Fern Leaf Rollers

Welcome back to Ask a Naturalist: your own personal “Google” for information on all things natural on PEI.  I’ve been getting quite a few questions about the rolled-up tips of fern leaves that are appearing around the Island now (Photo 1), so let’s take a look!

Photo 1: A typical rolled-up tip of a fern frond (Cinnamon Fern in this case) that's the sure sign of Leaftier Caterpillars.

At first glance these may look like fiddleheads, but they’re on the ends of mature fronds rather than young plants and – unlike fiddleheads – they’re spherical, not flattened.  These artfully rolled balls are the beautiful work of native Fern Leaftier caterpillars (Herpetogramma spp.).

These caterpillars overwinter in leaf litter and soil, emerging in spring to climb to the tips of nearby fern fronds.  There, each one starts folding a frond into a tight, cozy shell that protects it from weather and predators such as birds and parasitic wasps.  This shelter is so important to the caterpillar’s survival that it’s worth the tremendous expenditure of energy to build it; caterpillars actually lose weight during the construction process. A caterpillar that can’t find a good fern on which to build its temporary home is able to delay pupation rather than pupate in a more exposed location.

Photo 2: If you unroll a ball, you'll find frass (aka insect poop) as shown here. If you're lucky, you might also find the caterpillar that made it.

Once protected inside its leaf ball, the caterpillar feeds on the inner layers of frond leaving behind tell-tale ‘frass’ (aka insect poop, Photo 2).  If you’re lucky, you may find the yellow or green caterpillar inside before it pupates into a Herpetogramma Moth (Photo 3).

Photo 3: The adult moth that evetually emerges from those fern balls.

Each caterpillar makes more than one leaf ball as it develops, and so it’s not uncommon to see several balls on the same fern.  All this leaf rolling doesn’t harm the plant: only a few fronds on any one fern are in use, and only part of each frond is folded into a caterpillar home.  That leaves lots of surface area for the fern to continue photosynthesis and feed itself.

Weather-proof protection from predators is valuable real estate. As each leaf ball is vacated by the caterpillar, it may get taken over by other invertebrates (particularly spiders) similar to how secondary cavity excavators such as Owls take over old Woodpecker holes. In this way, Fern Leaftier caterpillars contribute to the ecology and diversity of the areas they inhabit.

There are dozens of species of Herpetogramma moths worldwide, but only three have caterpillars known to make these distinctive fern balls. Of those, only one – Herpetogramma theseusalis – has been confirmed on PEI, although other species could be present here. There doesn’t seem to be much fern specificity among the caterpillars. The leaf balls shown here were on Cinnamon Fern, but I’ve seen them on Interrupted, Ostrich, and Bracken Ferns as well.

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