In the European lineage of Lymantria dispar the female is too heavy to fly and stays near her cocoon, laying a mass of several hundred eggs covered in buff-coloured hairs. Adult dispersal is therefore limited to males, which do not carry eggs. The species' spread depends instead on the first-instar larvae.
On hatching, the tiny caterpillars climb to exposed twigs, release a strand of silk and hang from it. Wind catches the thread and lifts the larva, a behaviour called ballooning, carrying it typically tens to hundreds of metres and occasionally much farther. This is short-range compared with human-assisted movement of egg masses on vehicles, firewood and outdoor equipment, which is what drives long-distance spread.
The North American population traces to insects brought to Massachusetts in the 1860s for silk experiments and released or escaped. Outbreaks defoliate millions of hectares of oak forest in some years, and populations collapse periodically when a fungal pathogen and a nuclear polyhedrosis virus take hold. Predicting those collapses remains difficult. The common name was changed from the earlier ethnic slur in 2022.