That's a leaf-footed bug (look at the widened
tibiae on the hind legs). There are several species, and as far as know,
they all feed on a variety of plants. They might be attracted to the
fading flowers because of some odor produced that is not there in buds or crisp
flowers (just guessing). I've never had them on Iris, but there is a
species here that loves Cacti, and can become a problem one them.