Some, like the ivy aphid are already in North America, but I'm not sure how much of a dent it actually makes. There's great precedent for successful introduction of biocontrol agents for other invasive species. There's also great precedent for catastrophic failures(especially in instances where there's a native close relative of a target species), so it's definitely something that needs to be very carefully explored and not to be done haphazardly.

Seems a bit unnecessary when you could just dress the ivy up like schoolkids and the yanks would wipe it out in a week

(Yes, we're going to keep making these jokes until yous apathetic cunts DO SOMETHING)