Competition as a factor underlying the abundance of an uncommon phytophagous insect, the salt-marsh planthopper Delphacodes penedetecta

1. Recent reviews of experimental studies provide compelling evidence that competition should be retained as a potential factor influencing the success of phytophagous insects. In this context, the objective of the study was to determine the role of interspecific and intraspecific competition, both contemporaneous and plant mediated (feeding-induced resistance), in limiting the population density of a consistently rare insect in a guild of abundant potential competitors. 2. Competitive interactions were assessed experimentally between two phloem-feeding planthoppers, the abundant Prokelisia dolus and the rare Delphacodes penedetecta (Hemiptera: Delphacidae). Both species are monophagous on the cordgrass Spartina alterniflora and overlap broadly in their use of habitats in the intertidal salt marshes along the Atlantic coast of North America. 3. The two planthoppers partition their cordgrass host plant, with D. penedetecta feeding more on the basal stems (particularly females) and P. dolus occurring most often on the canopy leaves. Notably, there was no evidence for niche shifting in D. penedetecta because its distribution on the plant did not change in the presence or absence of P. dolus. 4. Interspecific interactions with P. dolus had very little effect on the performance (development time and body size) and survival of D. penedetecta, a result demonstrated in both the laboratory and field. This result occurred both in contemporaneous interactions and on plants fed on previously by P. dolus. Only the males of D. penedetecta experienced weak competitive effects from P. dolus, as evidenced by reduced body size and slightly protracted development. 5. By contrast, there were strong adverse effects of intraspecific crowding (both from contemporaneous interactions and on plants fed on previously by conspecifics), whereby the survival, development time, and body size of D. penedetecta were affected very adversely. 6. These results suggest that interspecific competition is a weak force influencing the abundance of D. penedetecta in the field. Rather, strong intraspecific competition, a high requirement for plant nitrogen, and intrinsically low lifetime fecundity combine to explain the rarity of D. penedetecta.