Parasitism and brood mortality in Alfalfa Leafcutting Bee (Megachile rotundata (Fabricius)), nesting in vacated comb cells of European Paper Wasp (Polistes dominula (Christ)).

Social paper wasps (Hymenoptera: Vespidae) construct comb nests of tens to hundreds of brood cells that are abandoned each year before winter. The nests are positioned where they are protected from inclement weather and may remain intact for several years. Here, I detail observations of nests provisioned by the non-native, solitary Alfalfa Leafcutting Bee (Megachile rotundata (Fabricius, 1787); Hymenoptera: Megachilidae) in individual brood cells in vacated combs of the invasive, social European Paper Wasp (Polistes dominula (Christ, 1791)) on a green roof in Toronto, Ontario. A total of 12 paper wasp combs were dissected and 280 M. rotundata nests (one per wasp comb cell) were recovered; 22 nests were provisioned in 2013 consisting of 32 individual M. rotundata brood cells. Parasitism by Melittobia and Monodontomerus wasps accounted for 46.9% of M. rotundata mortality in the cells in 2013; mortality from all causes, including parasitism, was 78.1%. In contrast, total mortality of M. rotundata in brood cells provisioned in a human-made bee nest box on the same roof in 2013 was 4.2% and there was no parasitism. Mortality by parasitism and total brood mortality in 391 brood cells provisioned in 41 nests in the bee nest box in 2011--2013 were 2.0% and 21.2%, respectively. Therefore, the use of vacated paper wasp comb cells resulted in an overall >20-fold increase in parasitism and >3-fold increase in brood mortality over that observed in the bee nest box when all years are combined. [ABSTRACT FROM AUTHOR]