INTRASPECIFIC AND INTERSPECIFIC COMPETITION IN ADULTS OF 2 ABUNDANT GRASSHOPPERS (ORTHOPTERA, ACRIDIDAE) FROM A SANDHILLS GRASSLAND

Recognizing the existence and importance of intra- and interspecific c ompetition in natural populations of insect herbivores remains problem atical. We performed field-cage experiments to detect the presence of both intra- and interspecific competition among adults of two common, grass-feeding grasshoppers, Ageneotettix deorum (Scudder) and Amphitor nus coloradus (Thomas). Based on increased mortality and lowered fecun dity at higher densities, strong evidence for intraspecific competitio n exists for both species. Interspecific competition was weaker than i ntraspecific competition in this study. When interspecific competition existed, it was asymmetric in its effect, with the larger species (A. coloradus) negatively impacting the smaller one (A. deorum). Although there was little evidence for interspecific effects on mortality, egg production was impacted in a size-dependent, asymmetric manner betwee n the two species. Significant density-dependent depression of availab le food was observed for A. coloradus, the larger species. Experiments were run at high densities (10-30 individuals per square meter), leve ls only irregularly reached under natural conditions (either in sporad ic patches or overall about every 10-12 yr). Because naturally occurri ng densities are typically below the competitive threshold, interspeci fic competition between these two species may occur only intermittentl y. Other forces may greatly impact adult mortality at this site, inclu ding predation.