Ant Foraging Reduces the Abundance of Beneficial and Incidental Arthropods in Citrus Canopies

Abstract

Abstract The abundance of nonpest arthropods in ant-free or ant-infested (Iridomyrmex rufonigergp. spp.) citrus tree canopies was evaluated monthly in a 2-year study (September 1994–August 1996) in southern New South Wales, Australia. Limb tap samples from ant-free trees yielded a total of 4029 beneficial arthropods (predatory beetles, bugs and mites, lacewings, spiders, parasitic wasps) and 3650 other nonpest arthropods (mainly beetles, psocids, cockroaches) compared to 1839 and 2300, respectively, from ant-infested trees. The largest differences occurred during spring (September–November) of both years (1994 and 1995) and summer (December–February) of the second year (1995) when there were 2.8- to 4-fold more beneficial arthropods in the ant-free tree samples. No differences occurred during summer, autumn, and winter of the first year and autumn and winter of the second year. Other nonpest arthropods were 1.6- to 6.2-fold more abundant in ant-free citrus during spring and 2.3- to 3.9-fold more abundant in autumn with no difference in summer and winter. Most taxonomic groupings of beneficials showed ant-related population reductions during spring and some (e.g., spiders and predatory mites) were more abundant on ant-free trees throughout spring–autumn in the second year. These data suggest that canopy-foraging by honeydew-seeking ants reducing the abundance of natural enemies is the major cause of outbreaks of soft and armored scales on citrus in southern New South Wales.