Biology of Xylocopa (Xylocopa) violacea (Linnè, 1758) (Hymenoptera: Apidae): giant nest!

Abstract

The present paper aims to find an explanation for the presence of exceptionally long and branched nests in the Xylocopini species. Two giant nests of Xylocopa violacea (L., 1758), excavated in a Prunus persica trunk, were found in 1994: M94/1 and M94/2. The former was formed by seven tunnels, four of which were ascending and 111 mm in average length, and 3 were descending and 108 mm in average length; the total length of the nest was 795 mm. M94/2 showed 9 tunnels, 7 of them were ascending and 98 mm in average length, and 2 were descending and 66 mm in average length; the total length of the nest was 856 mm. Nests of similar or slightly larger sizes have been reported for X. frontalis (social and multivoltine), X. hirsutissima (social and multivoltine), X. subvirescens and Lestis bombylans (communal nesting and multivoltine). Giant nests are common in both social and multivoltine species, since the founder female is helped by the daughters of the 1st generation in the construction of the cells of the 2nd generation, nest lengthening, and even oviposition. Moreover, the nest can be reused for several years and therefore subjected to further lengthening. The explanation for these two giant nests is not simple for the following reasons: 1) X. violacea is considered by all the authors as univoltine and solitary without any mother-daughters interaction; 2) the M94 nests were excavated in 1994, and therefore cannot be the result of lengthening due to its reuse; 3) the M94 nests had an entrance each, and therefore cannot be the fortuitous, or otherwise, result of the mergence of two or more nests. Therefore, five hypotheses are here formulated, based on my 10-year observations and literature data on this and other co-generic species. The main conclusion is that it is necessary to reconsider the whole life cycle of X. violacea, by carrying out further investigations on the life of the founder female after the nest has been completed, as well as on the fecundity of the Xylocopini species from the evolutionary standpoint.