A simulation model of wind-driven dispersal of Helicoverpa moths

A simulation model is described that simulates the dispersal of Helicoverpa spp. moths over a heterogeneous landscape. The model divides the region under consideration into a scalable grid of square cells, each of which may contain one or more landuse units. Moth flight between cells and their component landuse units is largely governed by wind speed and direction. Flight is simulated for each cell in turn, once each day. For each cell, the flight paths of emigrating moths are distributed into sectors radiating from the cell centre and flanking the wind direction. The proportion of moths allocated into each sector is based on an assumed normal distribution, where the arc of each sector represents one standard deviation from the mean wind direction. The angle of the flight sectors on either side of the wind direction is inversely proportional to wind speed. If wind conditions are calm, the flight sectors encompass a full 360° arc and are allocated equivalent numbers of moths. The moths within each flight sector are moved out from the cell centre on a front which forms the arc of the sector. As the arc intersects with cells on the underlying grid, a proportion of moths land in each cell. The proportion of moths alighting is determined on the basis of the overall attractiveness of the habitats within each cell, and the proportion of the cell that is intersected by the flight sector. Alighting moths are distributed into the landuse units in target cells according to the relative attractiveness of each landuse, and its relative area within the cell. Wind velocities may vary between cells or time steps.