Help me identify this pest

hi, I am a totally novice gardener,

i am planting a new mixed border and planted about 100 perennials in april

all are doing great until the leaves of my rudbeckia - 5 @ 2 ltr plants started to collapse 

on inspection they are partly sliced through right at the base - almost like a really sharp knife 

nothing is being eaten on the leafs and no other plants are being touched --- can anyone help me identify and deal with this pest please 

thank you 

brian