Comparison of ant community structure (Hymenoptera: Formicidae) in citrus orchards under chemical and biological control of red scale, Aonidiella aurantii(Maskell) (Hemiptera: Diaspididae)

Abstract

AbstractEpigaeic ants were sampled by pitfall trapping in three South African citrus orchards supporting low levels of Aonidiella aurantii(Mask.). All three were similar except that chemical control of the scale was in progress in one, biological control in the second, while the third was in transition from chemical to biological control. Within-orchard variation was statistically zero at the chemical and biological control sites, both of which had constant species abundances and rank-abundance sequences throughout. Within the transition site, rank-abundance sequences were constant but species abundancies varied. Despite the statistical constancy of rank, within-orchard samples at the transition and chemical control sites varied in their dominant ants. The biological control site was dominated throughout by Pheidole megacephala(F.). Individuals of the dominant species made up between 59 and 69% of the total, and as the dominant species varied, this high figure suggested strong interspecific competition. Log-normal models were fitted to the site rank-abundance plots. The interquartile index, Q, was higher when based on an underlying log-series model (α) than on a log-normal one; it was about 2·0, which is low compared with most other studies. The total numbers of individuals and species of ants were higher in the biological control than chemical control orchard. Diversity, as measured by α or Q log-normalwas lowest in the chemical control site, medium in the biological controlone, and highest in the transition site. Between-site variation showedvariation in general ant abundance. As with the within-orchard results, those between orchards were relatively constant. Ant management in relation to community structure is critically appraised on the basis of limiting factors (especially abundance of honeydew as food for potentially dominant ants) and rank-abundance sequences.