photo by: Dan Coleman

Hackberry emperor butterflies take flight at the entrance to the Roth Trailhead at the University of Kansas Field Station.

A crowd slowed traffic one recent morning on a road north of Lawrence, but it dispersed in a cloud of fluttering wings, as thousands of hackberry emperor butterflies took to the sky. Asterocampa celtis, also known as the hackberry emperor, is common during summer months in the eastern third of Kansas where hackberry trees (Celtis occidentalis) are numerous, but this spring’s brood is exceptional.

“You’ll see it every year, but not to this extent,” says Ryan Rastok, forest health coordinator for the Kansas Forest Service. “This is above and beyond.”

photo by: Dan Coleman

Hackberry emperor butterflies gather to drink from a puddle on a rock near the Roth Trailhead at the University of Kansas Field Station.

The exact cause this season is unclear, but Rastok says such population booms are not unheard of, and they usually result from fluctuations in climate, decreased predator populations and lots of healthy hackberry trees.

While the sheer number of the butterflies can be overwhelming, they don’t pose a threat to the environment. Hackberry trees, around which the life cycle of the hackberry emperor revolves, bear the brunt of the damage, as larvae sometimes devour every last leaf. But most of these hackberry trees survive by drawing on reserves of energy to grow new leaves, and as summer progresses they will host a second brood of the butterflies.

photo by: Dan Coleman

A hackberry tree defoliated by hackberry emperors is pictured inside the gate to the Fitch Natural History Reservation.

Hackberries are considered a native species, but some folks would prefer fewer of these shade-tolerant trees in Kansas oak-hickory woodlands. Rastok has even heard landowners express disappointment when told defoliated hackberries probably won’t die.

Each spring, butterfly larvae from the previous summer emerge from a winter-long dormancy on the forest floor, and climb back up the nearest hackberry to gorge on its new leaves. A month or two later they enter the pupal stage of their life cycle, attaching to the tree in chrysalis form. In a week to 10 days they emerge as male or female butterflies (males are smaller, with darker, more slender wings). Then they mate, and the females lay eggs on hackberry leaves. In a few weeks they will die, and the cycle repeats.

photo by: Dan Coleman

Male, left, and female hackberry emperor butterflies are pictured at the Roth Trailhead at the University of Kansas Field Station.

Each summer is time enough for two generations of hackberry emperors, as eggs laid by the butterflies we see now will be adults by July and August. The eggs of the second generation will hatch, and when winter comes, caterpillars not eaten by predators like raccoons, assassin bugs and spiders will go dormant, to emerge as the first generation next spring.

Rastok calls hackberry emperors “inefficient pollinators,” because the adults rarely consume nectar from flowers. To them, no drink is tastier than hackberry sap, but even if they don’t do much for our gardens and orchards, they can be charming. A need for the sodium in our sweat draws them to humans, and it’s not unusual for arms, shoulders, necks or even noses to become landing spots for the butterflies in places where large numbers are found, such as the Fitch Natural History Reservation, Clinton Lake Overlook Park or Lone Star Lake. While some people are enchanted by this behavior, Rastok reminds that “it’s actually kind of gross because of the other stuff they feed on: carrion, rotten fruit and feces.”

photo by: Ryan Rastok

A screenshot from a YouTube video featuring Ryan Rastok, forest health coordinator for the Kansas Forest Service, with a swarm of hackberry emperor butterflies.

Predators and cars take their toll on the butterflies, especially when they gather on roads to drink from puddles, but Rastok sees an evolutionary strategy aimed at surviving predation through abundance. “So many emerge that they are successful at the species level, even though a lot of individuals don’t survive.”

As summer progresses, he says, one can find all four stages of the butterfly, from egg to caterpillar to pupa to adult, on the same hackberry tree. And while the hordes of early June may be gone, “there are a lot more all around that we’re just not seeing,” he says. “They are in the woods and all around.”

See a video of Rastok discussing the extraordinary butterfly population this year.

photo by: Dan Coleman

A male hackberry emperor butterfly is pictured at the Roth Trailhead at the University of Kansas Field Station.