The "fern ball" is a structure found at the end of a Christmas fern frond. It is made of the terminal leaflets (pinnae) that have been rolled up and tacked together by silk.
|A mysterious "Fern ball"|
Over at BugGuide there is a single photograph of a fern ball posted in their "unidentified leaf shelter" category.
|Caterpillar frass inside fern ball|
We collected three fern balls today and all contained quantities of frass, but not caterpillars. The caterpillar occupant had left the house.
I consulted an online database of lepidopteran food plants and found only a single species recorded as having fed on the Christmas fern (Polystichum acrostichoides), namely, a moth, Herpetograma aeglealis. Googling this name led, indirectly, to a recent paper describing a new species of Herpetogramma, H. sphingealis.*
The following quotation from that paper reveals a University of Georgia connection:
Larvae of a species of Herpetogramma were reared on Christmas fern in Athens, Georgia (Ruehlman et al. 1988)**; the species was identified as H. aeglealis, but through the courtesy of Dr Matthews, one male specimen was loaned to the CNC, and a complete dissection including the vesica was prepared by Don Lafontaine, and it proved to be a specimen of the new species. All specimens for that study were determined to be a single species, due to their larvae, habits, host plant, and appearance of the adults, we believe they all belong to Herpetogramma sphingealis and not to H. aeglealis. According to these authors, the larvae are solitary leaf rollers and live on the terminal leaflets of young fronds, passing the winter as eggs, emerging at the beginning of the spring and eating the fronds within a silk shelter for around a month, and then pupate to emerge about 12 days later.
So the caterpillar we never saw is likely to be the newly described moth species Herpetogramma sphingealis (there is no common name).
* Handfield L, Handfield D (2011) A new species of Herpetogramma (Lepidoptera, Crambidae, Spilomelinae) from eastern North America. In: Schmidt BC, Lafontaine JD (Eds) Contributions to the systematics of New World macro-moths III. ZooKeys 149: 5–15. doi:10.3897/zookeys.149.2344
** Ruehlmann TE, Matthews RW, Matthews JR (1988) Roles for structural and temporal shelter-changing by fern-feeding lepidopteran larvae. Oecologia (Berlin) 75: 228–232. doi:10.1007/BF00378603]