John, Oxford ragwort has the same dual personality. It damages the livers of horses and cows, but it's essential food for the caterpillars of the cinnabar moth, lovely yellow and black stripey things. I'm now wondering if your 'tansy' is the same plant?
I've heard of a recent arrival here in UK of the 'processional caterpillar' which inhabits oak trees. It does the same thing with its hairs, total agony. (It's called that because they walk along the ground in along line. Very strange!)
As to chainsaws, they make me shudder. I feel one should have the training and the safety equipment in order to use them.
I once badly skinned all my toes on one foot with a rather fierce strimmer. I was wearng flipflops (twit!) The blood was amazing.