While walking in the Jurassic Garden at the South Carolina Botanical Garden, I noticed the ends of many fern fronds clearly engineered to make globe-like structures. When I took a close-up of this globe, I noticed delicate strands of silk crisscrossing the surface and holding the fonds closed tightly. What could be inside?
After some research, I discovered this is called “leaf-tier” behavior, which is the practice of moth caterpillars. At least three native moth species employ this defensive/feeding strategy on ferns. Inside the fern ball, the caterpillar safely lives its life protected from parasitoids. They feed on the fronds, then metamorphose into an adult. When I carefully opened two of these structures, I discovered a green caterpillar in one and a tiny adult in the other, and both were filled with large amounts of frass (poop). Despite their activity, there was very little damage to the fern, and the overall plants were healthy. Researchers found these globes do double duty. Once the moth leaves, the shelter often becomes a spider home. It is simple to remove the spheres, but why do so when the caterpillars do so little damage, the globes look so interesting, and the moths and spiders feed the birds?
For more information, see BYGL Oddball Fern-balls.