Factors involved in trap design, trap construction, and trapping protocol in the field were examined to determine their influence on accuracy of an established monitoring program for the carrot rust fly,Psila rosae(F.). Changing the adhesive from Stikem Special to Stiky Stuff did not alter trap efficacy significantly. Catches ofP. rosaedecreased linearly with decreased trap surface area, but reducing coverage of standard-sized traps coated with adhesive by up to 75% did not alter trap efficacy significantly. Lengthening trap inspections from 4 d to 7 or 10 d would have underestimated counts of flies on traps, leading to omissions or delays in spray recommendations. Traps situated near outer perimeters of fields generally captured more flies than traps placed inside fields. This trend was sometimes reversed when crops were young, and catches were equal on inner versus outer traps in a mature field late in the growing season. Traps placed at corners of fields generally captured more flies than those placed between corner traps along field perimeters. Traps coated on both north- and south-facing surfaces captured significantly more flies on the north face in all three flight periods. Based on the results, a modified monitoring program forP. rosaeis described that would be as effective, but more economical to operate than the existing program in British Columbia.