Evaluating a novel core-and-perimeter delimiting trapping survey design for insects. II. simulations and case studies

Authors: CATON, B - Animal And Plant Health Inspection Service (APHIS); Manoukis, Nicholas; PALLIPPARAMBIL, G - North Carolina State University; Nelson, Rosalie; HAIN, E - Animal And Plant Health Inspection Service (APHIS); FANG, H - North Carolina State University

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/2/2025
Publication Date: 5/7/2025
Citation: Caton, B.P., Manoukis, N.C., Pallipparambil, G.R., Nelson, R., Hain, E., Fang, H. 2025. Evaluating a novel core-and-perimeter delimiting trapping survey design for insects. II. Simulations and case studies. Journal of Economic Entomology. 118(3):1235-1250. https://doi.org/10.1093/jee/toaf090.
DOI: https://doi.org/10.1093/jee/toaf090

Interpretive Summary: "Delimitation" is often a key first step in the response to an invasive insect pest; it involves setting up traps to confirm the presence of the pest and to determine the potential extent of its spread. This study is the second in a two-part series that tests a novel approach to setting the trap positions for delimitation, which we term the "core-and-permieter" ("C&P") method. Under C&P traps are set up in an area around the first detection(s) and then a parimeter is set at a distance estimated to be larger than the spread of the population. In part 2 we apply field results from part 1 and determine key parameters such as insect movement to enable simulations to study alternative delimitation designs. We also analyzed costs of a fully trapped grid (evenly spaced traps over the whole area) vs C&P and found that the latter saved between 60 and 88% of the costs but with similar efficacy for the goals of delimitation.

Technical Abstract: We proposed a novel, “core-and-perimeter” (C&P) delimiting trapping survey design for introduced insects, with traps only near the epicenter and in a perimeter set at an optimal distance for zero captures. We compared C&P and fully trapped (FT) designs in the field with Ceratitis capitata in 2022. Trials verified some flaws in FT designs and supported the C&P concept generally but was undersized. Thus, we used the empirical data here to create a random-walk dispersal model to evaluate the C&P design more fully. Key model parameters were the daily dispersal probability (PDisp) and step distance (DDay), and the directional correlation, 'Max (maximum turning degrees). The estimated value of 'Max was 90° (correlated movement), based on recent results for a different fruit fly. We quantified the likelihood of inner traps to “intercept” insects along their paths, which depended on inter-trap distance, and used those findings in validation models when relevant. In calibration with our empirical data, PDisp values from 0.3 to 0.5 were reasonable. Validation against three independent datasets for total or specific service day dispersal densities was successful, and predictions also reproduced the varying shapes of distributions. The best model used the baseline DDay distribution, PDisp = 0.4, and 'Max = 90°. In C&P evaluation tests with the simple dispersal model, Medflies very rarely traveled 3.2 km in 30 days, even with forward-biased movement ('Max = 45°). A trapping version of the validated model gave zero perimeter captures with the C&P design, but single flies were captured in the core in 70 percent of iterations. The model with the standard FT grid captured proportionally fewer flies than the C&P design, despite having over 1,400 more traps. Finally, we compared FT and C&P survey costs using published plans for C. capitata and four other species. The costs of the C&P designs were 37–77 percent lower at standard sizes but were 60–88 percent lower if they used optimal (shorter) radii. The combined field, simulation, and case study results indicated that C&P designs can effectively meet delimitation goals whilst often substantially reducing costs compared to FT grids.