During the Ohio Department of Natural Resources Teacher Field Day held in Hocking Hills this past Thursday, we went on a walk-about in Conkle’s Hollow State Nature Preserve. The Preserve encompasses one of the deepest gorges in Ohio noted for some of the highest Black Hand Sandstone cliffs found in the state.
Walk-about participants viewed beautiful displays of ferns made all the more interesting by some plants having tightly curled tips. A casual observer could possibly mistake the odd structures for emerging frond fiddleheads.
However, a closer look revealed that the tips of the fronds were not just curled; they were rolled and tied together to form tight ball-like structures. The culprits were so-called Fern Leaftier caterpillars. I first came across these oddball “fern-balls” in 2019 in a landscape planting of ostrich fern (Matteuccia struthiopteris).
Although the caterpillars of Herpetogramma theseusalis (family Crambidae) appear to be one of the most common architects of fern-balls, the scientific literature notes that at least two other members of this genus also practice this unusual leaftier feeding behavior on ferns. Some online references link the moths to specific ferns; however, I have found no research that defines the host ranges of the different moth species. It's possible each moth species feeds on a number of different fern species.
The secretive caterpillars vary from cream-colored to an almost translucent light green. They pull the tips of the fronds together with silk to form a hollow ball structure. The tightly bound fronds provide protection from various enemies including parasitoid wasps targeting both the caterpillars and pupae.
The odd-ball fern-balls were made all the more interesting by rows of exposed fern spore structures known as sori (singular = sorus) on the undersides of the fronds that resembled octopus suckers. The sori were the subject of a brief but fascinating discussion led by one of the teachers about fern reproduction. You can learn a lot going on a walk-about with school teachers; I highly recommend it.
The fern-ball caterpillars feed on the innermost fronds within the structure. They even defecate within their frond-ball-abodes presumably to further conceal their presence from enemies.
The caterpillars construct multiple frond-balls during their development. Finding 2 – 3 silk-bound balls tied closely together is not unusual. However, they appear to cause no significant harm to the overall health of a fern colony. Usually, the vast majority of the fern fronds were unaffected and the caterpillars only damage the tips of the fronds with a significant portion of the affected fern leaves remaining functional for photosynthesis.
Of course, if control is deemed necessary, the caterpillars can be dispatched by simply squeezing the fern structures. However, you should open up a few to see if the fern-balls are in fact still occupied by the moth caterpillars or pupae.
That's because the fern leaftier 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 are important for the survival of a number of different species of spiders.
Selected References
Ruehlmann, T.E., Matthews, R.W. and Matthews, J.R., 1988. Roles for structural and temporal shelter-changing by fern-feeding lepidopteran larvae. Oecologia, 75, pp.228-232.
Morse, D.H., 2009. Four‐level interactions: herbivore use of ferns and subsequent parasitoid–hyperparasitoid performance. Ecological Entomology, 34(2), pp.246-253.
LoPresti, E.F. and Morse, D.H., 2013. Costly leaf shelters protect moth pupae from parasitoids. Arthropod-Plant Interactions, 7, pp.445-453.
Jennings, D.T., Longcore, J.R. and Bird, J.E., 2017. Spiders (Araneae) inhabit lepidopteran-feeding shelters on ferns in Maine, USA. Journal of the Acadian Entomological Society, 13.
https://acadianes.org/journal/papers/jennings_16-3.pdf
Morse, D.H., 2017. Where should I lay my eggs? Oviposition choices of a shelter‐building moth and the shifting danger of being parasitized. Entomologia Experimentalis et Applicata, 165(1), pp.1-8.
Morse, D.H., 2021. Spider use of caterpillar shelters. The Journal of Arachnology, 48(3), pp.284-287.
Photo: Joe Boggs
Photo: Joe Boggs
Photo: Joe Boggs