Tephritid fruit flies below the radar: How sub-detectable populations persist and spread in California

Early, sub-detectable phases of spread are a critical but often overlooked and little understood aspect of invasion biology. Nonnative tephritid fruit flies in California are a key example. Remarkably many tephritid species are considered threatening agricultural pests, sometimes invading aggressively outside their native ranges. To date, 17 nonnative tephritid species across four tropical genera have been found in California since the 1950s. Previous research demonstrates that multiple species have established in the state though often remaining below detection thresholds for years or decades between captures. We hypothesized that early, extended periods of sub-detectability may be more ubiquitous than previously assumed, even in highly invasive insects. Our spatial analysis of a long-term (60+ years) spatiotemporal dataset of tephritid detections in California, collected by the CA Dept. of Food & Agriculture supports the following conclusions. First, despite the tendency for some nonnative tephritid species to be highly invasive, populations in the state usually remain low density and spread slowly. Second, the large-scale pattern of many of these species is likely the result of persistence of small-scale, patchy pockets of individuals. Further analysis will reveal whether species-specific patterns are correlated with local adaptation to their ancestral environment. These findings have important implications for invasion theory and nonnative insect management and policy.