DENSITY-RELATED DISPERSAL IN PLANTHOPPERS - EFFECTS OF INTERSPECIFIC CROWDING

In dense populations, most planthoppers (Homoptera:Delphacidae) produce fully winged migratory forms that can escape to new habitats. Under less crowded conditions, flightless morphs with reduced wings result. For two sympatric species, Prokelisia marginata and Prokelisia dolus, interspecific crowding was found to be as strong a stimulus for the production of migrants as intraspecific crowding. The effects were reciprocal for both species and were demonstrated both in the laboratory and field. However, because migratory forms were triggered at a much lower density in P. marginata than P. dolus, interspecific interactions were asymmetric, with P. dolus having a far greater influence on the wing form of P. marginata. Interspecific interactions did not directly influence survival, development time, or body size, but reproductive capability was indirectly affected because migratory females are less fecund than their flightless counterparts. Intraspecific crowding adversely affected the survivorship, development time, and body size of P. marginata, but had no influence on the fitness of P. dolus. P. marginata is a much more migratory species than P. dolus. Consequently, mobility is associated with the extent that each species' fitness is reduced by intraspecific crowding. Our results demonstrate that for wing dimorphic insects, the effects of interspecific competition can be extended to include altered wing form, a character that directly influences dispersal capability as well as reproductive potential. We suggest that when wing-dimorphic species share a common habitat, interspecific interactions can influence population dynamics by increasing emigration and decreasing population growth.