They succeeded, but there is always a risk with biological control like any other. Host specificity in a new environment is not known, not even for viruses. It was difficult at first to pin the species down. Even Williamson, an expert on invasive plants, cannot give a straightforward definition of weediness. Weediness is seen differently depending on who is involved.
Agronomists define a weed as a plant that is unwelcome in their fields.
But is this valid. There is evidence that if monocultures are not pursued so rigidly, natural insect predation on pests is enhanced. As yet unfinished experiments in wheat fields with strips of weeds hosting a rich insect biodiversity seem to point support this. Genetic engineering too has a very great potential for attaining a more ecologically sound agriculture. For instance, it could be used to transfer genes from traditional though disused cultivars into the varieties used today.