The genus Bactrocera (Diptera: Tephritidae) includes ≈70 polyphagous species that are major pests of fruit and vegetable crops. Most Bactrocera species have limited geographic distributions, but several species are invasive, and many countries operate continuous trapping programs to detect infestations. In the United States, California maintains ≈25,000 traps (baited with male lures) specifically for Bactrocera detection distributed over an area of ≈6,400 km² (2,500 miles²) in the Los Angeles area. Although prior studies have used male lures to describe movement of Bactrocera males, they do not explicitly relate capture probability with fly distance from lure-baited traps; consequently, they do not address the relative effectiveness of male lures in detecting incipient populations of Bactrocera species. The objective of this study was to measure the distance-dependent capture probability of marked, released males of Bactrocera dorsalis (Hendel) and Bactrocera cucurbitae (Coquillett) (methyl eugenol- and cue lure-responding species, respectively) within the detection trapping grid operating in southern California. These data were then used to compute simple probability estimates for detecting populations of different sizes of the two species. Methyl eugenol was the more powerful attractant, and based on the mark—recapture data, we estimated that B. dorsalis populations with as few as ≈50 males would always (>99.9%) be detected using the current trap density of five methyl eugenol-baited traps per 2.6 km² (1 mile²). By contrast, we estimated that certain detection of B. cucurbitae populations would not occur until these contained ≈350 males. The implications of the results for the California trapping system are discussed, and the findings are compared with mark—release—recapture data obtained for the same two species in Hawaii.