O. leucostigma is a North American species with a wide distribution in the eastern part of the USA and Canada. O. leucostigma is a polyphagous species recorded on more than 160 hosts, including deciduous and coniferous trees, as well as bushes and herbaceous plants including weeds. Outbreaks of O. leucostigma can last 1-4 years and are periodic, with major outbreaks every 20 years. Severe outbreaks have been reported mostly in northeastern USA and eastern Canada, where defoliation of large areas of deciduous and coniferous trees has occurred. Defoliation over several years may result in economic and environmental impact (wood loss, tree death). Impacts have been reported in forests in both the USA and Canada, in Christmas tree plantations (Abies balsamea) in Canada, and occasionally in fruit crops. O. leucostigma appears to be a minor pest in parts of its range. Furthermore, larvae may cause social impact by dispersing, and dropping frass and their allergenic hairs. In its native range, O. leucostigma is regulated by a complex of natural enemies, but control methods are sometimes applied. 

For the purpose of the assessment, the EWG considered all host plants and categorised them into ‘main hosts’ and ‘other hosts’. The likelihood of entry was rated as high for plants for planting (except seeds, bulbs, corms, tubers, rhizomes, tissue cultures, pollen) of main hosts (except Fragaria, Zea mays, Triticum). It was rated moderate for plants for planting of other hosts and for round wood with bark of main hosts. The likelihood of entry on cut branches (incl. Christmas trees), sawn wood with bark, bark of main hosts, and round wood with bark of other hosts was assessed to be low, and on all other pathways was assessed as very low. A moderate uncertainty was associated with most ratings of likelihood. For most pathways, transfer to a suitable host was the main constraint for entry. 

Hosts are widespread in the EPPO region, and suitable climatic conditions exist in a large part of the EPPO region. The likelihood of establishment outdoors was rated as high, with a low uncertainty. 

The magnitude of spread was rated as moderate with a moderate uncertainty. Females are flightless, and dispersal occurs primarily by ballooning of young larvae. Human-assisted spread would be the main mode of long-distance spread. Nevertheless, if new infestations are detected early, measures may be put in place and would limit further spread. 

Impact in North America was assessed as moderate with a moderate uncertainty. Outbreaks of economic significance are periodical and there can be many years between severe outbreaks. In addition, populations causing economically-significant damage have not been documented throughout the current range, although O. leucostigma is still considered a pest throughout North America. If considering only Canada and only outbreak years, impact is higher. 

The potential impact (for the endangered area) in the EPPO region was assessed to be moderate with a high uncertainty. Many potentially suitable hosts and climates are present in the EPPO region. O. leucostigma could cause defoliation in various habitats (such as forests, Christmas trees plantations, amenity trees, private gardens, fruit orchards). In forests, both economic and environmental impacts would be expected. As in North America, there may be fluctuation in impact depending on locations and years. Although natural enemies are present in the EPPO region, it is not known whether they would regulate populations of O. leucostigma. The same natural enemy complex that exists in North America and regulates populations does not occur in the EPPO region.