Ask Mr. Smarty Plants

From Wikipedia: "Hosta is a genus of about 23–45 species of plants commonly known as hostas, plantain lilies (particularly in Britain) and occasionally by the Japanese name giboshi. Hostas are widely cultivated as shade-tolerant foliage plants. The genus is currently placed in the family Asparagaceae, subfamily Agavoideae,  and is native to northeast Asia."

The Lady Bird Johnson Wildflower Center, home of Mr. Smarty Plants, is committed to the growth, propagation and protection of plants native not only to North America but to the area in which they evolved; in your case, Macomb County, in southeast Michigan.

Therefore, this plant is out of our area of expertise, but we can at least address your question as to whether they are attracting flies. All we could learn was that the only insects the hostas attracted were snails and slugs. There is a native member of the group called "carrion flowers" - Lysichiton americanus (American skunkcabbage). They smell like rotting meat and thus draw in the flies that happen to be the pollinators of the plant.

However, we found no indication that the genus Hosta smells that way nor that it is pollinated by flies. It is more likely the too-fresh manure in some recently-installed fertilizer or a small dead animal in your garden is what is attracting flies.