Competition among Sympatric Erythroneura Leafhoppers (Homoptera: Cicadellidae) on America Sycamore

Eight species of leafhoppers (Homoptera: Cicadellidae) in the genus Erythroneura (maculata group) breed on the American sycamore, Platanus occidentalis L.; several of these species occur sympatrically in many areas throughout the range of their host plant. The eight species are E. arta Beamer, E. bella McAtee, E. hymettana Knull, E. ingrata Beamer, E. lawsoni Robinson, E. morgani (DeLong), E. torella Robinson, and E. usitata Beamer. Population densities of these leafhoppers in mid—September reached an average of 72 adults per 10 leaves at the West Vienna site in southern Illinois USA. This was 28 time greater than at the Golconda site approximately 48.3 km away where weather severely reduced spring population levels. By early August at West Vienna food resources at the customary feeding sites, within 2 mm distance of the main veins, had been 95% depleted. This apparent food shortage was experienced by 86% of the total leafhoppers collected during the year. By mid—September the total leaf surface area utilized by feeding leafhoppers reached 98%. Similar levels of feeding damage were common throughout Illinois. Feeding was concentrated in the lower canopy. In the upper canopy (12 m aboveground) of a tree which had supported a relatively low leafhopper population throughout the season, there was no damage to leaves while in the lower portion (2 m aboveground) 59% of the leaf surface area was utilized. We infer that during the period of peak leafhopper abundance there are severe food limitations on these insects. Single species and mixed species rearing experiments in the field demonstrated that these leafhoppers compete at population densities comparable to those commonly observed throughout Illinois from August to October. At West Vienna 86% of the total leafhopper population probably experiences competition for about 9 wk in the year. For each species the number of progeny produced per ♀ under single species conditions was reduced even at low densities (2 ♂ ♂/leaf), at 6 ♀ ♀/leaf reproductive success was reduced by >70%, at 14 ♀ ♀/leaf no progeny matured. For mixed species rearings at comparable densities trends were similar. However, certain species pairs were less adversely affected by high density than others, suggesting slight differences between these species in the manner in which they utilize to explain coexistence by niche segregation. The relative rates of dispersal and extinction among competing species may explain coexistence in the sycamore leaf—feeding guild. The competitive ability of the Erythroneura leafhoppers may be frequency—dependent since at high densities each species adversely affected its own fitness more than it did that of its competitor in mixed species rearings.