Flower Failures

I have a large dead spot at the front (street side) of my prairie plantings.  Late June last year, I had liatris, gallardia, echinacea and butterfly milkweed all flowering after the evening primrose and calylophous were done.  One of my neighbors had admitted that it was beginning to look good.

June 9, 2012

May 22, 2013.  Note border and lack of plants in the foreground.

Oenothera. Two days after planting a few days prior to being declared dead.

A year later and the prairie garden is well defined, but everything that was in that part of the garden except one aster, plus all of the poppy mallow, gaura and several oenotheras I have added is dead.  Dead dead.  My neighbor did a walk-by weeding yesterday morning.
Flea beetles, I believe, are one culprit.
Have any suggestions?