With plants stinging nettle releases HUGE amounts of formic acid. Sometimes it appears to hang off the stems like dew. I remember one day I was out hiking, saw this plant that looked like it had alot of dew on it and my hands were sticky with pine sap so I RUBBED MY HANDS ON IT. It felt like I had just stuck my hand into a fire ant nest.
ANyways, back on topic, if you're trying to controll varroa all I use is a SBB and powdered sugar.