Fall Webworm

Hyphantria cunea — the fall webworm — is one of the most-encountered late-summer caterpillars in North America and a globally significant tree pest since accidental introduction to Europe and East Asia in the mid-20th century. The species is widespread across North America, Mexico, and now Europe (since the 1940s arrival in Hungary), East Asia (since 1979 arrival in Korea, then Japan and China), and parts of South Asia. Females lay egg masses on leaves in midsummer; caterpillars hatch and immediately begin spinning silk together, weaving a large communal web that envelops several leaves at the END of a branch (this distinguishes the species from the eastern tent caterpillar Malacosoma americanum, whose tents are in the CROTCH of branches in spring). The web grows as the caterpillars grow and feed, eventually reaching 1+ meter in length and enclosing entire branch tips. Inside the web, hundreds of caterpillars feed communally on the enclosed leaves; new leaves are incorporated into the web as the caterpillars deplete the existing food. The species feeds on over 600 documented host tree species — one of the broadest diets of any North American Lepidoptera — including cherry, apple, willow, walnut, mulberry, persimmon, maple, and elm. Damage is mostly cosmetic; trees usually recover in the following growing season. Adult moths emerge from soil pupae the following spring and are pure white in the northern subspecies (the northern fall webworm is one of the only completely white moths in North America) and white-with-black-spots in the southern subspecies. The species' invasive expansion across Eurasia since the 1940s has been driven by a combination of long larval-period polyphagy, easy egg-mass transport on shipped nursery stock, and lack of native parasitoid suppressors in introduced range.