Yeah I wish you could have a sort of checklist within a single observation field.
https://forum.inaturalist.org/t/observation-field-checklist/12837
My workaround has been to have several observation fields with “visible”/“not visible” options (basically “yes”/“no”) and when I add observations to a project I just select add for all the fields I want to add. It defaults to “visible” (the first option) so that’s easy. I don’t really use “not visible” but it could be used.
Do you mean that the observations already have a host plant observation field that’s incorrect? There’s lots of similar fields so you could pick a different one, say, that doesn’t have any Trillium observations.
https://www.inaturalist.org/observation_fields?utf8=✓&q=host+plant&commit=Search
But I’d think asking the user to correct the observation field would be better. Also, if it’s a field associated with a project you manage, you should be able to edit the fields.