International audienceThe nesting biology of the leafcutting bee Megachile (Pseudocentron) gomphrenoides Vachal (Hymenoptera: Megachilidae) was studied in an agro-ecosystem in the Province of Buenos Aires, Argentina. Data were obtained from trap-nests placed in the field margin during two agricultural years (2008/2009 and 2009/2010). Females made an average of 7.55 cells per nest, and used leaves of at least three plant species to build their nests. Provisions of cells were principally of Asteraceae pollen. Adult emergence showed a bimodal pattern suggesting a facultative bivoltinism life cycle. Approximately, 30 % of all offspring failed to complete development to the adult stage and an additional 10 % were killed by natural enemies. These included parasitic wasps (Eulophidae: Melittobia and Horismenus), a cleptoparasite bee (Megachilidae: Coelioxys), and a bristle beetle (Meloidae: Tetraonyx). The host/cleptoparasite association between M. gomphrenoides and Coelioxys remissa constitutes the first such record for both species, and the Megachile-Tetraonyx interaction was previously unknown. M. gomphrenoides possesses some characteristics that make it an interesting potential opportunity to use this species for pollination of commercial sunflowers in the Pampean region

Nests of the leafcutting bee Megachile (Chrysosarus) catamarcensis Schrottky (Hymenoptera: Megachilidae) were obtained during a 24 month trap-nesting programme in a xeric forest in the Province of La Pampa, Argentina. Nests were constructed in December and January; females made an average of 4.0 ± 2.2 cells, and used mud with flower petals and/or leaf pieces to build their nests. Provisions of cells were practically pure pollen mass of Proposis caldenia. Adult emergence showed a unimodal pattern, suggesting a univoltine life cycle. However, two individuals that emerged a month after the traps were collected indicate the existence of a small early-summer generation. This could indicate that M. catamarcensis, in some years (warmer) or in some populations (at lower latitudes), could exhibit a bivoltine life cycle. Approximately 15.3 % of all offspring failed to complete development to the adult stage, and an additional 13.9 % were killed by natural enemies. These included diverse parasitic wasps (Chrysididae, Sapygidae, and Leucospidae), a cleptoparasite bee (Megachilidae: Coelioxys), and a bee fly (Bombyliidae: Antrhax). Ecología de nidificación de Megachile (Chrysosarus)catamarcensis Schrottky (Hymenoptera: Megachilidae), una abeja especialista de Prosopis Resumen Nidos de la abeja cortadora de hojas Megachile (Chrysosarus) catamarcensis Schrottky (Hymenoptera: Megachilidae) fueron obtenidos durante 24 meses en un estudio de trampas-nidos en un bosque xérico en la Provincia de La Pampa, Argentina. Los nidos fueron construidos durante diciembre y enero; las hembras hicieron un promedio de 4.0 ± 2.2 celdillas, y utilizaron barro con pétalos y/o piezas de hojas para construir sus nidos. Las provisiones de las celdillas fueron masas de polen prácticamente puras de Proposis caldenia. La emergencia de los adultos exhibió un patrón unimodal sugiriendo un ciclo de vida univoltino. Sin embargo, dos individuos nacieron un mes después de que las trampas fueron colectadas indicando la presencia de una pequeña generación de principios de verano. Este hecho podría estar indicando que M. catamarcensis en algunos años (más calurosos) o en algunas poblaciones (a menores latitudes) podría exhibir un ciclo de vida bivoltine.Aproximadamente, el 15.3 % de toda la descendencia no logró completar el desarrollo hasta el estadio de adulto y un 13.9 % adicional fue asesinado por enemigos naturales. Estos incluyeron diversos avispas parásitas (Chrysididae, Sapygidae, y Leucospidae), una abejas cleptoparásita (Megachilidae: Coelioxys), y una mosca abeja (Bombyliidae: Antrhax).

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Megachile is the largest genus of the family Megachilidae, being widely distributed throughout temperate and tropical areas of the world. The validity of M. (Chrysosarus) and M. (Dactylomegachile) was investigated based on a cladistic analysis of 63 characters of adult morphology of 14 taxa. The single most parsimonious tree (181 steps, CI=0.60; RI=0.63) resulting from an implied weighting analysis recovered M. (Chrysosarus) and M. (Dactylomegachile) as monophyletic groups. Both subgenera are redescribed and the value of the most frequently used characters in the taxonomy of Megachile is discussed.