I received an e-mail message a couple of weeks ago with images showing the tips of ostrich fern (Matteuccia struthiopteris) fronds rolled into tight ball-like structures. One of the images showed a "fern-ball" opened to reveal a translucent caterpillar.
I visited the site this past Friday and found the caterpillars had pulled the tips of the fronds together with silk to form a protective hollow ball structure. A casual observer could mistake the odd structures for emerging frond fiddleheads.
The secretive caterpillars feed on the innermost fronds within their protective structures. They even defecated within their frond-ball-abodes presumably to further conceal their presence from enemies. Unfortunately, all of the caterpillars had pupated; I could find none to aide in making an identification of the fern-ball culprits.
The ferns were planted in a landscape and despite the frond fern-balls being readily apparent, it was clear the caterpillars were causing no significant harm to the overall health of the fern planting. The vast majority of the fern fronds were unaffected and the caterpillars were only damaging the tips of the fronds with a significant portion of the affected fern leaves remaining functional for photosynthesis.
Caterpillars of at least three native species of moths belonging to the genus Herpetogramma (family Crambidae) practice this unusual leaftier feeding behavior on ferns. They are H. aeglealis (serpentine webworm moth); H. theseusalis; and H. sphingealis. Apparently, these fern leaftier caterpillars construct multiple frond-balls during their development. This may explain why I sometimes found that 2 – 3 silk-bound balls tied closely together.
Although some online references link the moths to specific ferns, I could find no research quantifying the host ranges of the different moth species. It's possible each species may feed on a number of fern species as well as other forest understory herbaceous plants.
H. sphingealis is the newest member of the genus having been described in 2011. The specific epithet refers to the resemblance of the male moths to sphinx moths (family Sphingidae). The describers of the species make an observational case for this moth being restricted to Christmas fern (Polystichum acrostichoides); however, they also make allowances for the possibility this moth can make a living on other ferns. You can read the scientific paper describing the species by clicking this hotlink:
A paper published in 2013 titled, "Costly leaf shelters protect moth pupae from parasitoids" revealed that enemy avoidance is the most likely motivation behind H. theseusalis spending time and energy to create their fern-ball abodes. The researchers used sensitive fern (Onoclea sensibilis) in their studies noting a strong connection in their region of Maine between this fern and H. theseusalis. Field and laboratory studies showed a significant connection between thicknesses of the protective rolled-frond structures and parasitism of the pupae. You can read the entire paper by clicking this hotlink:
Fern leaftier (leafroller?) moth caterpillars aren't the only arthropods to benefit from the odd-ball fern-balls. Apparently, leaf shelters constructed by these and other lepidopterous larvae play an important role in the ecology of eastern forests. Spiders take over the protective leaf-abodes once the caterpillars pupate and vacate the premises. The shelters play an important for the survival of a number of different species of spiders.
A research paper published in 2017 titled, "Spiders (Araneae) inhabit lepidopteran-feeding shelters on ferns in Maine, USA," describes studies done in and around Bangor, ME, on the relationship between fern-feeding nest-making caterpillars and spiders. H. theseusalis on royal fern (Osmunda regalis) and cinnamon fern (O. cinnamomeum) was included in the studies. You can read this fascinating paper by clicking this hotlink: