Maybe they're hover flies not house flies?

"In summer, hover flies are found nectaring on woodland or meadow wildflowers or in gardens alongside butterflies, bumble bees and other flower-dependent insects. Males emerge and mature earlier in summer than females. Hover fly larvae are beneficial to garden crops by eating pests including aphids."

Claire

This post was edited by claire on Sat, Jun 29, 13 at 17:46

Asclepias species seem to attract an incredible diversity of insects. So many insects seem to love the foliage or flowers of milkweed, or the insects that reside there, from slugs, ants, earwigs, spiders, beetles, flies, leaf hoppers, lady beetles, to bees, flies, wasps. Several butterflies and moths like Monarchs, Queens, and Tussock moths use the foliage as a larval food plant.

Flies can be important pollinators. But I admit I go on regular patrols to remove assorted insects from the milkweed - especially those carnivorous insects that will prey on Monarch eggs and larvae, such as earwigs, beetles, and ants. Usually I use a container of soapy water to drown the insects.

I am trying to keep the plants clean and in good shape for the Monarch's arrival.

Thank you, Claire! I think you might be right. It is still unsettling to see "flies" all over my flowers, but if they are the beneficial hover flies, I'm okay with it.

And Terrene, the asclepias plants also attract asclepias beetles...which I dispatch...Have you seen them? They look like giant lady bugs. I've not seen many this year...
Still not sure about the flies...they don't look exactly like the hover flies I found on line, but are smaller than house flies.