I'm glad Brian raised the issue, as the source has never been identified
(on BEE-L).

From BEE-L archives:
"Dick Allen asked:
In late summer or early fall workers are often seen with a yellow
marking on their thorax that looks as though someone had marked them.
What is that marking showing up on workers at the end of the season? 

Mike Palmer answered:
My bees get a white stripe on the dorsal side of their thorax. I
affectionately call them 'stripers'. I've always thought they got it
from working chicory.  The way the bee gets nectar from the chicory
causes the pollen to be deposited on the thorax.  I could be wrong of
course."

This is the time of year that the 'stripers' show up.  As far as I know,
we have never definitively identified the source.  Mike speculated
Chicory (Cichorium intybus).  I often wondered if it might be Touch me
Not, also called Jewelweed (Impatiens capensis), which is blooming
abundantly in these parts right now.

McGregor writes about chicory, "The composite 1 1/2 inch flower opens
early in the morning (5:30 to 7:30 a.m.) and closes about noon
(Dinakaran and Sundaraj 1960)."  I have seen the "stripers" late in the
afternoon, so I would be inclined to rule out chicory.  Jewelweed is not
listed in McGregor, so I would be inclined to rule that out too,
although I found the following passage in cyberspace: "Spotted Jewelweed
is often visited by nectar-loving animals, such as hummingbirds, bees,
and butterflies", so perhaps it's still in the running.  At the same
site I found this warning and interesting tidbit: "While Jewelweed needs
moisture to grow, it often grows alongside another plant that grows just
about anywhere: Poison Ivy. The sap of jewelweed is even used to relieve
itching from Poison Ivy rashes".  Nature never ceases to amaze me!

Bottom line: the culprit painting our honeybees remains at large and
unidentified!  Does anyone know who the culprit might be?

Aaron Morris - thinking mysteries abound! 

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