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  have been found in California since the 1950s. Previous research utilizing a long-term (60+ years) dataset of tephritid detections, collected by the California Dept. of Food & Agriculture, demonstrates that multiple species have established in the state, often remaining below detection thresholds for years or decades between captures. Although detections occur consistently, particularly for certain species such as Bactrocera doralis, each detection event is a rare data point in space and time, which presents methodological challenges. We developed a spatiotemporal model to overcome obstacles associated with analyzing spatiotemporally rare data. By classifying detection events as related or unrelated to spatiotemporal neighbors, we are better able explore the deeper dynamics, and characteristics of persistence/non-persistence, of these populations. Our preliminary analysis suggests 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 these spatial patterns are correlated with attributes of the anthropogenic landscape.