For quite some time I have been documenting butterflies nectaring on a wide range of cultivated and native plants. This effort has helped me gain a better appreciation of which species of butterflies use which plants. Every so often, I encounter a butterfly nectaring on a plant I never realized they visited.
For example, a few days ago my wife and I checked our bronze fennel for black swallowtail eggs and/or eggs. Much to our chagrin, we did not find either. However, my wife did make a fascinating discovery. When she called me over to look at what she had found, I was surprised to see six red-banded hairstreaks nectaring on a bronze fennel’s pale yellow blossoms. 
Although we have been growing bronze fennel in our garden for a number of years, we never considered the well-known black swallowtail host plant a source of nectar for butterflies. Oh sure, we routinely see the blossoms routinely visited by sweat bees and other native pollinators, but never a red-banded hairstreak other butterfly. Yet, here were half a dozen beautiful red-banded hairstreaks so engrossed in sipping nectar they never attempted to fly away in spite of the fact we were standing only a few feet away from them.
A quick check of the literature and Internet failed to uncover any mention of red-banded hairstreaks using the plant as a source of nectar. In fact, most authors simply mentioned it was visited by a number of pollinators; however, none said it was source of nectar for butterflies.
While my wife’s sighting may not be an
important scientific find, it was important to us. It
advanced our understanding of the unbelievably
complex relationships that exist between the plants
and animals that live just outside our backdoor.