Checking in at bee hotels: trap-nesting occupancy and fitness of cavity-nesting bees in an urbanised biodiversity hotspot

With urbanisation leading to loss of nesting resources, and increasing public interest in helping bees, bee hotels (trap-nests) are becoming popularised. However, their success is relatively understudied. The influence of habitat type in determining occupancy and emergence is also poorly known. Over two years across 7 bushland remnant and 7 residential garden sites, trap-nests were installed and completed nests collected monthly over spring-summer. Bees appeared to take a month to find the trap-nests, irrespective of month they were installed. A small percentage (13% and 6% in the two years) of tubes were occupied, but this was within the range of other trap-nesting studies. Smaller 4 and 7 mm diameter nests had a higher occupancy rate than 10 mm diameter tubes. An impressive number - 24 bee species - occupied the trap-nests. Representation however was dominated by five species. The species composition (species and their relative abundances) of cavity-nesting bees differed greatly between those using the trap-nests compared with those observed in the field. Bushland remnants tended to have more bee hotels occupied, and male body size of emerged bees was larger in this habitat. Unexpectedly native and total flower diversity reduced bee hotel occupancy, whereas native flower parameters tended to positively influence bee fitness. Overall installing bee hotels can provide additional nesting resources for native bees in urbanised areas, and providing high proportions of native flora in the vicinity should enhance fitness of the bees using them.