Caterpillars are common sights in gardens, forests, and woodlands. They are unusual-looking wildlife that is known for transforming into beautiful butterflies or moths. You can look under leaves, in hedges, among nettles, and in the grass to find them.
A sign you may have caterpillars in your yard is leaves with holes in them. There are numerous types of caterpillars in Pennsylvania. As we cannot list all of them here, we have highlighted the most common caterpillars you can expect to counter in the State.
The most common caterpillars you are likely to encounter in Pennsylvania include:
1. Monarch Caterpillar
The Monarch Caterpillar (Danaus plexippus) transforms into the monarch butterfly, a milkweed butterfly that is a common sight in Pennsylvania. The caterpillars go through five life stages, known as instars. They molt at the end of each life stage with each stage lasting up to five days.
In the first instar, the caterpillar emerges from the egg. They are light green or gray-white with large black heads. They eat the egg casing and feed on milkweed in a circular motion, leaving arc-shaped holes in the leaves. As they age, they develop dark stripes on their green background with small bumps that become their front tentacles.
The second life stage is where the caterpillar develops a white, black, and yellow pattern of bands. There is a yellow triangle on their heads with two yellow bands around the triangle. At this stage they have short hairs over their bodies and the black tentacles start growing.
In the third instar, the caterpillar has distinct banding with longer tentacles. They feed on the edge of leaves and can be 1.5 centimeters in length. After molting they have different banding patterns with white spots on the prolegs, close to the back. They can grow an additional centimeter in the fourth instar.
Their banding becomes more complex in the fifth life stage, with white spots on the prolegs and small front legs close to the head. In the fifth stage, they can grow to 4.5 centimeters in length. In laboratory settings, this caterpillar has shown aggressive behavior in the fourth and fifth instars when food availability is scarce.
2. Eastern Tiger Swallowtail Caterpillar
The Eastern Tiger Swallowtail (Papilio glaucus) is a butterfly that is one of the most familiar species you see in Pennsylvania. The caterpillar eats the leaves of the host pad, resting on a silk pad attached to the leaf with the leaf edges folded over.
This caterpillar is brown in the first three growth stages with a white spot, known as a saddle, on the abdomen. In the fourth instar, they become green with a swollen thorax with two yellow, blue, and black eyespots.
There is a distinct yellow and black stripe between the first and second abdominal segments. The abdomen has light blue spots. They turn dark brown just before pupating, growing to 5.5 centimeters in length.
3. Banded Woolly Bear
The Banded Woolly Bear (Pyrrharctia isabella) transforms into a tiger moth. The caterpillar was formally named in 1797 as the woolly bear or woolly worm. These caterpillars have thirteen segments covered in brown hair in the mid regions and black hair on the posterior and anterior areas. In the light, the brown hair has a red tinge.
The hairs are not venomous and do not brush off, therefore they do not cause inflammation or irritation. Handling the caterpillars is not encouraged as people with dermatitis may experience discomfort caused by the spiny hairs.
The caterpillars feed on a variety of plant species, this includes trees and herbs. As a result, moths can be found anywhere where plants are grown. Folklore states that the amount of brown and black hair on these caterpillars determines the severity of the upcoming winter.
If the caterpillar has a wide brown band, it is believed that the winter will be mild. A narrow brown band means a severe winter. There are other variations to the folklore stories that say the color of the stripes will determine the winter weather with dark stripes indicating a severe and cold winter.
Caterpillars that come from the same clutch of eggs can be very variable in color. The caterpillar’s brown band tends to get wider with each molt. Some believe that the direction in which the caterpillar crawls determines the winter weather. If you see one of these caterpillars crawling south, they are trying to escape a cold winter.
There are numerous festivals throughout the country associated with the woolly bear. In Pennsylvania, there is the Oil City Pennsylvania Woolly Bear Jamboree, which started in 2008 to predict the winter weather.
4. Black Swallowtail Caterpillar
The Black Swallowtail Caterpillar (Papilio polyxenes) transforms into a common butterfly encountered in Pennsylvania. The female butterfly lays pale yellow eggs that hatch into young caterpillars.
The young caterpillars are white and black with a saddle. Older caterpillars are green with black banding. The bands contain yellow spots. These caterpillars absorb toxins from the host plant, making them taste bad when bird predators attempt to eat them.
These caterpillars have an orange forked gland, known as the osmeterium. When the caterpillar feels threatened, the forked gland looks similar to a snake’s tongue, releasing a foul odor that deters predators.
5. Small White Caterpillar
The Small White Caterpillar (Pieris rapae) transforms into a medium-sized butterfly, known as the cabbage white or cabbage butterfly. The caterpillars are better known as the imported cabbageworm. They are blue-green with a black ring around the spiracles and a row of yellow dashes.
These caterpillars can be found resting under leaves. This makes it harder for predators to see them. Their heads are black in the first and second instars. In the third instar, they have yellow on their black heads. There is a dark green-yellow dot behind each eye with the rest of the head remaining black in the fourth and fifth instars.
They are pests to numerous host plants, including kale, cabbage, horseradish, broccoli, and radish. The voracious caterpillars eat the leaves of the host plants, boring into the interior of the cabbage and feeding on any new sprouts.
They adjust their feeding to maintain a constant nitrogen uptake rate. They feed faster in low-nitrogen environments. These caterpillars can cause hundreds of thousands of damage to commercial crops each year.
6. Silver-spotted Skipper Caterpillar
The Silver-spotted Skipper Caterpillar (Epargyreus clarus) transforms into a butterfly that is known as the skipper in Pennsylvania. The caterpillar consumes the leaves of shrubs, trees, vines, and herbs in the pea family.
Caterpillars can be found in swamps and meadows. They are yellow with black stripes. The first thoracic segment is black with a brown shield. Their heads are black or red-brown with two orange spots, that mimic eyes.
All stages of the caterpillar build shelters on their host plant. They build five shelters throughout their growth and development. Each one is constructed based on the caterpillar’s size.
7. Red-spotted Admiral Caterpillar
The Red-spotted Admiral Caterpillar (Limenitis arthemis) is the larvae of a North American butterfly. The caterpillars feed on several tree species, including poplar, willow, and aspen. They are also known to feed on black cherry and yellow birch.
These caterpillars have dark brown heads with yellow-bordered bodies. The older caterpillars are dark brown-olive with white midsections on the dorsal side. The prolegs and legs are red-brown. They hibernate at the start of winter.
8. Hickory Tussock Moth Caterpillar
The Hickory Tussock Moth Caterpillar (Lophocampa caryae) is long and hairy. It is mostly white with some black tufts of hair down the center of the back. There are four hair pencils, two located near the front and two at the rear.
They have black spots on their sides with black heads. Their hairs are known to cause contact dermatitis in humans, particularly those prone to allergies. The hairs are barbed, though they do not cause serious medical conditions. In more than three hundred and fifty cases, the hairs were transferred from the hands to the eyes.
These caterpillars feed in groups of up to one hundred in their early life stages. They are known to skeletonize leaves. As the caterpillars age, they become solitary, growing to 5 centimeters before they pupate.
The caterpillars primarily feed on hickory, walnuts, and pecans. They are known to also enjoy elm, oak, ash, and willow trees.
9. Spicebush Swallowtail Caterpillar
The Spicebush Swallowtail Caterpillar (Papilio troilus) transforms into a green butterfly, which is commonly encountered in Pennsylvania. Female butterflies lay green-white eggs, placing one or two on a spicebush leaf.
The young caterpillars chew through the edges of the leaves to the midrib. They lie on the midrib and produce silk, which contracts and causes the leaf to fold on itself, creating a shelter for the caterpillar.
The caterpillars are brown in the first growth stages. They hide during the day in their folded leaf to avoid predators, coming out at night to feed. In the later growth stages, the caterpillars turn green-yellow before pupation.
The older caterpillars also live in a rolled-up leaf. They have two stages of mimicry. When they are in their early stages, they are dark brown, resembling bird droppings, helping to discourage predators.
In the fourth and fifth growth stages, they turn yellow-green with two black dots. The dots’ location gives the caterpillar a snake-like appearance, which warns off any predators. In the older stages, they are known to rear up and retract the caterpillar’s head to make them look similar to common green snakes.
10. Pearl Crescent Caterpillar
The Pearl Crescent Caterpillar (Phyciodes tharos) turns into the beautiful pearl crescent butterfly. The caterpillars are chocolate brown, black, and white. They are covered in bristles. They feed in groups.
The groups of caterpillars start at the top of the host plant, working their way down. They are commonly seen on roadsides, vacant areas, pastures, open pine forests, and fields where they feed on several aster plants.
11. Giant Woolly Bear
The Giant Woolly Bear (Hypercompe scribonia) is the caterpillar of the giant leopard moth, the largest tiger moth in Pennsylvania. Young caterpillars are orange on their fourth, fifty, ninth, and tens abdominal segments with their first, third, sixth, and eighth abdominal segments being dark brown. There is a mid-dorsal and lateral line running the length of the body.
The caterpillars are covered in black hairs. Fully grown caterpillars can reach up to 7.5 centimeters in length. The older caterpillars are black with red spiracles with shiny black hairs. They feed on several low-growing woody plants.
They only stay on the plant for a short period before moving to the next plant, often a different species. The mixing of plants leads to them absorbing chemicals that they use as a defense against predators.
These caterpillars are nocturnal and are often seen crossing roads during the falls, searching for a suitable site to spend the winter. They are often encountered under leaves and in woodpiles.
12. Great Spangled Fritillary Caterpillar
This caterpillar (Argynnis cybele) transforms into a common North American butterfly. The caterpillars are very selective about what they eat and will not consume milkweed. This caterpillar prefers violets.
They are large caterpillars in a velvet black with black spines. The spines have orange or red bases. They have a bulb-shaped gland under the head that emits a foul odor, that is used as a defense.
In the first growth stage, they look like fluffy balls, hiding in leaf litter close to violet plants. The larvae feed on newly sprouted violet leaves.
13. Banded Tussock Moth Caterpillar
This caterpillar (Halysidota tessellaris) transforms into the banded tussock moth, which is a tiger moth. The caterpillars are covered in long hairs, formed in tufts. They vary from orange to dark gray, or yellow. The long hair pencils are white, black, and/or orange and can be found at the front and rear of the caterpillar.
Their heads are bright orange. They are often seen on the upper surface of leaves. They can grow to 3.5 centimeters in length. Their preferred hosts include ash, blueberry, elm, hackberry, oak, willow, alder, birch, chestnut, grape, hazel, and willow. They do not cause serious injury to the trees.
These caterpillars come in a variety of colors from gray to yellow, rusty brown, to white, or yellow. Their heads extend forward. There is a dark line running down the center of the body.
14. Red Admiral Caterpillar
The Red Admiral Caterpillar (Vanessa atalanta) transforms into a medium-sized butterfly with black wings, white spots, and red banding. The caterpillars grow to 2.5 centimeters in length and vary in color. They are often black with white spots and spines.
In a laboratory setting with various temperatures, they had different colorations. The higher the temperature resulted in bright scarlet pupae and at low temperatures, they were black with a small scarlet area.
These caterpillars prefer feeding on stinging nettle, though they can also be seen on false nettles and other species within the nettle family.
15. Spongy Moth Caterpillar
Spongy Moth Caterpillars (Lymantria dispar) are seen feeding on a variety of trees, including maple, hickory, aspen, birch, oak, apple, basswood, pine, hemlock, and more. They are found in forest areas throughout Pennsylvania.
They grow to just over six centimeters in length. They have five pairs of blue spots that are raised. There is a further six pairs of red spots that are raised on their backs. Their hairs can cause skin irritation in humans.
A female moth can lay up to seven hundred eggs in a single mass, that is light brown with fuzzy patches. You are likely to find egg masses on tree branches, trunks, and on firewood.
After hatching, these caterpillars leave the egg mass, climbing up to the end of a branch. They drop a single strand of silk, dangling from the silk waiting for the wind to blow. This process is called ballooning. The tiny caterpillars have long hairs on their bodies, helping them disperse in the wind.
Once the young caterpillars land on suitable hosts, they start to feed. If they do not land on a suitable host, they climb up and go through the process again. In the majority of cases, the caterpillars travel no further than one hundred and fifty yards from their hatching site.
These caterpillars feed during the day in their early growth stages, turning to night feeding from the fourth growth stage.
16. Ailanthus Webworm
The Ailanthus Webworm (Atteva aurea) transforms into a moth, a common sight in Pennsylvania. The webworm is native to South Florida. They feed on the Allanthus plant, known as the “tree of heaven,” an invasive species sold by nurseries as a yard plant.
The caterpillars form nests on the host plant. They then consume the bark and leaves. The caterpillars have a green-brown stripe down their backs and alternating white and green stripes on their sides.
These caterpillars vary in color from dark black to light brown. Their lifecycle is only four weeks from egg to egg. It is not uncommon to find a large web of caterpillars, which has become a communal web, home to multiple generations in one season. These large webs contain eggs for caterpillars of all stages.
17. Eastern Comma Caterpillar
This caterpillar (Polygonia comma) transforms into a North American butterfly. The eggs are green and lay under the leaves of host plants. The caterpillars vary in color from white or black to light green. They are solitary caterpillars that feed at night.
As the caterpillar ages, they construct leaf shelters to hide in during the day, which is a single leaf pulled together using silk. While these caterpillars are covered with spikes, they are harmless to humans.
18. Polyphemus Moth Caterpillar
This caterpillar (Antheraea polyphemus) is a silk moth larva, emerging as a small yellow caterpillar when they hatch. As they age, they go through five life stages. Each life stage is different. In the last life stage, they are bright green with silver spots on the sides.
They feed heavily on the host plant, growing to ten centimeters in length. Older caterpillars have less prominent hairs. In the fifth life stage, they have a yellow band around the dorsal and lateral areas behind the head.
Also in the last life stage, the males are different from the females. Males have a black pit on the ventral aspect of their ninth abdominal segment, something the females are lacking.
19. Fall Webworm
Fall Webworms (Hyphantria cunea) transform into moths. The caterpillars form webbed nests on the limbs of trees in a variety of hardwoods. This can be seen from late summer and into the fall. They are pests, though they do not harm healthy trees.
The caterpillars vary in color and can range from dark gray with yellow spots to pale yellow. They have two cream stripes on their sides. They grow to 3.5 centimeters in length.
The caterpillars feed inside their tents until their later growth stages. Young caterpillars only feed on the upper surface of leaves. Later they will consume the entire leaf. The caterpillar stage can last up to six weeks.
These caterpillars are unique in that they wiggle vigorously at periodic intervals and in synchrony. How they synchronize these movements when they are distributed over a wide area is unknown.
20. Milkweed Tussock Moth Caterpillar
The Milkweed Tussock Moth Caterpillar (Euchaetes egle) feeds during the late summer. They prefer milkweed and dogbane. They absorb chemicals from the host plants which are used for their defense against predators.
Young caterpillars are gray and slightly hairy. They eat the entire leaf, leaving a lacy leaf behind. They are gregarious until their third life stage. As they age they develop tufts of white, orange, and black hairs. They have blackheads.
As they age they tend to wander more, often seen in groups or alone. Mature caterpillars can be seen from June onwards, growing up to 3.5 centimeters in length. Female moths lay their eggs on milkweeds and dogbane, where the caterpillars feed on older shoots.
The milkweeds and dogbane produce sticky latex that encourages caterpillar feeding. Young caterpillars avoid the veins and eat the rest of the leaves, while older caterpillars eat the veins that supply the latex.
21. Common Buckeye Caterpillar
The Common Buckeye Caterpillar (Junonia coenia) is a butterfly that is often seen in Pennsylvania. The caterpillars prefer plants with a bitter compound, activating their digestive systems.
These are beautiful caterpillars with black backs with light colored markings in white, gray, brown, or beige. They vary considerably. The sides have white markings with orange to red spots.
Their heads are orange-red with black markings on their face. The spines run lengthways down the back and sides. There can be more than seven hairs per segment. There are small spines on the top of their heads.
These are solitary caterpillars that feed in isolation. They are not aggressive and even if there is more than one caterpillar feeding on the same host plant.
22. Yellow Woolly Bear
The Yellow Woolly Bear (Spilosoma virginica) transforms into a Virginian tiger moth. The caterpillars are best known as yellow woolly bears or yellow bear caterpillars. These caterpillars vary in color with tufts of different colored hair.
They feed on low-growing plants, including clover and grass. The caterpillars skeletonize the leaves, enough to cause damage to crops.
23. North American Luna Moth Caterpillar
The North American Luna Moth Caterpillar (Actias luna) is a giant silk caterpillar that is green in color. The caterpillars have sparse hairs. In the first growth stage, they are eight millimeters in length, but in the fifth growth stage, they are nine centimeters in length.
They have colorful dots in magenta or yellow with spots on their sides in the final life stages. Each life stage takes approximately four to ten days with five life stages before they cocoon.
These caterpillars feed on various species of broadleaf trees. The caterpillars do not reach population densities that are enough to cause damage to the host trees.
24. American Dagger Caterpillar
The American Dagger Caterpillar (Acronicta americana) transforms into the well-known American dagger moth. The caterpillars are covered in yellow hair when they are young. As they age, they can be white or pale yellow.
There are thin black hairs on the first and third abdominal segments and a tuft of black hair on the eighth segment. They can grow to five centimeters in length.
While they do not possess venom, there have been reports that the hairs of this caterpillar can cause skin irritation in humans, therefore it is not recommended to handle one of these caterpillars if you encounter one.
25. Hummingbird Clearwing Caterpillar
The Hummingbird Clearwing Caterpillar (Hemaris thysbe) is a moth species. These caterpillars feed on cherry trees, along with cranberries, dogbane, and honeysuckle. They burrow into the soil over the winter.
Female moths lay their eggs under the leaves of the host plant, which take approximately one week to hatch. Caterpillars are plumb with spiky tails. They are yellow-green with some being brown.
They pupate in a thin cocoon in the leaf litter during the cold months and emerge as a moth in late spring to early summer.
26. Eastern Tent Caterpillar
The Eastern Tent Caterpillar (Malacosoma americana) transforms into a tappet moth. These are social caterpillars. Female moths lay eggs in a single bath of up to three hundred eggs. Caterpillars emerge in the spring, chewing out the egg.
Young caterpillars create a silk tent after they emerge from the egg. They expand the tent daily to accommodate their increasing size. Caterpillars feed three times a day, when they emerge from their tent, add silk to their structure and move to feed, before returning to the tent.
These caterpillars are covered in hair. Younger caterpillars are dark with two yellow stripes on their backs. Older caterpillars have a medium white stripe on their backs with two yellow stripes. The body is mottled black, yellow, and blue.
27. Greenstriped Mapleworm
The Greenstriped Mapleworm (Dryocampa rubicunda) transforms into a small moth, known as a great silk moth. These caterpillars go through five growth stages. Each stage has a unique coloration and eating behavior.
Young caterpillars have large black heads and light green-yellow bodies with green stripes. There are two dark green to black tubercles on the second thoracic segment. They molt at around six to eleven days. The second molt is around twelve days after hatching with the third molt being around twenty days.
As they age the black head becomes smaller with a darker stripe. The head changes to yellow. By the last growth stage, the body is yellow-green with white to green stripes. There are two prominent horns on the second thoracic segment and two rows of short spines on both sides of the body.
28. Imperial Moth Caterpillar
The Imperial Moth Caterpillar (Eacles imperialis) can grow to ten centimeters in length. They are orange with black bands and large spines.
At the end of each growth stage, they spin silk into the major vein of a leaf. The caterpillar attaches to the silk, helping its molt. The caterpillar emerges from its old exoskeleton, puffing itself up and hardening. They are known to eat their old exoskeleton from protein.
These caterpillars feed on coniferous and deciduous trees and shrubs.
29. Wild Indigo Duskywing Caterpillar
The Wild Indigo Duskywing (Erynnis baptisiae) belongs to the Hesperidae butterfly family. Young caterpillars are white-orange, changing to pale green-yellow as they age. Older caterpillars have short hairs and small white tubercles.
Their heads are mottled in orange and brown. Mature caterpillars will overwinter and pupate in the spring months.
30. Evergreen Bagworm
This caterpillar (Thyridopteryx ephemeraeformis) is often referred to as the common basketworm or bagworm. It spins a cocoon in its caterpillar stage and decorates it with plant materials. The case grows to over six centimeters and is tapered, and open on both ends.
The young caterpillars are black and turn brown and then tan as they age. They have a yellow line on their thorax and heads and can grow to 3.2 centimeters when fully grown. They thrive in urban areas where there are not many predators.
When they are disturbed, the caterpillars retract their heads into the case. Older caterpillars remain in host trees, dragging their case nearby before they attach for the pupa stage.
Caterpillars feed on leaves and buds of red cedar, arborvitae, cypress, pine, apple, black locust, maple, oak, willow, and sycamore trees.
31. Variegated Fritillary Caterpillar
This caterpillar (Euptoieta claudia) transforms into the variegated fritillary butterfly. The caterpillars feed on the flowers, leaves, and stems of the host plant. They are red with black stripes and white spots.
Some caterpillars have a red middorsal stripe that has white or black oval-shaped spots, which are shown as one per segment.
32. Question Mark Caterpillar
This caterpillar (Polygonia interrogationis) transforms into the popular question mark butterfly that is commonly encountered in city parks, open spaces, and wooded areas in Pennsylvania. Fully grown caterpillars can grow to 3.5 centimeters in length.
They have red-brown heads with short hairs and branching spines. The body is black with yellow or white lines and spots. In some caterpillars the black is obscured by the lines, giving them a yellow appearance. The spines range in color from orange to black, or yellow.
These caterpillars prefer feeding on elm trees.
33. Yellow-collared Scape Moth Caterpillar
The Yellow-collared Scape Moth Caterpillar (Cisseps fulvicollis) was first described in 1818. This caterpillar is brown, black, or yellow with long soft hairs. There are dark stripes on the sides and back that are surrounded by orange or yellow stripes.
The caterpillars, which can grow to three centimeters, feed on sedges and grasses.
34. White-marked Tussock Moth Caterpillar
This caterpillar (Orgyia leucostigma) is common in the late summer months throughout Pennsylvania. They are brightly colored with tufts of hair. They have bright red heads and bodies. There are yellow or white stripes with a black stripe down the center of the back.
There are red defensive glands on the hind end. There are four white hairs on the back, which mimic the external cocoon of parasitic wasps. The hairs can cause an allergic reaction in humans.
They feed on a variety of trees, including apple, cherry, birch, and elm.
35. Sycamore Tussock Moth Caterpillar
This caterpillar (Halysidota harrisii) has an orange-yellow head and body that is covered in hair. They grow to 3.5 centimeters in length. There are two pairs of orange hair pencils and two pairs of white hair pencils at the front of the body. One pair of white hair pencils can be seen at the rear.
In large groups, they can damage sycamore trees. They have also been documented for causing urticaria. Urticaria is a raised itchy rash on the skin, it can be welts, weals, or hives.
36. Snowberry Clearwing Caterpillar
These caterpillars (Hemaris diffinis) grow to 4.5 centimeters in length. They are green with black spots. They have prominent granules that extend over the head. They have yellow collars.
They often have a pointy tail, which is black with a yellow base.
37. Delicate Cycnia Moth Caterpillar
The female moth (Cycnia tenera) lays a clutch of up to one hundred eggs. The caterpillars feed in groups of up to seven caterpillars in their first growth stages. These caterpillars feed on milkweed and Indian hemp.
They are covered in white hair and feed at night.
38. Io Moth Caterpillar
These (Automeris io) are gregarious caterpillars, often seen traveling in a single-file procession over a host plant. They are orange when they first hatch, which turns to bright green with hairs and spines, as they age.
The green caterpillars have two stripes, the upper is bright red, and the lower one is white. They can grow to seven centimeters in length. They crawl to the base of the tree to create their cocoon among the ground leaf litter.