The spurious claim is sometimes made that the concern of environmentalists on the ragwort issue concerns the declining Cinnabar moth. This isn't the case. Ragwort is an important plant for a number of reasons and it supports much more wildlife than just the moth. Ragwort Invertebrates are an important part of biodiversity and are much more than a single moth.
There is also the question of the irritating myths and falsehoods which scientifically minded people always dislike anyway.
Specifically on the Cinnabar Moth there are a number of issues to be dealt with the first is the ludicrous myth that a massive crash in moth numbers has caused a plague of ragwort. This is plain errant nonsense. Yes, the moth has declined but it has not crashed and there is no plague of ragwort as a result. Ragwort is not increasing despite all the claims to the contrary.
The other silly assertion is that the Cinnabar moth can survive on just groundsel alone. This shows fundamental ignorance about the moth's ecology. Cinnabar moths lay their eggs in large batches and it is quite true that the caterpillars can and do eat groundsel but the plant is much smaller than ragwort and there needs to be a large number of groundsel plants not just one as can be the case for Ragwort. Furthermore, groundsel is a much more ephemeral plant and doesn't necessarily persist over long periods at a site.
Related to this is the fact that Cinnabar moths , like many insects, exist in metapopulations. These are groups of populations where the moths can interchange and where individual populations become extinct and are recolonised from the neighbouring population. In this system metapopulations composed largely of groundsel using populations, would be more unstable due to the ephemeral nature of groundsel populations and less likely to last.
In fact the decline in the moth is probably caused by the loss if interconnectivity between ragwort populations caused by agricultural intensification. Metapopulation theory, a mainstay of modern population ecology , tends to indicate that the declines occur sometime after the loss of the habitat due to the lags in the system. Extinct sites don't get recolonised so easily and small populations don't get rescued by immigration from nearby sites so easily leading to greater, but delayed decline.