Quantifying insect dispersal distances from trapping detections data to predict delimiting survey radii

The spread potential of invasive pests is a major concern for delimitation, quarantine and eradication efforts. We analysed trapping survey detections data for five insects and one low-dispersing mollusc to quantify 30-day dispersal kernels (mean total distance [MTD], m). We hypothesized that MTD would increase with species' reported diffusion coefficients (D, m(2) per day), and that D could be used to predict containment radius lengths for delimiting surveys for exotic pests. We collected trapping data for the following six invasive pest species: European grapevine moth (EGVM; Lobesia botrana [Denis & Schiffermuller]), Giant African land snail (GALS; Lissachatina fulica [Bowdich]), Japanese beetle (JB; Popillia japonica [Newman]), Mediterranean fruit fly (Medfly; Ceratitis capitata [Wiedemann]), Mexican fruit fly (Mexfly; Anastrepha ludens [Loew]) and Oriental fruit fly (OFF; Bactrocera dorsalis [Hendel]). We used K-means clustering to group detections that were proximate in space and time, then finalized them manually. We calculated MTD from the cluster centroid for each detection. Probability histograms for MTD were fit to a two-parameter exponential function, and from those functions we estimated species' dispersal limits for four percentiles (99th to 99.99th). The least vagile species, GALS and EGVM, had the greatest decay rates, and smallest MTD and percentile distances. OFF, with the greatest reported D, had the smallest decay rates and greatest MTD and percentile distances. Medfly, Mexfly and JB had intermediate MTD and percentile distances. Each regression of percentile distance as a logarithmic function of D fit the data well. The best regression used adjusted 99.9th percentile distances. These empirical results indicated that many delimiting surveys currently in use are oversized; adopting the new recommendations could yield significant cost savings. These results could be a general solution for estimating delimiting survey radii (for durations of ca. 30 days) across a wide range of insect dispersal abilities.