Inferior herbivorous competitors avoid traces of superior competitors to prevent losing competition

Species sharing overlapping ecological niches are expected to experience unstable coexistence due to interspecific competition. Non-web-building Panonychus spider mites share host plant species with most web-building Tetranychus spider mites. Previous research demonstrated that Panonychus mites are hindered by the three-dimensional webs produced by Tetranychus species, and may experience competitive exclusion. In this study, fecundity of Panonychus citri was found to be reduced on leaves bearing Tetranychus kanzawai webs, indicating that the webs impose a fitness cost on P. citri. Such selective pressure would favor behaviors that allow Panonychus mites to avoid direct competition. Consistent with this prediction, migrating P. citri avoided T. kanzawai traces at a bifurcation, suggesting that they can detect and evade the presence of superior competitors. Moreover, P. citri avoided settling onto leaf surfaces with T. kanzawai traces including silk threads but no feces or eggs, indicating the ability to recognize competitor establishment from traces on the leaf. This study provides the first evidence that spider mite traces can have a repellent effect on competing heterospecific spider mites. In contrast, fecundity of T. kanzawai was not influenced by the presence of P. citri webs, and T. kanzawai did not avoided P. citri traces either at bifurcation or on leaf surfaces. These results suggest that only inferior competitors avoid encountering superior competitors to prevent the amensalistic competitive relationship between them.