Cephalic labial gland secretions of males as species recognition signals in bumblebees: are there really geographical variations in the secretions of the Bombus terrestris subspecies? (Hymenoptera: Apidae: Bombus).

Abstract

In order to grow artificial colonies and to obtain fresh males, spring queens of Bombus terrestris subspecies (B. terrestris audax, B. t. canariensis, B. t. maderensis, B. t. terrestris and B. t. dalmatinus) were collected from different localities. In addition, males of B. terrestris lusitanicus from Portugal, B. t. terrestris from southern France and Germany, B. t. xanthopus from Corsica and B. t. sassaricus from Sardinia were collected in the field. Cephalic labial gland secretions of 18 specimens were analysed by gas chromatography/mass spectrometry (GC/MS). About 70 compounds were identified: a mixture of acyclic sesqui- and diterpenes (alcohols, aldehydes, acetates) and various straight-chain fatty acid derivatives (alcohols, aldehydes, esters, and both saturated and unsaturated hydrocarbons with C21 - C29). The main component in all specimens was 3,7,11-trimethyldodeca-(6E),10-dien-1-ol (2,3-dihydrofarnesol). A few specimens from the field collections also contained considerable amounts of esters (connected to the aging of males), particularly 3,7,11-trimethyldodeca-(6E),10-dien-1-yl dodecanoate and, as these compounds (not involved in communication) decreased the peak area percentages and made comparisons difficult, all substances with a molecular mass larger than heptacosane (C27 hydrocarbon) were excluded from the analysis. With the exception of specimens of B. t. xanthopus from Corsica and B. t. sassaricus from Sardinia, which were devoid of ethyl dodecanoate (available in all of the other specimens at about 10 % of the peak area), no significant geographical variability was observed. The variability in the pattern of substances of each of (1) single males from the same colony, (2) specimens from the same geographical region, and (3) specimens from localities separated by a large geographical distance was within the same order of magnitude. These findings are discussed and compared with differences found in other published results. Because of their low variability and stability over large geographical distances, labial gland secretions of B. terrestris are very well suited to functioning as ‘species recognition’ signals.