Modeling intra-sexual competition in a sex pheromone system: how much can female movement affect female mating success?

Mating disruption theory predicts that high concentrations of female pheromone, and/or large numbers of release sites, should confuse males orienting to "calling" females, reduce the number of successful matings, and decrease the reproductive potential of the population. In this scenario, females are regarded as stationary point sources of pheromone. Past behavioral observations, however, have shown virgin female grape root borers, Vitacea polistiformis Harris, significantly alter their behavior in mating disruption treatments. Treated females call at different heights, move less before call initiation, and move more after call initiation than control females. Pheromone gland dragging and wing fanning also increase significantly during pheromone treatments. These behavioral differences are significant only if they alter the mating success of females. Because long-term field studies are impractical, we used known behavior of male and female GRB to build a Fortran language time step model, adding the effects of female movement to past models of male pheromone plume following. Females were distributed randomly, and then assigned a conditional movement strategy. If females were within the competitive portion of another female's plume, the downwind female moved. Except in the lowest population density tested, females moving upwind and crosswind when in a competing female's pheromone plume mated significantly more often than females remaining stationary. In all population simulations, mating success was significantly reduced when females moved downwind. These field and simulation studies provide strong evidence for female movement as a previously overlooked potential mechanism for resistance to mating disruption treatments, as well as a shaping behavior in the evolution of pheromone communication systems.