Dispersal of a highly vagile insect in a heterogeneous environment

In total, similar to 7 000 000 marked adults of Heliothis virescens, a noct uid moth pest of cotton, were released from multiple sites in western Missi ssippi during the springs of four years, and the dispersing populations of males were sampled in pheromone traps over similar to 2000 km(2). The large scale of these observations and some innovative analytical techniques allo w extension of the use of diffusion-based models for dispersal to a highly vagile species moving in a heterogeneous environment. Temporal variation in catches of released individuals was removed by using total catch per trap over the lives of released populations of moths as the, dependent variable. Spatial variation in total trap catch due to site-specific factors was rem oved using variation in catches of wild males. The adjusted data were fitte d to the leptokurtic dispersal model that results when a constant loss term is added to the Fickian diffusion equation in two dimensions and the resul t is integrated across time. Effective area sampled per trap was estimated to be similar to 20 ha, with twofold variation among years. On the order of 10 wild males/ha emerged from overwintering, but there was over sixfold va riation among years. Density dependence of pheromone trap efficiency was ma nifested as a delay in trap catch of males in the area of release in one of the four years. The typical distance moved by a male from point of release to point of capture was similar to 10 km, with nearly threefold variation among years,Variation in movement among years was not correlated with varia tion in population density. Variation in population density due to habitat selection was apparent at a spatial scale of at least 0.5 km. The movement results are consistent with the low values of F-ST (0.002) for this species observed in a previously published genetic study at similar spatial scales . However, levels of movement similar to those observed do not generally pr eclude rapid response to strong selection, because this species has repeate dly developed resistance to insecticides used against it on cotton in the s tudy area.