REPRODUCTIVE INTERFERENCE AND ITS CONSEQUENCES FOR THE COMPETITIVE INTERACTIONS BETWEEN 2 CLOSELY-RELATED PANONYCHUS SPIDER-MITES

Two genetically incompatible Panonychus mites (Panonychus citri and Pa nonychus mori) geographically segregate their distributions in Japan. They occasionally co-exist on deciduous fruit trees at the intermediat e latitudes, but their occurrences are unstable and P. mori is often e xcluded. Interspecific matings between the two species interfere with a subsequent effective intraspecific mating, to a greater extent far P . mori than for P. citri. Panonychus mori males show a strong preferen ce for conspecific females, whereas P. citri males do not show any mat ing preference. This suggests that under the co-existence of the two s pecies the deleterious effect through reproductive interference is mor e intense for P. mori than for P. citri. On peach trees where the two species co-exist with various density ratios, the proportion of ineffe ctive females (females that did not produce any female offspring) was higher in P. mori than in P. citri and the proportion increased with i ncreasing density of the other species relative to that of its own spe cies. A simple differential equation model incorporating the above rep roductive interference demonstrated that the stable co-existence of th e two species is unlikely and that P. citri will have a much higher ch ance of excluding P. mori, due to a greater reproductive interference effect and higher reproductive rate than the latter. However, the outc ome of competition depends on the initial density ratios, which will b e influenced by various stochastic events, thereby reducing the probab ility of one-sided exclusion over a wider area.