Eremapis parvula, Ogloblin, 1956

Vossler, Favio Gerardo, 2013, The oligolecty status of a specialist bee of South American Prosopis (Fabaceae) supported by pollen analysis and floral visitation methods, Organisms Diversity & Evolution (New York, N. Y.) 13 (4), pp. 513-519 : 516-518

Eremapis parvula : a narrowly oligolectic bee of Prosopis

Taking into account the consistency of the results in the present study, it is highly probable that the data recorded by other authors belong to nectar hosts. It is likely that the mistaken identification of the generalist feeding habit of this bee species is due to the lack of discrimination between male and female foragers and/or nectar versus pollen gathering when recording floral visitation. Moreover, Neff (1984) found that males visited several floral hosts but females visited only Prosopis flowers. Pollen analysis of nest pollen samples of E. parvula taken from seven nest aggregations at different sites and times showed that it is a bee associated with host plants of the Prosopis genus alone. This type of pollen specialization can be classified as monolecty (sensu Robertson 1925) as well as narrow oligolecty (sensu Cane and Sipes 2006) (i.e., the former term was suggested to include bee species that are pollen-specialized in a plant species only while the latter in a genus). However, because Prosopis species of the section Algarobia found near the nests have pollen of similar morphology, differentiation between the Monolecty and narrow oligolecty categories was not possible using the pollen analysis method alone.

Most oligolectic bees do not discriminate among congeneric hosts, and so they should readily colonize new sister taxa ( Wcislo and Cane 1996). This could be the case with Prosopis —a genus quite uniform in floral and inflorescence phenotype ( Burkart 1937; Palacios and Bravo 1981). Examples at population level include a Vaccinium ( Ericaceae ) specialist that readily adopts another Vaccinium species outside its native range ( Cane and Payne 1993). Moreover, in areas where closely related pollen hosts bloom together (as observed for the seven species of Prosopis ), specialists typically forage on all of them ( Wcislo and Cane 1996).

Considering the great availability of floral hosts in the field (Table 1), pollen analysis of nest samples proved that E. parvula is a specialist bee of Prosopis . Moreover, Neff (1984) found that all nest provisions studied were composed solely of Prosopis and suggested that E. parvula is apparently oligolectic on this plant genus.

Thus, as many Prosopis species were recorded as hosts for E. parvula throughout a large geographical area ( P. alba , P. chilensis , P. flexuosa , P. strombulifera and P. torquata ), the narrow oligolecty category is supported. In different bee families, others narrowly oligolectic genera have been found, such as Dufourea ( Halictidae family), Perdita and Andrena ( Andrenidae ), Melitta ( Melittidae ), Proteriades ( Megachilidae ), and Diadasia ( Apidae , Emphorini ) ( Cane and Sipes 2006; Parker 1978; Rust et al. 1974; Torchio et al. 1967).

Synchronization between Eremapis parvula emergence and Prosopis blooming

During the three spring periods studied, a large and diverse spectrum of floral hosts was available. The constancy of only Prosopis pollen in all nest samples suggests that there is a strong correspondence between E. parvula and Prosopis in the Chaco forest, similar to those found in the Monte desert by Neff (1984).

Unlike annual plants that rely on rainfall for germinating and flowering, Prosopis species are woody perennials and their flowering is a predictable event. Conversely, emergence of E. parvula is unpredictable because it seems to be associated with rainy episodes when the soil is optimal for building its nests. Therefore, synchronization between them might occur only in rainy periods. The nest aggregations observed for E. parvula in different months suggest that it is a multivoltine bee (i.e., a bee species with emergence several times in a year). As this exomalopsine is a multivoltine bee and flowers of some of the more than seven Prosopis species are always available during spring, synchronization between them during spring rainy periods is highly probable. Thus, it is unlikely that E. parvula has to forage on alternative pollen hosts. Therefore, it is impossible to prove whether it is a facultative [i.e., a bee species that might turn to alternative pollen taxa, but only when their regular pollen hosts are temporarily absent ( Linsley and MacSwain 1958)] or an obligate [i.e., a bee species that refuse to provision or even nest in the absence of their floral host ( Cane and Sipes 2006)] oligolege bee. Pollen grains of Prosopis contain starch in the cytoplasm of the vegetative cell and pollenkit between the intine and exine walls ( Hoc et al. 1994) as well as a high percentage of protein (>20 %) ( Andrada and Tellería 2005; Vossler 2012). This would mean that Prosopis pollen could supply all nutrient requirements for the offspring of any bee species.

Although many monolectic and oligolectic bees appear to depend on a particular plant species or genus, host plants are generally not dependent for pollination on their monoleges or oligoleges ( Michener 1979, 2007). Therefore, the lack of synchronization between this specialist bee and Prosopis flowering would not have a negative consequence for the reproductive success of Prosopis species.

The importance of Prosopis for several bee groups

Bees are the most important group of pollinators of Prosopis , and several bee groups are highly attracted to Prosopis flowers for both pollen and nectar resources wherever their species are found ( Genise et al. 1990, 1991; Golubov et al. 2010; Keys et al. 1995; Michelette and Camargo 2000; Neff 1984; Pasiecznik et al. 2001; Simpson et al. 1977). These resources are collected intensively and stored in nests by different bee groups. In South American xeric areas, these are: Caupolicana mendocina Jorgensen , C. ruficollis Friese ( Colletidae , Diphaglossinae , Caupolicanini ), Xylocopa splendidula Lepeletier ( Apidae , Xylocopini ) ( Genise et al. 1990; 1991), Apis mellifera Linnaeus ( Apidae , Apini ) ( Andrada and Tellería 2005; Basilio and Noetinger 2002; Cabrera 2006; Costa et al. 1995; Fagúndez and Caccavari 2003, 2006; Salgado 2006; Salgado and Pire 2000; Tellería 2000; Tellería and Forcone 2000), Geotrigona argentina Camargo and Moure ( Apidae , Meliponini ) ( Vossler et al. 2010). However, only E. parvula has been associated with Prosopis alone using pollen analysis of nest provisions.

Over 80 and 160 bee species have been found visiting flowers of Prosopis in the deserts of South and North America, respectively ( Moldenke and Neff 1974; Simpson et al. 1977). Among them, the most common bee genera found are Colletes , Bicolletes , Oediscelis , Liopoeum , Megachile , Centris , Eremapis , Exomalopsis , Svastrides and Xylocopa in South America and Dialictus , Evylaeus , Nomia , Perdita , Megachile , Chalicodoma , Centris and Melissodes in North America ( Simpson et al. 1977). Oligolectic bees of Prosopis have been recorded only tentatively for the South American Colletes , Bicolletes , Oediscelis , one Megachile species and Eremapis parvula ( Neff 1984; Simpson et al. 1977) and for North American Perdita species.

Flowers of Prosopis have relatively exposed nectar and pollen resources ( Arroyo 1981), being an important source for both generalist and specialist bees. In the Chaco region, diverse bee groups forage on Prosopis . It is probable that a high number of oligolectic bees are waiting to be discovered in this plant genus.

For that reason, analyses of pollen provisions from nests, as was done in the present study for E. parvula , are necessary to provide more information about the actual diversity of pollen host plants associated with a bee species and, thus to reveal its pollen specialization category.