Osmia atriventris Cr. and Megachile relativa Cr. are the most abundant bees accepting trap nests in the Black Sturgeon Lake area of Ontario. Hylaeus ellipticus (Kby.), H. verticalis verticalis (Cr.), Hoplitis albifrons albifrons (Kby.), and H. cylindrica (Cr.) are common while Hylaeus basalis (Sm.), Osmia proxima Cr., M. inermis Prov., and M. gemula Cr. are comparatively rare. All except Osmia atriventris are univoltine and overwinter as prepupae. O. atriventris is both univoltine and biennial. The major flight and nesting period for all bees is during July although M. relativa is active throughout August. Hylaeus spp. construct nests of silken membranous, secreted materials; Hoplitis spp. utilize macerated plant materials and pebbles to divide their nests; Osmia spp. partition their nests with macerated green plant material; and Megachile spp. line and plug their nests in typical leafcutter fashion. All bees provision their nests with pollen moistened with varying amounts of nectar. The sexes are probably almost equally divided in all species. Major parasites are Gasteruption assectator assectator (L.) for Hylaeus spp.; Chelynia subemarginata (Cr.) for Hoplitis spp.; Sapyga sp., for Osmia atriventris; and Coelioxys modesta Sm., and C. lucrosa for Megachile relativa. Melittobia chalybii Ashm. destroyed a few cells occasionally.
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