The biological study of wild non-Apis bees can provide useful information that may help with the pollination of food crops and native plants in areas where the keeping of honey bee colonies is restricted or affected by CCD. Here, we describe the nesting biology of the Oriental large carpenter bee, Xylocopa (Biluna) nasalis Westwood, 1838. An aggregation of more than 80+ bamboo nests of X. nasalis was discovered in Suan Pheung district, Ratch Buri province, Thailand on the 25^th of May 2012. We collected 27 nests from the site to dissect, measure the external and internal nest architecture, and analyze the pollen composition of the pollen masses. X. nasalis constructs linear unbranched nests with nest entrance mostly located at the open-end of the bamboo culms. The nest length and the branch diameter of the nest entrance (excluding nesting edge) are 25.40 ± 6.95 cm and 17.94 ± 6.00 mm, and the maximum number of provisioned cells is 8. A biased sex ratio of 8♀: 1♂ is reported, with up to 7 adults inhabiting in a single nest. 29 pollen types were identified from 14 pollen masses using an acetolysis method and visualization under both light microscope and scanning electron microscope. 13 pollen types were considered as major pollen sources (contribute ≥ 1% in total pollen volume); however, only 10 can be identified to family and generic levels. The dominant pollen sources are of the families Elaeagnaceae (Elaeagnus cf. latifolia), Euphorbiaceae (Croton), Fabaceae (Senna siamea and Cassia), Fagaceae (Lithocarpus and Castanopsis), and Lythraceae (Trapa) which are mostly native to the region of Southeast Asia. The nesting architectural details should prove to be beneficial to beekeepers and researchers who are interested in trapping and studying X. nasalis, and the polylectic behavior of X. nasalis can be highly valuable for future crop pollination strategies, particularly for plants that require sonication of their poricidal anthers.

Carpenter bee, nesting biology, Thailand, bamboo, pollen

Because of the declining honey bee population worldwide resulting from the condition known as Colony Collapse Disorder (CCD; Oldroyd 2007, van Engeldorp et al. 2008, Ratnieks and Carreck 2010), the use of widespread pesticides (Hopwood et al. 2012), climate changes (Bartomeus et al. 2011), and the increase in monotonous agricultural landscapes that reduce the biodiversity and the availability of foods for bees, the study of wild and/or domesticated non-Apis bees can provide useful information for complementary bee species that may help with the pollination of food crops in areas where keeping of honey bees colonies are being affected or restricted (Chagnon et al. 1993, Wilmer et al. 1994, Javorek et al. 2002, Hoehn et al. 2008, Brittain et al. 2013). Until now, only a handful of non-Apis bee species have been used extensively in agriculture, e.g., Bombus terrestris (Linnaeus, 1758), Megachile rotundata (Fabricius, 1787), Nomia melanderi Cockerell, 1906, Osmia rufa (Linnaeus, 1758), and some stingless bee species (Westerkamp and Gottsberger 2000, Hogendoorn et al. 2006, Greenleaf and Kremen 2006, Slaa et al. 2006, Hoehn et al. 2008). These bees have been shown to be effective pollinators, as good as, if not better than, honey bees on certain crop plants (Greenleaf and Kremen 2006).

The large carpenter bees of the genus Xylocopa Latreille, 1802 (Hymenoptera; Apoidea) have recently received attention due to their pollination capabilities. The use of large carpenter bees to assist with pollination of greenhouse tomatoes and honeydew melons in Australia and Israel has been reported (Hogendoorn et al. 2000, Sadeh et al. 2007, Keasar 2010). In Brazil, where passion fruits are one of the main exported fruit crops of the nation, studies of using X. (Neoxylocopa) grisescens Lepeletier, 1841 and X. (N.) frontalis (Olivier, 1789) to pollinate the flowers, instead of using manual labor, have shown promising results in increasing the fruit sets and quality and reducing the production costs (Junqueira et al. 2012, Yamamoto et al. 2012).

Carpenter bees can be found throughout the tropical and subtropical parts of the world (Hurd and Moure 1963, Gerling et al. 1989). These are large and robust bees that many novices regularly confuse with bumble bees (Bombus) due to their similar sizes and shapes. There are currently ca. 470 species described with 32 subgenera recognized in a single genus (Michener 2007, Ascher and Pickering 2013). Most Xylocopa species are known to excavate their nests in dead or decaying woods, with the exception of the subgenus Proxylocopa Hedicke, 1938 which excavates nests in the soil (Gottlieb et al. 2005). There are two main types of nests among the wood-nesting Xylocopa species: (1) unbranched or linear nests in which the tunnel runs in the same direction as the nest entrance or at most with a single right angle corner from the nest entrance and (2) branched nests which consist of at least two tunnels or more although with only one nest entrance (Gerling et al. 1989).

One subgenus of Oriental Xylocopa, Biluna Maa, 1938, comprises five to nine species (Michener 2007, Ascher and Pickering 2013). Its distribution ranges from India and Sri Lanka to Southeast Asia and Japan. Species of Biluna are only known to construct unbranched nest in bamboo culms (Maa 1946, Maeta et al. 1985; Hurd and Moure 1963). Xylocopa (Biluna) nasalis Westwood, 1838, is a species commonly found throughout Southeast Asia. It superficially resembles the sympatric species X. (Mesotrichia) latipes Drury, 1773 and X. (M.) tenuiscapa Westwood, 1840 because of the presence of black pubescence on the mesosoma and their large size (21–35 cm in length). Males of Biluna lacks both a basitibial fig and a spine on the outer apex of the hind tibia, while the females have a dense mat of short setae on the middle tibia and lack an apical middle tibial spine. The behavior, biology and natural history of X. nasalis, is poorly known even though it is commonly found throughout rural and agricultural areas in Southeast Asia. Boontop et al. (2008) briefly described the nesting biology of X. nasalis studied in Kasetsart University, Kamphaengsaen Campus, Nakhonpathom province, Thailand, though their account lacks many nest architectural details and, importantly, the palynological data on plant food sources. Here, we extend Boontop et al. (2008)’s work via reporting the finding of a nest aggregation of X. nasalis in Suan Pheung district, Ratch Buri province (~100 km southeast of Nakhonpathom province), Thailand, along with details of other nest architectural components, its floral preferences, and some behavioral observations at the nest entrance. We anticipate that by providing such detailed nesting biology and pollen food sources of a local large carpenter bee from an area with poorly known mellitological data (such as Southeast Asia), it will stimulate interest and provide practical information for local bee keepers and bee researchers to consider Xylocopa to be an important native pollinator for certain crops and endemic plants in the near future.

Boontop et al. (2008) provided a brief nest architectural description of 20 Xylocopa nasalis communal nests collected from Nakhon Pathom province ~100 km southeast of our collecting site, though they only reported the nest total lengths (described as “Internode length”), size of the nest entrances, diameter of the bamboo nests, and the sex ratio. Here, we also reported additional detailed nest characteristics that were undescribed previously. The total nest length averaged at 38.35 cm, whereas Boontop et al. (2008) found theirs to be 32.63 cm. The branch diameters of the nests are also similar between our work and that of the previous authors, 17.94 and 15.60 mm, respectively; however, the sex ratio between female and male bees from this observation is about twice to what was earlier described (8♀: 1♂ vs. 4♀: 1♂). The difference in the number of female to male bees can be explained by the collecting date, which may correspond to a later period of colony development, where most of the sister bees have emerged and stay together inside the nest, whereas male bees may have departed right after emerging from their cocoons or there is a sex ratio bias in egg-laying by the mothers. Observations on such activity are needed to test these hypotheses about the skew sex ratio in the nest. The three unconsumed pollen masses have an average fresh weight of 1.37±0.13 g compared to 1.16 and 1.09 g in X. (Ctenoxylocopa) sulcatipes Maa, 1970 and X. (Koptortosoma) pubescens Spinola, 1838, species found in the desert area of the Middle East (Gerling et al. 1989).

Maeta et al. (1985) reported finding of a nesting aggregation of another Biluna species, Xylocopa tranquebarorum tranquebarorum, in Szechungchi near Henchun, Taiwan. This Biluna species also nested in bamboo culms though the nest entrances were excavated exclusively from the underside (the authors found only five nests in successive internodes of a single culm. In contrast to the previous finding, we found the nest entrance of X. nasalis to be mostly at the end of the bamboo culms, but we also observed that a couple of the nest entrances were on the underside of the culms excavated by the bees as well. This observation suggests that both X. nasalis and X. tranquebarorum tranquebarorum can excavate nest entrance from the underside of the bamboo culms, which might be a behavior shared by members of the subgenus Biluna. This tedious nest entrance excavation of a smooth and hard surface such as the sides of bamboo culm may be explained by the compensation that the bees will receive after the initial perforation of the culm with the omission of the need for later heavily burrowing (Iwata 1938). However, if the ends of the bamboo culms are open and exposed, the bees may choose not to allocate their energy in the excavation of the undersides of the culms as seen in this study.

Xylocopa nasalis is polylectic with a diverse group of pollens collected. This is consistent with the described foraging behaviors of other carpenter bee species (Hurd and Moure 1963, Gerling et al. 1989, Burgett et al. 2005). Interestingly, the family Fagaceae, particularly of the genera Lithocarpus and Castanopsis, constitutes abundant pollen sources for X. nasalis. Both pollen types can be accounted for 26.43% and 13.77%, respectively, in terms of total grains count; however, their pollen sizes are twice or thrice smaller than other pollen types found in this study (Table 2), thus the total percentage pollen volumes contribute to the bee diets are rendered to only 7.65% and 3.22%. Lithocarpus and Castanopsis are evergreen genera of large shrubs and trees that can reach more than 20 m in height. Records show that there are 56 species of Lithocarpus and 33 species of Castanopsis indigenous to Thailand (Phengklai 2006), though there is no available information pertaining to a reliable identification of the pollen species of both plant genera in the area of study. Many species of the lowland Lithocarpus and Castanopsis flowers bloom abundantly during the beginning of the dry season to the end of monsoon period (March–October), which corresponds to the time of our collecting. It is known that species of both Lithocarpus and Castanopsis are pollinated via insects (Nixon 1989, Manos and Steele 1997, Manos et al. 2001), therefore it is evident that we should consider X. nasalis as one of the important pollinators for genera of large endemic trees that constitute the deciduous and evergreen landscape in the area of central Thailand. In addition, Burgett et al. (2005) reported the importance of Lithocarpus and Castanopsis pollens as two of the top food sources of the night-flying carpenter bee X. (Nyctomellita) tranqueberica (Fabricius, 1804) as well. They suggested that these pollen types serve as the primary pollen sources for X. tranqueberica found in northern Thailand, second to the introduced plant species of Casuarina Linnaeus, 1753, which is heavily planted throughout Thailand for reforestation (Burgett et al. 2005).

Another group of large trees that also benefit from Xylocopa nasalis visitation is Senna siamea (Fabaceae), and other related but unidentified species in the genus Cassia. Senna siamea is an indigenous evergreen tree found throughout Thailand and other neighboring countries in South and Southeast Asia; locals use its leaves mainly for consumption; it is seldom used as fodder for animals and intercropping. The flowering period of this species is documented to be during March to September or otherwise year round, if the hot and humid weather permitted (Hanum and van der Maesen 1997, Sosef et al. 1998). Both S. siamea and Cassia have poricidal anthers, which require a sonication or “buzz-pollination” from floral visitors to extract pollen from their anthers and thus eventually affect pollination (Buchmann and Hurley 1978, Buchmann 1983). Visitations by carpenter bees, which are known for their abilities to vibrate their thoraces at the pores of the flowers’ anthers to release the pollens (King et al. 1996, King and Buchmann 2003), are crucial for the reproductive successes in these plant genera. Xylocopa nasalis may as well be an important pollinator of this group of large trees in this area.

From the analyses of the pollen volume, we found that Croton (Euphorbiaceae) contributes the highest volume (14.95%). It is important to note that though this genus was found for only 2.80% of the total pollen count (Table 2), the relatively large size of Croton makes it become one of the most important food sources for Xylocopa nasalis. In Thailand, there are about 30 species of Croton (Chayamarit and Welzen 2005). The genus has a reputation of containing biomedical-active compounds such as alkaloids and terpenoids (Rizk, 1987) that have potential values to the pharmaceutical industries.

One genus of an annual floating-leaved aquatic plant is also frequently visited by Xylocopa nasalis. The pollens of water chestnut of the genus Trapa (Lythraceae) contribute 13.36% of the total pollen volume in the bee diets. Smitinand (2001) listed only three Trapa species in Thailand: T. bicornis Osbeck, 1771, T. incisa Siebold & Zuccarini, 1845, and T. natans Linnaeus, 1753. However, the taxonomy of Trapa is still in flux (Kadano 1987; Cook 1996; Takano and Kadano 2005), and a thorough survey of this common aquatic plant in Thailand is needed, since the fruits of T. bicornis are one of the important food crops in Thailand. Identification of Trapa pollens to specific level can provide important information regarding which species X. nasalis visit and pollinate.

Lastly, the main shrub species that X. nasalis visits for pollen is Elaeagnus cf. latifolia (Elaeagnaceae), a prominent shrub that has a native range in northern Thailand, although it can be found throughout the country due to its high adaptability to various soil conditions and habitats (Smitinand 2001). Other minor pollen-providing plants that can be identified in this work such as Sambucus and Ziziphus, which are possibly introduced into the area as ornamental plants, and which contribute less than 1% of the pollen volume in a given pollen mass.

Our observations of nest-defending by resident females are consistent with the nest defending postures reported in Xylocopa sulcatipes and X. pubescens in Israel (Gerling et al. 1989) and in X. (Ctenoxylocopa) fenestrata (Fabricius, 1798) in India (Kapil and Dhaliwal 1968a), though whether the guarding females are the progeny of the founding female still needs to be investigated in X. nasalis.

In summary, our observations and dissections of Xylocopa nasalis nests agree with known reports of other Xylocopa species (Hurd 1958, Hurd and Moure 1960, 1961, 1963, Kapil and Dhaliwal 1968a, 1968b, Michener 1974, Mordechai et al. 1978, Gerling et al. 1989, Boontop et al. 2008). The X. nasalis nest is strictly unbranched. The provisioned cells are separated via partitions made from bamboo particles excavated by the founding female. The nesting architectural details provided within this work should prove to be of beneficial to beekeepers and researchers who are interested in trapping and studying X. nasalis. For further genetic and social behavioral studies, we found that in a given nest, sister bees can tolerate and live inside the same nest with up to 7 individuals along with their mother. Kinship analyses using molecular markers such as microsatellite DNA will reveal interesting details pertaining to the social structure in a single nest and the population structure of the bees living communally in the same vicinity in Ratch Buri province, Thailand (W. Hongjamrassilp and N. Warrit (unpublished data)). As for the pollens foraged by X. nasalis, the broad host plants range can be highly beneficial for many crop pollinations, particularly for plants that require the “buzz” pollination method by their pollinators (Keasar 2010). Flower constancy and other related pollination studies are required for further justification of using X. nasalis as future potential pollinator for agricultural and forest plants in Southeast Asia.

We would like to thank two anonymous reviewers and the editor: Dr. Jack Neff, Drs Deborah Smith, and Charles Michener for helping us to improve the manuscript. This research cannot be completed without the assistances of undergraduate and graduate students of the Department of Biology, Chulalongkorn University: Pattarawit Engkananuwat, Suttimon Narongchaisarid, Thanawan Duangmanee, Narin Chomphuphung, Nantikarn Thongcharon, Wassamon Suwannarat and Chatphagorn Rangsri. Pollen identification was guided by Dr Chumpol Khunwasi, Department of Botany, Chulalongkorn University, and Ms. Nungruthai Wichaikul. Dr Ajcharaporn Piumsomboon graciously contacted the SEM facility at the Center of Nano-imaging, Mahidol University, for us to produce the pictures of the pollen grains. We are grateful to the assistance of Dr Orawan Duangphakdee and Mr. Preecha Rod-im of King Mongkut’s University of Technology Thonburi for their hospitality and for providing an opportunity to the authors to study the carpenter bees in Ratch Buri province. This research is partially funded by the Thailand Research Fund (TRF#MRG5380139) and Grants for the Development of New Faculty Staff, Chulalongkorn University, Thailand, to NW.