Hackberry Emperor (Asterocampa celtis)

In this guide

1. Description and identification
2. Life cycle: egg, caterpillar, chrysalis and adult
3. Where the hackberry emperor lives
4. Host plants and diet
5. Behavior: perching, chasing and puddling
6. Similar butterflies and how to tell them apart
7. Quick facts
8. Did you know?
9. Pictures
10. Frequently asked questions

Description and identification

The hackberry emperor belongs to the brush-footed butterfly family and, within it, to the emperors — the subfamily Apaturinae, a small group of fast, sap-feeding woodland butterflies whose Old World relatives include the purple emperors. Like every brush-foot it walks on four legs, the front pair reduced to short, hairy brushes. It looks plain at first glance, but eyespots, bars and a hackberry tree overhead identify it quickly.

Adult butterfly

• Wingspan: 1.4–2.5 in (35–63 mm), a medium-sized butterfly with a slightly angular forewing tip.
• Upperside: tan, gray-brown or warm reddish-brown. The forewing carries two or three short black bars in the cell near the leading edge, a scatter of white spots and dashes towards the tip, and — the key mark — a single conspicuous black eyespot, usually with a pale ring, near the outer margin towards the rear corner. The hindwing has a submarginal row of small dark eyespots, typically five or six, some with faint blue or violet pupils.
• Underside: a mottled mixture of gray, lavender and brown that matches lichen-covered bark almost exactly, with the same eyespot pattern showing through more faintly. A butterfly that settles on a trunk with its wings closed can be very hard to find.
• Sexes: contrary to many older accounts, the sexes are distinguishable with practice. Females are noticeably larger, paler and broader-winged, with rounder forewing tips; males are smaller, darker and more sharply pointed at the forewing apex.
• Regional variation: populations differ a good deal. Southern and western individuals tend to be paler, grayer and more strongly marked, and several named subspecies have been described across the range.

How to identify a hackberry emperor in the field

Start with the tree. If a medium tan butterfly is flying fast and erratically around a hackberry, landing head-down on the trunk and shooting out at anything that passes, you are almost certainly watching this species. Then check the forewing: one bold black eyespot near the outer margin, with white spots above it, separates the hackberry emperor from the tawny emperor, its closest relative, which flies in the same woods but is warmer orange-brown and has no forewing eyespot. The question mark and the eastern comma share the bark-like underside but have deeply ragged wing edges and a silver mark beneath the hindwing, while the common buckeye has huge multicoloured eyespots and orange bars on a brown ground.

Life cycle: egg, caterpillar, chrysalis and adult

Everything happens on or under a hackberry. Females rarely leave the tree they emerged from, and the whole cycle — egg, five larval instars, pupa and adult — can be completed on a single trunk. In the north there is one generation a year, in the middle of the range two, and in Texas and the Gulf states often three, with adults on the wing from May into October.

Eggs

The eggs are pale cream to greenish-white, dome-shaped and about 1 mm across, with fine vertical ribs running from base to tip. They are placed on the underside of hackberry leaves, on twigs, or on the bark, usually singly or in small groups of a few to a few dozen — not in the enormous single-layer rafts of several hundred that the closely related tawny emperor builds. They darken to gray shortly before hatching, which takes between four days and a week in warm weather.

Caterpillar

The full-grown caterpillar is one of the most distinctive in eastern woodland: bright green, about 1.4 in (35 mm) long, with pale yellow or white raised bumps and two yellowish lines running the length of the body, a pair of branched, antler-like horns on the head, and the tail end drawn out into two sharp points. It tapers at both ends and lies flat along a leaf midrib, where the green body and yellow lines disappear into the foliage. Young larvae feed side by side in loose groups, sometimes on a lightly webbed leaf, and become solitary as they grow.

The winter is passed as a half-grown larva, and the way it is done is remarkable. In autumn the caterpillars change colour from green to a dull brown that matches dead foliage, crawl into a curled hackberry leaf or drop with the falling leaves, and hibernate in small clusters in the litter at the base of the tree. In spring they climb back up, turn green again within a few days, and resume feeding on the new leaves.

Chrysalis

Pupation takes place under a hackberry leaf or on a shaded twig. The chrysalis is pale green, flattened from side to side and sharply keeled along the back, with a pointed head end and fine yellowish-white diagonal lines and pale dots along the abdomen. Hanging beneath a leaf it reads as a curled leaf blade rather than an insect. The adult emerges after roughly one to two weeks.

Adult

Adults live only about six to fourteen days, so a population turns over quickly, and a good hackberry can produce butterflies continuously for weeks. Males emerge slightly before females in each brood, take up perches on trunks and branches, and are already waiting when the first females appear.

Where the hackberry emperor lives

The hackberry emperor is far more widespread than older accounts suggest; it is not a southeastern speciality. Its range covers most of the eastern and central United States, from southern New England, New York and southern Ontario west across the Great Lakes states to the eastern Great Plains, and south through the whole of the Southeast to Florida, across Texas and into Arizona and New Mexico, continuing south into northern Mexico. It is absent from the Pacific states and from the higher Rockies.

The map is really a map of hackberry trees. Common hackberry (Celtis occidentalis) covers the north and midwest, sugarberry (C. laevigata) the south, and netleaf hackberry (C. reticulata) the dry southwest, and the butterfly follows each of them. Typical habitat is therefore riverside and floodplain woodland, wooded roadsides, forest edges, limestone bluffs, fencerows and — very often — city parks and residential streets, because hackberry is a tough, widely planted shade tree. Colonies can be astonishingly dense: a single large tree in a suburban park may support hundreds of adults at once.

Because the species does not migrate and rarely disperses far, it is genuinely absent from apparently suitable woods that contain no hackberry, and equally it turns up in the middle of towns wherever the tree has been planted.

Host plants and diet

Caterpillar host plants. Hackberry emperor larvae eat only hackberries, the genus Celtis in the elm family. Common hackberry, sugarberry, netleaf hackberry and dwarf hackberry are all used, along with planted ornamental hackberries in gardens and streets. Nothing else is accepted, which makes the butterfly a strict specialist and ties its fate directly to a single group of trees.

Adult diet. Adult hackberry emperors essentially never visit flowers. They feed on sap running from wounded trunks and broken branches, on fermenting fallen fruit, on animal dung and carrion, and on damp soil, mud puddles and moist gravel, taking up salts and dissolved minerals. They are also notorious for landing on people, walking over an arm or the brim of a hat and drinking perspiration for its sodium — a habit that makes them one of the easiest butterflies in North America to photograph at close range. Our guide to what butterflies eat covers this nectar-free way of life in more detail. The claim, repeated on many websites, that they feed on hackberry flower nectar is a misunderstanding: hackberry flowers are small, green and wind-pollinated, and offer no useful nectar at all.

Behavior: perching, chasing and puddling

Flight. The flight is fast, low and erratic, with sudden changes of direction, short glides on flat wings and abrupt landings. It is quick enough that an alarmed hackberry emperor is usually out of sight in a second, and it is one of the reliable field marks of the species.

Territorial males. Males perch head-downwards on a tree trunk, a low branch or a leaf in a sunlit gap, often only a few feet above the ground, and watch. Anything that flies past — another butterfly, a bee, a dragonfly, a falling leaf, a person's hand — triggers a rapid pursuit, after which the male returns to the same spot. Several males may hold adjacent perches on one trunk. Late morning and afternoon are the busiest hours.

Landing on people. This is the behaviour most people remember. Hackberry emperors will settle on clothing, skin or a camera and probe with the proboscis for salt, and they often stay put for minutes at a time. They cannot bite or sting, and the interest is purely in sweat and mineral traces.

Puddling and group feeding. Groups gather at damp gravel, at the edge of puddles, at sap flows and at rotting fruit beneath the tree, sometimes with red-spotted purples, mourning cloaks and question marks, all of which share the same taste for fermenting material. Several species feeding together at one sap run is a common sight in eastern woods in midsummer.

Defence. The butterfly relies on speed and on camouflage: the bark-coloured underside makes a resting adult very hard to see, and the eyespots along the wing margins draw a bird's strike away from the body, so worn individuals often have neat beak-shaped notches out of the hindwing edge. Unlike the meadow-loving pearl crescent, which spends its life among flowers in the open, the emperor stays in the dappled light of the canopy and its edges.

Similar butterflies and how to tell them apart

Quick facts

Did you know?

• Hackberry emperors are among the few butterflies that routinely land on people. They are after the sodium in sweat, and a hiker standing still in a hackberry grove on a hot afternoon may collect several at once.
• The caterpillars change colour twice a year. Green in summer, they turn brown before hibernating among dead leaves, then turn green again within days of climbing back into the canopy in spring.
• Because they feed on dung, carrion and fermenting fruit rather than nectar, hackberry emperors do almost no pollinating, but they are efficient recyclers of nutrients on the forest floor.
• The eyespots along the wing margins are cheap insurance. A bird aiming at a ring of colour takes a piece of wing instead of the body, which is why so many worn adults fly perfectly well with a neat bite missing.
• The butterfly has one of the tightest host relationships in North America. Plant a hackberry in a bare yard within its range and the species will usually find it within a season or two.

In summary

The hackberry emperor, Asterocampa celtis, is a tan to gray-brown brush-footed butterfly of eastern and central North America, identified by the single black eyespot near the outer margin of each forewing, a row of smaller eyespots on the hindwing and white spots towards the wingtip. Its wingspan is 1.4 to 2.5 inches, its bright green, horned caterpillar feeds only on hackberry trees, and it overwinters as a brown half-grown larva in curled dead leaves. Adults live under two weeks, fly fast and erratically, perch head-down on trunks, take no nectar at all, and are famous for settling on people to drink sweat.

Hackberry Emperor butterfly pictures

Frequently asked questions about the hackberry emperor butterfly

What does a hackberry emperor butterfly look like?

The hackberry emperor is a medium tan to gray-brown butterfly with a wingspan of 1.4 to 2.5 inches. Its forewing shows short black bars, white spots near the tip and one bold black eyespot near the outer margin, and the hindwing carries a row of five or six smaller dark eyespots. The underside is mottled like lichen-covered bark.

What does a hackberry emperor caterpillar look like?

It is a bright green larva up to about 1.4 inches long. It has pale yellow raised bumps, two yellowish stripes along the body, a pair of branched antler-like horns on the head and two sharp points at the tail. It lies flat along a hackberry leaf, where its green body is almost invisible.

How does the hackberry emperor survive the winter?

It overwinters as a half-grown caterpillar. In autumn the larvae turn from green to dull brown, crawl into curled hackberry leaves and hibernate in small clusters in the leaf litter at the base of the tree. In spring they climb back up, turn green again within days and finish feeding on fresh foliage.

Why are hackberry emperor butterflies not found on flowers?

They do not drink nectar. Hackberry emperors feed on sap oozing from wounded trees, fermenting fallen fruit, dung, carrion, damp soil and human sweat, so they have no reason to visit blossoms. Hackberry trees themselves are wind-pollinated and their small green flowers produce no nectar worth collecting.

Where do hackberry emperors live?

They live throughout eastern and central North America, from southern Ontario and New England south to Florida, west to the Great Plains, Texas, Arizona and northern Mexico. They occur wherever hackberry trees grow, including floodplain woods, forest edges, wooded roadsides, limestone bluffs, city parks and planted residential streets.

Can you tell the difference between a male and female hackberry emperor?

Yes, with practice. Females are noticeably larger, paler and broader-winged, with rounded forewing tips, while males are smaller, darker and more sharply pointed at the forewing apex. Behaviour also helps: males perch head-down on trunks and dart out at passing objects, while females fly quietly through the canopy looking for egg sites.

What does the chrysalis of a hackberry emperor look like?

The chrysalis is pale green, flattened from side to side and keeled along the back, with a pointed head end and fine yellowish-white diagonal lines and pale dots along the abdomen. It hangs beneath a hackberry leaf or a shaded twig, where it looks like a half-curled leaf blade, and the adult emerges in one to two weeks.

What do hackberry emperor caterpillars eat?

They eat only the leaves of hackberry trees in the genus Celtis, including common hackberry, sugarberry, netleaf hackberry and dwarf hackberry, plus planted ornamental hackberries. No other plant is accepted, so the butterfly cannot survive anywhere its host tree is missing, which is why colonies are so sharply tied to individual trees.

Why do hackberry emperors land on people?

They land on people to drink perspiration for its salt. Sodium is scarce in a diet of sap and rotting fruit, and males in particular need it, so they probe skin, clothing, hats and camera straps with the proboscis. They cannot bite or sting, and the sensation is a faint tickle at most.

How do you tell a hackberry emperor from a tawny emperor?

Look at the forewing. The hackberry emperor has one clear black eyespot near the outer margin and a scatter of white spots towards the tip; the tawny emperor has neither, and is a warmer, more uniform orange-brown. The tawny emperor also lays eggs in large single-layer clusters of hundreds, while the hackberry emperor lays singly or in small groups.