Tephritid fruit flies as ghost invaders: 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, often invading aggressively outside their native ranges. To date, 17 nonnative tephritid species across four tropical genera have been found in California since the 1950s. Examination of long-term spatiotemporal detection data collected by the CA Dept. of Food & Agriculture has revealed some important properties of this invasion. Many of these tephritid species have established in the state and, contrary to their often extremely invasive behavior, are spreading slowly at ultra-low densities, with populations often remaining below detection thresholds for decades between captures. We hypothesized that early, extended periods of sub-detectability, even with highly invasive insects and heavy post-detection treatment, may be more ubiquitous than previously assumed. We investigated this using a meta-analysis approach. Another crucial facet of the invasion is that each tephritid species is always found first in southern California, then years or decades later in the northern California. To explore how this pattern is possible, we tested large-scale spread hypotheses based on a paradigm of frequent sub-detectability.
Previous research confirmed that the majority of tephritid species that are established in California have individual captures within a few miles of their original detection point but separated by decades. This indicates both large- and small-scale low-density persistence. We tested whether these reocurrence patterns were unique to tephritids using a list of invasive insects threatening California compiled by the Invasive Species Council of California and the UC Davis Information Center for the Environment. We preliminarily found that many other insect species exhibit similarly long periods of sub-detectability, regardless of their degree of invasiveness.
To connect this finding to large-scale spread, we tested three south-to-north tephritid spread hypotheses: (1) de novo (each find is a separate introduction); (2) I-5 (human transportation from southern CA to the Bay Area/Sacramento); and (3) natural spread. We found that natural spread is likely responsible for the majority of the state-scale spatial patterns. However, we also noticed species-specific differences in spread routes (e.g. coast vs. Central Valley). Further analysis will reveal whether these patterns are correlated with local adaptation to their ancestral environment. These findings have important implications for both invasion theory and management, especially for early nonnative species spread behavior.