Recently there is a concern in the Pacific Northwest about an invasive species, the Houdini Fly (Cacoxenus indagator) . which is a kleptoparasite which invades mason bee homes and destroys the developing larvae and source of food pollen.
Brian Guzda of Brian’s Mason Bees (https://www.instagram.com/briansmasonbees/?hl=en ) provided this picture. Note that the white maggots are not contained within the cocoon as they are with the Mono wasp, another kleptoparasite which I profile in other posts here. It is essential to clean out whatever kind of tubes your mason bees have capped any time from October to December in order to catch these flies before they mature and cause further damage. 
The following video gives more information about this invasive species and emphasizes the importance of taking good care of your bees.
