Feeding-induced changes in plant quality mediate interspecific competitionbetween sap-feeding herbivores

Feeding-induced plant resistance is a well-documented phenomenon for leaf-c hewing insects. Furthermore, feeding-induced resistance provides the mechan istic basis for many cases of delayed interspecific competition, whereby pr evious feeding by one species diminishes the performance of other herbivore s which attack the same plant later in the season. This phenomenon, however , has been very poorly investigated for sap-feeding insects. The results we present here fur salt marsh-inhabiting planthoppers (Prokelisia dolus and P. marginata) provide one of the few known examples of delayed, plant-media ted interspecific competition between two sap-feeding insects. Three lines of experimental evidence from the laboratory, field cages, and open field plots provide support for the detrimental effects of previous fe eding by one planthopper species on the subsequent survival and performance of the other. Laboratory experiments showed that prior feeding on cordgras s by one congener resulted in reduced performance of the other in the follo wing generation. However, the effect was asymmetric. Prior feeding by P. do lus resulted in prolonged development and reduced body size (a correlate of fecundity) in P. marginata, whereas only development was protracted in P. dolus when plants were previously exposed to P. marginata. Consequently, P. dolus appears to be the superior competitor in the context of delayed, pla nt-mediated interactions. The negative effects of previous feeding by P. do lus on the development time, body size, and survival of P, marginata obtain ed in the laboratory were confirmed both in cages and on cage-free islets o f cordgrass in the field. Feeding-induced reductions in host-plant quality by P. dolus may provide additional impetus for P. marginata to migrate from shared habitats on the high marsh to nutritionally superior plants in the low marsh rarely occupied by P. dolus. The mechanism underlying the delayed competitive effects between Prokelisia planthoppers is most likely diminished plant nutrition, because feeding by P, dolus significantly reduces the concentration of essential amino acids in cordgrass. The asymmetry of plant-mediated competition between the Proke lisia species may be due to the ability of P. dolus to better tolerate feed ing-depleted levels of plant nitrogen via compensatory feeding. Even though these two planthoppers do not suffer significant fitness reduct ions during contemporaneous interactions, they compete severely in the cont ext of feeding-induced plant resistance which is expressed later in the sea son. This result, coupled with the fact that most studies of interspecific interaction between herbivorous insects are contemporaneous, indicates that interspecific competition may be profoundly underestimated as a structurin g force in phytophagous insect communities.