Simulation-based evaluation of two insect trapping grids for delimitation surveys

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

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2022
Publication Date: 6/30/2022
Citation: Fang, H., Caton, B.P., Manoukis, N., Pallipparambil, G.R. 2022. Simulation-based evaluation of two insect trapping grids for delimitation surveys. Scientific Reports. 12. Article 11089. https://doi.org/10.1038/s41598-022-14958-5.
DOI: https://doi.org/10.1038/s41598-022-14958-5

Interpretive Summary: Delimitation surveys are often used after detection of an invasive pest for which there is an effective trap and lure. These are usually set to a given density of traps per unit area. Here we conduct simulations to compare different designs and were able to reduce costs while keeping delimitation efficacy high. We find that most captures happen near the center of the grid, and that grids may be over-sized for their stated goals.

Technical Abstract: 1. After an exotic pest is detected, a delimitation survey is typically used to determine the spatial extent of the population. In the United States, published square trapping grids have been used for decades for different insects, but their performance has not been rigorously evaluated. 2. We used the TrapGrid simulation model to investigate the effectiveness of two representative designs with the most common sizes. These were the 8-km square grid for the leek moth (Acrolepiopsis assectella) with 900 traps, and the 14.5-km square grid for the Mediterranean fruit fly (Medfly, Ceratitis capitata) with 1,700 traps. We investigated grid compositions and design factors, measuring performance as the mean probability of pest capture over all traps, p(capture), over a 30-day duration. We also designed improved grids for each species, in terms of performance, containment, and cost. 3. For the default grids, p(capture) was 0.86 for leek moth and 0.71 for Medfly. Better performance of the leek moth grid, despite its lower trap density and smaller size, was due to greater trap attractiveness. For both species, captures in the core were 86 percent or more of maximum mean p(capture); outer bands generally contributed little. Egress testing indicated that both grids were oversized for pests with lower dispersal abilities, and the Medfly grid was oversized for all except highly motile insects. We also used simulations to test how to optimize the two grids for their targets. The most efficient grid for leek moth used 80 traps (9.7 traps per km2) in 3.2-km circular grid. This decreased mean p(capture) to 0.73 but reduced the total cost by 91 percent, and thereby gave greater return-on-investment (percentage points per $1000 spent). The best grid for Medfly was a 4.8-km circular grid used 232 traps, which gave mean p(capture) of 0.66 and reduced total costs by 86 percent. 4. Many trapping survey designs have been in use for decades without changes or evaluation, despite being based initially on very little science. We demonstrated that common trapping survey plans may be improved using simulation results, and that significant cost savings may often be achieved while maintaining survey performance.