Gasoline, kerosene, alcohol, bleach or any nasty harmful substance will do.
The only trick is to apply it while they are still walking around on you within an hour or two of being infested..I scrub and scrape like crazy with alcohol immediately after being exposed and then take a hot shower. That still won't get 'em all. Contrary to some thinking, you can definitely see chiggers.
Most people apply their favorite OTC stuff the next day or a few days later thinking they are going to kill them. At that point they are long gone. You can only treat the itch after the first day.
They bite within a few hours of hopping on board and after a little blood snack, they are gone. You react to their anticoagulant saliva for the next week or two or more if you are real sensitive.
I got a load Monday night picking wild cherries.
Oh well.