There are several insects that will munch on thistle, as well as a few insects that use thistles as an alternative to the prefered plant.

Here is a link that might be useful: Thistle munchers

Thanks but I don't think that is it unless these are eggs. I wonder why only the thistle and only the stems? I'll get a picture as soon as I can get outside. :)

There is a thistle aphid, Brachycaudus cardui...check out the link below.

Here is a link that might be useful: thistle aphid

That would be it. I wonder why I haven't seen them in the past and we have had 2 years of rather soggy summers. I thought aphids like it dry?

Aphids come, aphids go. It may just be a good year for them or they finally found your plants.

honalee, you must be thinking about spider mites (liking it dry...so they say). Aphids thrive in all kinds of weather, climates, temperatures, humidity levels, etc.