning before scent-marking commences while the smallest amounts are found around noon which is the time when the patrol-flying behaviour replaces the scent-marking.
To conclude, there should no longer be any doubt that the compounds originating from the cephalic portion of the male labial gland are those used for scent-marking and most likely are important as signals in the premating behaviour of Scandinavian bumblebees.
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