Insecticide susceptibility and resistance in populations of pear psylla,Psylla pyricolaFoerster, and the mirid predator,Deraeocoris brevisKnight, were evaluated through biochemical and computer simulation analyses. Pear psylla were resistant to azinphosmethyl and fenvalerate, whereasD. breviswere susceptible. Although esterase detoxification activity was higher in pear psylla than inD. brevis, glutathione S-transferase and cytochrome P-450 monooxygenase activities were similar in predator and prey. Sensitivity analyses predicted that high fecundity and low immigration of susceptible individuals into populations selected by insecticides contributed greatly to the development of rapid resistance in pear psylla. Conversely, lower fecundity and high immigration of susceptible individuals contributed greatly to the lack of resistance inD. brevis. Thus, it appears that life history and ecological factors explain resistance in pear psylla and the lack thereof inD. brevisbetter than do detoxification attributes.