35 insect species totally rely on ragwort for food including 7 moth and 7 beetle species;-Another 83 species are recorded as using Common ragwort and often it forms a significant food source. There are then a further estimated 50 species of parasite in turn feeding on those.
On top of those 133 speciesragwort is a significant source of nectar for others including bee species that specialise in feeding on yellow Asteraceae (daisies) and many species of butterfly. Government research shows that of over 7,000 plant species in Britain Common ragwort is the 7th most important nectar-producing plant.(1) Lost of habitat in general is a major problem for UK wildlife. Moth numbers have declined by over a third over the last 30 years and a major cause of this is habitat loss. This has knock on effects on other creatures such as bats and birds which use the insects as food. These pages below document a few of the species affected and some of the science involved in the ecology of ragwort. There are many many more species than this list which is being added to as more material is researched.
References:-
Friends of the Earth Briefing sheet. https://cdn.friendsoftheearth.uk/sites/default/files/downloads/Ragwort%20-%20Problem%20plant%20or%20scapegoat%20-%20Briefing%20Nov%202016.pdf