I recently received an email from a park naturalist located in eastern Ohio who asked about ferns showing tightly curled, ball-like tips. A casual observer might mistake the odd structures for emerging frond fiddleheads. Of course, as the naturalist noted, it's too late in the season for fiddleheads.
However, a closer look will reveal that the tips of the fronds are not just curled; they are rolled and tied together with silk to form tight ball-like structures. The caterpillar culprits are so-called Fern Leaftiers. 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 that each moth species feeds on a number of different fern species.
The iNaturalist map for H. theseusalis shown below indicates the moth is widely distributed across the eastern U.S., southern Canada, and south through Mexico, Central America, and into northern South America. However, some of the southern reports are tagged with “needs ID,” so confirmation may be questionable.
Opening an “active” fern ball will reveal a secretive fern leaftier caterpillar which varies from cream-colored to an almost translucent light green. The caterpillars are solitary feeders, with each fern-ball occupied by a single individual.
The leaftier caterpillars primarily focus their construction efforts on the frond tips. They use silk to create hollow structures by tying together several leaflets. Sometimes, the leaflets are pulled down and attached to the frond’s midvein to produce a hollow club-shaped structure.
The odd-ball fern-balls may be made all the more interesting by rows of exposed fern spore structures known as sori (singular = sorus) on the undersides of the leaflets that resemble octopus suckers. Indeed, encountering the handiwork of fern leaftier caterpillars can reveal many interesting facets of fern morphology and reproduction.
The fern leaftier caterpillars feed on the innermost leaflets within their ball-like structure. They even defecate within their frond-ball-abodes, presumably to further conceal their presence from enemies.
As the fern leaftier caterpillars consume all of the tightly bound leaflets in their fern-ball, they leave to construct new fern-balls. Thus, they commonly construct multiple fern-balls during their development. Finding 2 – 3 silk-bound balls tied closely together is not unusual. Indeed, we commonly see clusters of fern-balls, which implies a larger caterpillar population than is actually found by opening the structures.
On the other hand, the image below shows multiple fern-balls involving pinnate leaflets rather than just the frond tip. Each structure housed a caterpillar. Based on pure speculation, this variation in feeding habit may be associated with the multiple tips available to the caterpillars on a pinnate fern, or a high population of caterpillars. Or both.
The tightly bound leaflets protect from various enemies targeting the caterpillars and pupae. Their primary nemesis is the braconid wasp, Alabagrus texanus (family Braconidae). However, the immatures may also become meals for other parasitoids and a few predators, including crab spiders (family Thomisidae). This happens if the caterpillars leave a “door” open by failing to firmly bind the leaflets.
The caterpillars spend the winter as early instars in the soil beneath their hosts. The literature notes that the young caterpillars face a high mortality risk owing to predators and environmental challenges.
Fatality rates rise further when they leave the soil in the spring to climb to the tips of the newly emerging fronds to construct their protective fern-abodes. Predation is such a strong threat, the caterpillars will not feed until they construct their ball-like strongholds. Indeed, according to the literature, the caterpillars are so focused on creating their ball-like structures before feeding that they sometimes succumb to starvation.
The fern leaftier caterpillars complete their development in June to early July, and pupae appear in the fern-balls. Moths emerge to lay eggs in July, meaning the current fern-balls are most likely empty.
However, moths lay eggs back onto fern-balls from which they developed. Consequently, the structures are once again used briefly as caterpillar feeding stations in late summer and expanded further until they drop to the soil to spend the winter.
Although the odd-ball fern-balls look weird, they appear to cause no significant harm to the overall health of a fern colony. Usually, the vast majority of the fern fronds are unaffected, leaving plenty of fronds functional for photosynthesis. Even the affected fronds are not a total loss.
Additionally, based on personal observations, localized caterpillar populations commonly rise and fall dramatically from year to year. As noted above, the caterpillars must run a gauntlet of significant threats, and they sometimes fail to make it through.
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. Although spiders may occasionally invade to feast on the developing caterpillars, once the fern-balls are abandoned, the structures can function as protective structures for other spiders. The shelters play an important role in the ecology of eastern forests by serving as protective abodes for a number of species of spiders.
Selected References
Jennings, D. T., Longcore, J. R., & 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
LoPresti, E. F., & Morse, D. H. (2013). Costly leaf shelters protect moth pupae from parasitoids. Arthropod-Plant Interactions, 7(4), 445-453.
https://link.springer.com/article/10.1007/s11829-013-9261-4
Morse, D. H. (2009). Four‐level interactions: herbivore use of ferns and subsequent parasitoid–hyperparasitoid performance. Ecological Entomology, 34(2), 246-253.
53.
https://resjournals.onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2311.2008.01073.x
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), 1-8.
https://onlinelibrary.wiley.com/doi/abs/10.1111/eea.12625
Morse, D. H. (2021). Spider use of caterpillar shelters. The Journal of Arachnology, 48(3), 284-287.
Morse, D. H. (2023). Factors confronting a shelter-building caterpillar at spring emergence. Journal of Insect Behavior, 36(3), 247-257.
https://link.springer.com/article/10.1007/s10905-023-09835-6
Ruehlmann, T. E., Matthews, R. W., & Matthews, J. R. (1988). Roles for structural and temporal shelter-changing by fern-feeding lepidopteran larvae. Oecologia, 75(2), 228-232.
https://link.springer.com/article/10.1007/BF00378603
