Abstract

Pear psylla, Cacopsylla pyricola (Foerster) (Hemiptera: Psylloidea: Psyllidae), is a western Palearctic species now present in all pear-growing regions of North America following its presumed arrival in the early-1800s. Pear psylla likely arrived in North America on imported pear trees or scions and then spread by dispersal of adults or by dissemination of infested nursery trees. This synthesis assembles a scattered historical literature to summarize arrival and spread by pear psylla in two growing regions: eastern North America, comprising the eastern seaboard and Great Lakes areas; and a western region extending from British Columbia into southern California. The geography and timeline of new detections are summarized, followed by an evaluation of when and where pear psylla possibly arrived in eastern and western regions. Patterns of spread differed between regions: slow and patchy in the East, while rapid and contiguous in the West. Reasons for these differences are examined. Pear psylla likely arrived in both regions earlier than assumed. Historical patterns in movement of imported pear trees and scions were examined to identify possible geographic sources of pear psylla and potential entryways into either region. The paper concludes with a discussion of challenges faced in disentangling the biological, sociological, and environmental factors that predispose an insect pest for establishment and spread outside of its natal region, particularly when the species in question – like pear psylla – arrived well in the past.