DISPERSAL OF A HIGHLY VAGILE INSECT IN AHETEROGENEOUS ENVIRONMENT

Abstract

In total, -7 000 000 marked adults of Heliothis virescens, a noctuid moth pest of cotton, were released from multiple sites in western Mississippi during the springs of four years, and the dispersing populations of males were sampled in pheromone traps over -2000 km2. The large scale of these observations and some innovative analytical techniques allow 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 removed using variation in catches of wild males. The adjusted data were fitted to the leptokurtic dispersal model that results when a constant loss term is added to the Fickian diffusion equation in two dimensions and the result is integrated across time. Effective area sampled per trap was estimated to be -20 ha, with twofold variation among years. On the order of 10 wild males/ha emerged from overwintering, but there was over sixfold variation among years. Density dependence of pheromone trap efficiency was manifested 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 -10 km, with nearly threefold variation among years. Variation in movement among years was not correlated with variation 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 FST (0.002) for this species observed in a previously published genetic study at similar spatial scales. However, levels of move- ment similar to those observed do not generally preclude rapid response to strong selection, because this species has repeatedly developed resistance to insecticides used against it on cotton in the study area.