In 2017, we found 17 dead females of Megachile frigida Smith in clear plastic bags of composted bark mulch in a residential yard in Bozeman, Montana, USA. Females apparently entered bags via small ventilation holes, then became trapped and died. To investigate whether this is a common source of mortality, we deployed unmodified bags of mulch and those fitted with cardboard tubes (as potential nest sites) at three nearby sites in 2018. We found two dead M. frigida females and five completed leaf cells in one of these bags of mulch fitted with cardboard tubes; two male M. frigida emerged from these leaf cells. In 2018, we also discovered three dead female M. frigida and three dead females of a second leafcutter bee species, Megachile gemula Cresson, in clear bags of another type of bark mulch. Both mulches emitted nearly identical blends of volatile organic compounds, suggesting their odors could attract females searching for nesting sites. These findings suggest that more research is needed to determine how common and widespread this is for Megachile species that nest in rotting wood and if there are simple solutions to this problem.