Yellow Petals - If it is slugs, and it looks that way ??? - you could try putting egg shell (a lot of it!) in the pot around the plant. Slugs and snails do not like egg shells as to them it is like walking on glass.
Having said that, if it were slugs, you would find their dead bodies in the pot or next to it (because of the pellets) - I hope you find the culprits !
:D