Simulation Modeling to Interpret the Captures of Moths in Pheromone-Baited Traps Used for Surveillance of Invasive Species: the Gypsy Moth as a Model Case.

Abstract

When pheromone traps are used for detection of an invasive pest and then delimitation of its distribution, an unresolved issue is the interpretation of failure to capture any target insects. Is a population present but not detected, a so-called false negative? Using the gypsy moth (Lymantria dispar) as an exemplar, we modeled the probability of males being captured in traps deployed at densities typical for surveillance (1 per 2.6km2 or 1 per mi2) and delimitation (up to 49 per 2.6km2). The simulations used a dynamic wind model generating a turbulent plume structure and varying wind direction, and a behavior model based on the documented maneuvers of gypsy moths during plume acquisition and along-plume navigation. Several strategies of plume acquisition using Correlated Random Walks were compared to ensure that the generated dispersions over three days were not either overly clumped or ranged many km. Virtual moths were released into virtual space with patterns mimicking prior releases of gypsy moth males in Massachusetts at varying distance from a baited trap. In general, capture rates of virtual and real moths at varying trap densities were similar. One application of this approach was to estimate through bootstrapping the probabilities of not detecting populations having densities ranging from 1 to 100 moths per 2.6km2 and using traps that varied from 25 to 100% in their efficiencies of capture. Low-level populations (e.g., 20-30 per 2.6km2) often were not detected with one trap per 2.6km2, especially when traps had low efficiencies.