I first saw one in a Kyoto garden, a jeweled creature moving with deliberate grace along a hackberry branch. It wasn't just a caterpillar; it looked like something a medieval illuminator would paint in the margin of a sacred text. This is the larval form of Sasakia charonda, the Japanese Emperor or Great Purple Emperor butterfly. But let's be honest, the butterfly, while majestic, gets all the press. The caterpillar's story is where the real drama is. It's a life of extreme specialization, hidden in the canopy, entirely dependent on one family of trees. Get that relationship wrong, and you won't find one, let alone raise it. This guide cuts through the basic facts you can find anywhere and dives into the gritty, practical details of this caterpillar's existence—details I've learned the hard way through years of observation and more than a few failed rearing attempts. Most sites will tell you it goes from egg to caterpillar to pupa to adult. That's true for every moth and butterfly. The Emperor's calendar is what's fascinating. It's a masterclass in timing. The adult butterflies lay eggs in late summer, typically August. The tiny caterpillars hatch, eat just enough to grow a little, and then… stop. They don't build a nest. They find a crevice in the bark of their host tree, spin a tiny silken mat, and go into hibernation. Right there on the tree. As first instar larvae. They spend the entire winter exposed to the elements. Come spring, when the hackberry buds break, they wake up. This is their first full meal in months. They then progress through four more molts (instars), each stage more impressive than the last. It's easy to misidentate. Here’s a quick breakdown of what to look for as they grow: The final instar is what people dream of seeing. After a few weeks of intense feeding, it will stop, empty its gut, and begin a restless wandering phase. This is the pre-pupation walk, and it's a critical behavioral cue most people miss. Here's the first major point of failure for enthusiasts: the host plant. You'll read "hackberry" and think that's it. The relationship is far more precise. The Japanese Emperor Caterpillar feeds only on plants of the genus Celtis, primarily Celtis sinensis (Japanese hackberry) and occasionally Celtis jessoensis. It's not a preference; it's a biochemical necessity. The caterpillar's digestive system is tuned to this specific plant's chemistry. I made the mistake once of trying leaves from a Chinese hackberry (Celtis sinensis var. ...) grown from foreign seed stock. The caterpillars refused to touch it. They'd rather starve. This level of specificity is why they are so vulnerable. You need to find a mature, healthy hackberry tree. Not a young one. The older trees have a denser canopy and a different leaf chemistry that the later instars seem to require. Look for trees in: The loss of these mature trees to development is a direct hit to their population. Forget walking through a forest and spotting one on a leaf at eye level. It doesn't work like that. Finding them is an exercise in indirect observation. Timing is everything: The best window is late May to late June in central Honshu. This is when the 5th instar caterpillars are large, active, and feeding high up. Look down before you look up: The most reliable sign is frass (caterpillar droppings). The frass of a large Emperor caterpillar is distinctive—dark green, pellet-like, about the size of a match head, and it accumulates in noticeable quantities on the ground or on lower leaves beneath a feeding site. Find the frass, find your tree. Scan the canopy: Use binoculars. Look for notches eaten out of the mature leaves at the top of the hackberry tree. The caterpillars are masters of camouflage, but their feeding damage is a dead giveaway. On a sunny day, you might catch the gleam of their bright green bodies. It's a hunt. It requires patience. The reward—seeing that living jewel in its natural habitat—is worth every minute. Rearing Sasakia charonda from caterpillar to adult is considered a pinnacle achievement among Japanese lepidopterists. It's also incredibly difficult and ethically fraught. I don't recommend it for casual hobbyists, but understanding the challenge is instructive. The Non-Negotiable Requirements: Frankly, the mortality rate is high. The ethical view, supported by organizations like the Japanese Ministry of the Environment which lists it as a near-threatened species, is to admire them in the wild and focus on conserving their habitat. Taking a caterpillar from a healthy population often does more harm than good. The Japanese Emperor is a national symbol, but its future isn't secure. The IUCN Red List hasn't assessed it globally, but in Japan, its status is a red flag. Habitat loss is the primary driver. The issue isn't just cutting down forests. It's the simplification of forests. Modern forestry and landscaping favor fast-growing monocultures. Mature hackberry trees are removed. The sunny forest edges they rely on disappear. Their habitat becomes fragmented, isolating populations and making them more vulnerable. Conservation isn't just about planting hackberries. It's about preserving entire woodland ecosystems with a mix of mature trees, sunny clearings, and connectivity. Some local prefectures have specific conservation programs, but it's a patchwork effort. Here are the questions I get asked most often, usually after someone has tried the basics and run into a wall. The Japanese Emperor Caterpillar is a reminder that the most spectacular things in nature are often the most fragile. Its story is tied to a specific tree, a specific place, and a specific rhythm of the seasons. Understanding it goes beyond identification—it's about appreciating a complex, ancient relationship that is quietly unfolding in the treetops, if only we know where and how to look.
What's Inside This Guide
The Five Transformations: A Caterpillar's Year
A Visual Guide to Instars
Instar
Size (Approx.)
Key Identifying Features
Activity & Diet
1st
3-4 mm
Dark brown/black, spiky. Looks nothing like the final form.
Feeds briefly in late summer, then hibernates on bark.
2nd & 3rd
Up to 1.5 cm
Develops green base color. Tubercles (fleshy bumps) start to appear.
Active spring feeders, staying on younger leaves lower in the tree.
4th
2-3 cm
Vibrant green body. Blue tubercles with black hair become clear.
Moves higher into the canopy. Becomes a voracious eater.
5th (Final)
5-6 cm
Stunning luminous green. Four pairs of vivid blue-black tubercles. Distinctive amber-brown head.
Feeds almost exclusively at the top of the tree. Prepares to pupate by late June/July.
The Host Plant Secret: It's Not Just Any Tree
How to Actually Find Them in the Wild
A Realistic Guide to Raising Them (And Why You Might Not Want To)
Conservation Status: Why They're Disappearing
Expert Answers to Your Trickiest Questions
What is the single biggest mistake when trying to find Japanese Emperor Caterpillars in the wild?
Looking too low on the trees. Most guides mention hackberry (Celtis sinensis) as the host plant, which is correct, but they fail to specify that the caterpillars, especially the larger instars, feed almost exclusively in the canopy on mature leaves. You'll rarely find them on saplings or low branches. Your best chance is to look for their distinctive frass (droppings) on the ground beneath large, healthy hackberry trees in late summer, then scan the higher branches with binoculars.
Can I raise a Japanese Emperor Caterpillar on a substitute plant if I don't have hackberry?
No, and attempting this is the fastest way to kill it. Japanese Emperor Caterpillars are obligate feeders on plants in the Celtis genus. They have evolved specific enzymes to process the chemicals in these leaves. Offering lettuce, oak, or even a different species of Celtis not native to Japan (like American hackberry) will lead to starvation. The caterpillar might take an exploratory bite but will not continue feeding. You must source authentic Japanese hackberry (Celtis sinensis) leaves, refreshed daily.
How do I tell a late-instar Japanese Emperor Caterpillar apart from other similar green caterpillars?
Focus on the tubercles and the 'face.' The fifth instar (final stage) has four pairs of fleshy, blue tubercles along its back, each tipped with black hair. The most distinctive feature is the head capsule: it's a striking amber-brown color, looking almost polished, with two short black stripes near the mouth. No other common Japanese caterpillar on hackberry has this combination. The body is a clean, almost luminous green without the stripes or spots common to many other species.
My Japanese Emperor Caterpillar stopped eating and is wandering. Is it preparing to pupate or is it sick?
This is the critical pre-pupation wander. It will leave the host plant, which is a healthy instinct to avoid predators. The mistake is to leave it in a rearing container with leaves at this stage. It needs a deep layer of slightly moist soil or peat moss (at least 15cm deep) to burrow into. If it's just wandering on a flat surface, it will exhaust itself and fail to pupate. Provide the soil, keep it in a cool, dark, and quiet place, and do not disturb it for the entire winter. It's not sick; it's following a 10-million-year-old program.
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