I have tried every thing that has been mentioned on this thread, but one, and that is onion/vinegar idea which I would try. In Australia, they have bottles of vinegar on the beaches to use for jelly fish stings so that might work on Chiggers.
Having said that, the only thing that has worked for me has been oral and topical antihistamines. Even then, scratching the heck out of the bites has helped quite a bit.
Overtime, I do seem to have developed less of a reaction to the bites from these devil spawn.
Later,
Dan