There is the similar observation set field, though I think that is usually used for multiple observations of the same organism, I have used it for a series of observations of toads in the same pond during their lifecycle.
See: Using the field “Similar Observation Set” for linking observations of lepidoptera when raising on
I’ve seen projects use observation fields to record this kind of extra info but that would be a single observation of a butterfly with a field like “feeding on = Buddleja davidii” rather than linking to the observation which it is feeding on.
I think the ideal would be “feeding on = observation id” and then have it display the community id of the plant it was feeding on (but I’m fairly sure that’s not possible currently).