Fern-balls aren't the only oddball leaf structures appearing in Ohio landscapes and forests. The odd looking "leaf-purse" handiwork of caterpillars of the hydrangea leaftier moth (Family Tortricidae; Olethreutes ferriferana) may be found on wild and cultivated hydrangeas.
Individual caterpillars apply silk along the edges of two newly expanding hydrangea leaves to cement or tie the leaves together. This creates a purse or envelope-like structure that surrounds both newly developing leaves and flowers.
The caterpillars then feed upon the leaves and flowers enveloped within these protective structures. The leaf structures created by this leaf-tier caterpillar tend to occur near the tips of plant stems and may be very obvious.
The two tied leaves fail to fully expand and become dark green, wrinkled and gnarled; the structure may superficially resemble a plant gall. Opening the tied leaves will reveal the caterpillars housed within silk littered with dark green frass pellets. The light green semi-transparent caterpillars have shiny black head capsules and a black thoracic shield on top of the segment just behind the head.
If you open some of the current leaf-nests, don't be surprised if you find a spider residing in the leaf-abode. As I noted in my "Oddball Fern-Balls" Alert, leaf shelters constructed by lepidopterous larvae play an important role in forest ecology by sheltering spiders. The spiders do not kill the original caterpillar tenant; however, they may quickly once the lep-architect vacates their leaf structures.
In fact, it appears that most if not all of the caterpillars have completed their development in southern Ohio. I've only found empty leaf nests over the past several days with the occasional pupal skin hanging from the leaf-purse structure. The moth only has one generation per season, so no new leaf-nests will appear. Of course, the current leaf structures will remain evident for the rest of the season.
The hydrangea leaftier appears to cause no harm to the overall health of its namesake host despite noticeably reworking normal leaf architecture. I've never seen damage to wild or cultivated hydrangeas progress beyond the "oddity category" by simply affecting plant aesthetics.
On the other hand, if control is desired, populations can be reduced earlier in the season when caterpillars are active by squeezing the leaf structures to mash the caterpillars. The leaf structures shield the caterpillars from direct exposure to a topical insecticide and there is no data on the efficacy of systemic insecticides.