Purple Milkweed Flowers

Purple Milkweed, Asclepias purpurascens, has finally started flowering! I've been growing this plant for three years and finally it blooms! I initially bought three of them, of which only one survived. As it turns out this species is a little finicky to grow. This one is growing in partial sun/mostly shade in ....uhh average, leaning more toward damp, soil. I have tried to start this plant from seed, which germinated, and I then planted them all over, only to have them all die over the winter time. I don't know what it is about this one plant or this one spot but I'm happy it's thriving. That said, this milkweed species isn't self compatable, so it will never pollinate itself and won't produce seeds until I manage to get another one established and flowering. 

No sooner does it start blooming, that Odorous House Ants, Tapinoma sessile, start raiding the flowers for nectar. I've noted how this ant species seems to specialize in robbing nectar from flowers in the past, and I'm quite annoyed they've chosen this plant to steal from. The flowers to Purple Milkweed are actually larger than those of Asclepias tuberosa. I've never seen these ants going nuts like this over milkweed flowers. Usually it's just a few workers, but this plant at times had almost five ants per flower! It suggests to me that this species produces a higher quality of nectar more deserving of insects that may actually pollinate it.