The spicebush swallowtail caterpillar is one of the most striking insects you can raise at home. Its later instar form looks so convincingly like a small snake that even people who know it is a caterpillar do a double take. The false eyespots, the inflated head end, and the way it tucks into a rolled leaf for shelter all work together to make one of nature’s best bluff displays.
This guide covers how to identify the caterpillar at different stages, how the snake mimicry actually works, what it eats, and how to raise one indoors through to the adult butterfly stage.
Key Takeaways
- Young spicebush caterpillars look like bird droppings, a common early defense among swallowtail larvae. Older instars shift to a striking green with large false eyespots.
- The eyespots mimic a green snake and are positioned to make the caterpillar’s inflated front end look like a snake head when threatened.
- Spicebush and sassafras are the primary host plants, and without one of these plants you cannot successfully raise this species indoors.
- The caterpillar creates a leaf shelter by folding the edge of a leaf over itself and resting inside, which provides physical camouflage during resting hours.
Identifying the Spicebush Caterpillar
Spicebush swallowtail caterpillars change appearance dramatically through their five instars. In the first and second instars, they are dark brown or black with a white saddle-like marking across the middle of the body. This coloration mimics a fresh bird dropping, which is a remarkably effective camouflage strategy. A small caterpillar that looks like something a bird just deposited on a leaf is unlikely to be investigated further by that same bird.
By the third instar, the caterpillar begins shifting toward the distinctive later-stage appearance. By the fourth and fifth instars, the body is bright green with a yellowish underside. Near the front of the body, behind the true head, the body swells into an enlarged, bulging section. On this swollen area sit two large, round spots with dark pupils and a pale ring, creating the appearance of eyes.
The true head is actually small and tucked somewhat underneath when the caterpillar is at rest. What appears to be the head of a snake in the threat display is actually the inflated thoracic segment bearing the false eyes. The overall body shape, the color, and the eye placement all contribute to the illusion. Compare this with other swallowtail species by checking out this guide to swallowtail butterfly species.
Full-grown fifth instar caterpillars reach about 4 to 5 centimeters in length. In the final days before forming a chrysalis, the caterpillar turns yellow-orange or gold as it prepares for pupation, which can come as a surprise if you are not expecting the color change.
The Snake Mimicry Defense
Mimicking a snake is an extreme strategy for an insect, but the spicebush caterpillar pulls it off with impressive specificity. The species it appears to mimic most closely is the rough green snake, a slender, bright green, non-venomous snake found across eastern North America. The coloring, the false eyespot pattern, and the general shape of the caterpillar’s front end all match this snake reasonably well at the scale of a songbird looking down from a branch.
When disturbed, the caterpillar enhances the display. It draws its legs up, inflates the front segment further, and sways slowly side to side. This swaying motion mimics the movement of an alert snake and adds a behavioral dimension to the visual deception. At the same time, it can evert its osmeterium, the forked orange chemical defense organ tucked behind the head, which adds an additional sensory deterrent.
Whether birds are actually fooled or simply learn to avoid the caterpillar through prior experience with real snakes is a question researchers have investigated. Studies on similar eyespot mimics suggest that naive birds can be deterred on first encounter, giving the caterpillar enough time to escape. Even if a predator eventually figures out what the caterpillar is, the initial hesitation can mean the difference between life and death.
The leaf shelter the caterpillar rests in also contributes to the defense. During daylight hours the caterpillar folds a leaf edge over itself and holds it closed with silk, resting inside the tube. This hides it from aerial predators entirely. The snake mimicry display is the backup defense when the shelter fails or when the caterpillar is found in the open.
What They Eat
Spicebush swallowtail caterpillars feed on spicebush, sassafras, swamp bay, and occasionally other members of the laurel family. Spicebush is by far the most commonly used host across most of the caterpillar’s range in eastern North America. Sassafras is a good alternative and is often more readily available for gardeners who want to grow a host plant.
The caterpillar is not a generalist feeder. It cannot survive on plants outside the laurel family, so having the right host plant is non-negotiable for anyone trying to raise this species. Finding the right plants for each swallowtail species is covered in depth in this guide to caterpillar host plants.
When feeding, the caterpillar consumes entire leaf sections, working from the edge inward. Older caterpillars eat a surprising amount, and a single fifth instar caterpillar can demolish a full leaf in a few hours. In the wild, populations rarely strip a whole spicebush because natural predator pressure keeps numbers in check. In a captive enclosure with multiple caterpillars, you need to keep a steady supply of fresh leaves coming.
Raising Them Indoors
Raising spicebush caterpillars indoors is straightforward if you have access to spicebush or sassafras. The biggest challenge is usually sourcing enough fresh leaves, since a fifth instar caterpillar is a significant leaf consumer. If you have a spicebush in your yard, you can harvest branches and keep them in water to extend their freshness, swapping them out every two to three days.
A simple plastic container with ventilated sides or a mesh enclosure works well for housing. Avoid glass tanks without ventilation since humidity builds up and can cause bacterial or fungal problems on leaves and frass. Line the bottom with paper towels that you can swap out daily to keep the enclosure clean.
When the caterpillar turns yellow-gold, it is entering its prepupal stage and will stop eating within 24 to 48 hours. At this point, add some sticks or rough surfaces to the enclosure at various heights. The caterpillar will select a spot to spin its silk pad and girdle, then form its chrysalis. Spicebush swallowtail chrysalides are typically green or brown depending on the surface chosen and the lighting conditions during formation.
The chrysalis stage takes 10 to 14 days in warm weather, or it will overwinter in diapause if it forms in late summer or fall. In a cool room or a refrigerator set to around 45 to 50 degrees Fahrenheit, you can simulate winter conditions and allow the chrysalis to complete its natural diapause cycle. Bring it back to room temperature in late winter and the adult butterfly should emerge within a few weeks.
Frequently Asked Questions
Are spicebush swallowtail caterpillars rare?
The spicebush swallowtail itself is not rare, but finding its caterpillars takes some searching. They are well-camouflaged and spend much of their time inside their leaf shelters. In areas where spicebush grows abundantly, patient searching will usually turn up caterpillars, especially in the late summer months when populations are at their peak for the season.
Can the caterpillar bite or sting?
No, spicebush swallowtail caterpillars cannot bite or sting. The osmeterium is alarming to look at and smells unpleasant, but it is not toxic to humans. You can handle the caterpillar safely, though gentle handling is better for the animal. If it everts its osmeterium during handling, simply set it down and give it a minute to retract it on its own.
How do I find caterpillars on spicebush in the wild?
Look for rolled or folded leaves held together with silk. The caterpillar’s shelter is the giveaway. Gently unrolling these shelters reveals the caterpillar inside. Early morning is often the best time to look, when caterpillars are resting after nighttime feeding. The shelter is usually made from a leaf on the plant itself rather than a nearby surface.
What does the adult spicebush swallowtail look like?
The adult butterfly is striking in its own right. Males are dark blue-black with a row of pale spots along the wing margins. Females are similar but with blue-green iridescence across the hindwings. Both sexes have orange and blue spots near the tail. The adult mimics the pipevine swallowtail, which is distasteful to predators, gaining protection through resemblance without the chemical cost.
How long does the spicebush caterpillar stage last?
At typical summer temperatures of around 75 to 85 degrees Fahrenheit, the caterpillar stage lasts about 21 to 28 days from hatching to the start of pupation. Cooler temperatures extend this timeline. The caterpillar spends roughly 4 to 6 days in each instar, with the fifth instar being the longest because the caterpillar is accumulating maximum body mass before transforming.
