This thesis addressed the potential of endemic predaceous and parasitic arthropods of the Hood River Valley, Oregon to suppress the pear psylla, Cacopsylla pyricola F8rster. Natural enemies adequately suppressed psylla in three of seven unsprayed orchards of differing vegetational settings, orchard age, and size. Relatively few psylla natural enemies dispersed to unsprayed pear mini-orchards, dspite abundant populations on surrounding noncultivated vegetation. Pear psylla natural enemies were more abundant on arboreal rather than herbaceous non-pear hosts. Selective programs of pear pest control based on diflubenzuron were moderately successful in controlling the pear psylla, with natural enemies aiding in suppression in six of fourteen commercial trials of selective programs. In both unsprayed and commercial pear orchards, late-season psylla densities appeared to be best suppressed when levels of natural enemies were high during early-season. Classification analysis confirmed that orchard site, chemical regime, type (mini- or commercial), and season affected proportions of natural enemies and their pear psylla prey. Complexes of natural enemies and pear psylla immatures from commercial orchards where biological control was successfully demonstrated were taxonomically similar. Effective natural enemy complexes in commercial orchards were characterized by earwigs, lacewing larvae, and moderate proportions of pear psylla immatures during early season and Deraeocoris brevis, earwigs, and lacewings during mid-season. Plagiognathous guttatipes (Uhler) or Diaphnocoris provancheri (Burque) dominated effective natural enemy seasonal complexes at each of two mini-orchards. The functional response to pear psylla eggs was measured for five predaceous mirids. Functional response parameters differed among species and their life-stages, but all destroyed large numbers of psylla eggs. Further experimental directions for the implementation of pear psylla biological control are proposed. General investigative strategies include: (1) augment natural enemies on non-pear vegetation adjacent to the target orchard, and (2) modify the orchard habitat to both encourage natural enemy colonization and allow permanent complexes of natural enemies to develop. Specific tactics include: plant hedgerows of filbert or willow, cultivate snakeflies, introduce Anthocoris nemoralis, adjust early season psylla densities with suitable timing and kind of delayed dormant sprays, using a more effective selective psyllacide, and reduce winter pruning to allow development of natural enemies which overwinter in the egg stage. / Graduation date: 1992