There’s probably some pessimism due to The Precautionary Principle and history. NZ bird populations are still suffering from the unforseen effects of previous biological control experiments (e.g. stoats introduced by colonists to counter the rabbits introduced by colonists).
Massey University has this to say about those linked weevils and flies introduced to combat Nodding Thistle:
“Three biological control agents have been released in New Zealand to attack this species, but at present they are having only limited success in controlling the weed.”
