Reproductive interference and its consequences for the competitive interactions between two closely related Panonychus spider mites

Two genetically incompatible Panonychus mites (Panonychus citri and Panonychus mori) geographically segregate their distributions in Japan. They occasionally co-exist on deciduous fruit trees at the intermediate latitudes, but their occurrences are unstable and P. mori is often excluded. Interspecific matings between the two species interfere with a subsequent effective intraspecific mating, to a greater extent for P. mori than for P. citri. Panonychus mori males show a strong preference for conspecific females, whereas P. citri males do not show any mating preference. This suggests that under the co-existence of the two species the deleterious effect through reproductive interference is more intense for P. mori than for P. citri. On peach trees where the two species co-exist with various density ratios, the proportion of ineffective females (females that did not produce any female offspring) was higher in P. mori than in P. citri and the proportion increased with increasing density of the other species relative to that of its own species. A simple differential equation model incorporating the above reproductive interference demonstrated that the stable co-existence of the two species is unlikely and that P. citri will have a much higher chance of excluding P. mori, due to a greater reproductive interference effect and higher reproductive rate than the latter. However, the outcome of competition depends on the initial density ratios, which will be influenced by various stochastic events, thereby reducing the probability of one-sided exclusion over a wider area. This revised version was published online in November 2006 with corrections to the Cover Date.