We live in the woods and have lots of Blackberries. Last year I made 23 pints of jam. My DH just loves it. Great find. BTW, I have never heard of packsaddles. This is the only insect listed on Google.
http://bowilliams.com/2007/03/beware...d-pack-saddle/. It seems they are quite large and colorful. They are the caterpillar that turns into the Regal Moth.