Influence of Avian Predation on a Grasshopper (Orthoptera: Acrididae) Assemblage That Feeds on Threadleaf Snakeweed

Abstract

A 2-yr study was initiated in south central Arizona in 1994 to determine the effect of avian predators on a threadleaf snakeweed, Gutierrezia microcephale (D.C.) Gray, feeding grasshopper assemblage. Two grasshopper species, Hesperotettix viridis (Thomas) and Melanoplus aridus (Scudder), made up 96 and 98% of the species composition during 1994 and 1995, respectively. Each of these species may play an important role in suppressing snakeweed when in high numbers. Grasshopper densities from caged and non-caged plots were compared to determine if predators were important in limiting the population dynamics of this grasshopper assemblage. Bird predation was a significant mortality factor for these species. Average densities of H. viridis and M. aridus were 2.2 and 2.9 times higher in bird exclosures than in controls in 1994 and 1995, respectively. M. aridus were preyed upon at a higher rate than H. viridis, probably because of the relative abundance and better erypsis of H. viridis. Female grasshoppers of both species appeared to be selected more frequently than males. Meadowlarks, Sturnella spp.; northern mockingbirds, Mimus polyglottos; and Cassin's sparrows, Aimophila cassinii were probably the most important insectivores at this site because of their abundance and food habits.