Answer: Based on your description, the caterpillar might be Florida fern caterpillar, which can sometimes be found on greenhouse grown ferns. These can be treated with carbaryl(ingredient in Sevin). A symptom of caterpillar damage is finding frass around the plant, and they usually hide at the base or in the tips of the new leaflets. (On the other hand, it could be something that is not actually infesting the fern and just happened to be there.)
Without knowing what kind of fern you have it is difficult to suggest when to divide it. If this is a houseplant type of fern, you could absolutely divide it now.
If it is a hardy garden type of fern, this is the most stressful time of year to divide it. If you do it now, take special care to prepare the soil deeply with ample organic matter and keep the area well mulched with organic mulch. Then water as needed to keep the soil evenly moist but not sopping wet. Also, keep the area well shaded for the next few weeks while they recover and establish. Good luck with your fern!
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