Nesting biology of two sympatric species of Megachile (Chrysosarus) (Megachilidae) in Argentina

Leaf cutter bees (genus Megachile) are a diverse group of pollinators of wild and cultivated plants. Many species nest in pre-existing cavities and use diverse building materials to construct their brood cells. For instance, species of subgenus Chrysosarus add a mud layer between two layers of leaves or petals. We studied similarities and differences in their nesting biology from 13 nests of Megachile (Chrysosarus) concava and 72 nests of Megachile (C.) diversa (Megachilidae) obtained in trap nests in northern Argentina. Females of M. concava used trap nests with higher diameters, shorter length, and built a lower number of brood cells per nest compared to females of M. diversa. In the latter species, the sex ratio was female-biased, but we observed male-biased progeny in shorter trap nests, thus the sex ratio of M. diversa could be context-dependent. Reproductive success was different between species: 40.5% for M. concava and 75.8% for M. diversa, desiccation and/or mold being the principal factor of mortality of immature stages. The attack from parasitic species represented 10.8% for M. concava and only 1.2% for M. diversa. Megachile concava, based on observations of a few nests collected during autumn, could be a bivoltine species and oligolectic of Faboideae, while M. diversa is a partially bivoltine or multivoltine, and polylectic species, with a marked turnover of pollen resource throughout its extensive reproductive season. Differences in the trophic niche and, to a lesser extent, in the inner diameter of trap nests used could allow their coexistence in the studied forests.