Splitting the ladies

[bill and Dale]

This not not a species post (although 6 Mourning Cloaks, 8 Cabbage
whites yesterday in N.Guilford) but rather a toss up question for the
group, if that's OK.
Suppose you get a fly-by look at a Lady  (Vanessa sp.) but not a good
enough look to tell which one: American Lady (V. virginiensis) or
Painted Lady (V. cardui ). This question has come up a couple of times
already this season and what I'm asking is which one is more likely at
this time of year? And what are the tricks to ID'ing these two on the
wing through binoculars? In other words would anyone care to comment on
whether we are having, this week, an infrequent invasion of V. cardui or
a typical emergence of the more common V. virginiensis?

Cheers,

Bill Yule