There are around 500 native species of bees in Illinois. One group of native bees, cavity-nesting bees, build their nests within holes in rotting wood. These bees lack suitable nesting spaces in urban and suburban environments where dead trees are routinely removed. To address this issue, bee hotels have emerged as a solution, providing artificial cavities for cavity-nesting bees. These structures consist of collections of nesting spaces crafted from various materials like plant stems, paper straws, or solid wood with drilled holes. 

While the intentions behind bee hotels are good, their value for conservation is not supported by much data. Some surveys of who is nesting in bee hotels, such as those done by Scott MacIvor in Toronto, Canada, and Benoît Gelsin in Marseille, France, have found bee hotels attracting bad guys alongside the good guys. The native bees were targeted by parasites while invasive bees thrived and proliferated. It is currently unknown what can cause bee hotels to be filled with bad guys, and how best to optimize them to be good homes for native bees. 

Parasites live on or in another organism negatively affected by this relationship. Parasitoids are a type of parasite that is distinguished by the fact that it kills its host. A large number of the natural enemies of bees are parasitoids. An example of a parasitoid that can be found in bee hotels is the Melittobia wasp. These tiny wasps lay their eggs on the pupa of bees and other insects. When the wasp larvae hatch, they feed on the pupa until there’s nothing left. The males and females mate and then the female wasps chew their way out of the host nest and leave in search of their next host. 

Kleptoparasites use the strategy of stealing food from other organisms. One kleptoparasite that can be found in bee hotels is the Sharp-Tailed Bee, which uses its cone-shaped abdomen to pierce the nests of cavity-nesting bees and lay eggs within. When the Sharp-tailed bee’s eggs hatch, the larvae kill the host larvae and eat the pollen the host’s mother left for it.

Other natural enemies of bees, like their predators and pathogens, can also affect bees in bee hotels. Birds such as woodpeckers can be predators of bee hotels, eating all of the larvae out of the nests. One pathogen that can affect bees in bee hotels is a fungal disease called chalkbrood, which kills bee larvae. 

There is concern that, since bee hotels have bees nesting in closer proximity than they often would in nature, they could be easy targets for all of these enemies of bees. Previous studies attempting to measure whether the dense groups of nests created by bee hotels are more or less easily parasitized have yielded opposite results.

Around 70% of introduced species of bees nest in cavities in wood. Previous surveys of who was nesting in bee hotels have found disproportionate numbers of invasive bees. Studies have suggested that invasive bees such as Megachile sculpturalis strongly deter native cavity-nesting bees. This disturbance caused by invasive bees poses a dual threat, impacting both native bee populations and native plant communities. Invasive bees tend to pollinate invasive plants well, aiding in their proliferation, while inadequately pollinating native plants, exacerbating their declines.  

There may be unknown factors at play causing bee hotels to be more or less likely to be filled with parasites and invasives. There is some evidence that urban areas host more invasive bees, so there may be a certain level of urbanization at which putting out a bee hotel will just make a home for invasive bees. Flower composition around the bee hotel could also be important, with native plants supporting native bees and ornamental plants possibly encouraging invasives.