there are only a few silver leafed senecios so of course they are the ones with a gaggle of these caterpillars on them! I can see two white dots on the head and some red spots on the body.
It does eat other daisies besides senecio but fireweed does seem to be a favourite - I gather they contain Pyrrolizidine alkaloids. These make the caterpillar unpleasant to taste, and poisonous to birds which would otherwise attack it. The magpie name comes from the colouring of the adult moth. The forward pointing"hairy horns" are one of the distinctive features.