Sub-lethal effects of abamectin suppressing colonies of the leaf-cutting ant Acromyrmex subterraneus subterraneus

An assessment of the sub-lethal effects of abamectin on queens of the leaf- cutting ant Acromyrmex subterraneus subterraneus (Hymenoptera: Formicidae) was carried out. A concentration-response bioassay was conducted by exposin g media workers (head capsule width ranging from 1.69 to 1.83mm) to filter paper impregnated with abamectin residue. This bioassay allowed selection o f the abamectin concentrations (0, 10, 50 and 100mg AIml(-1)) to be used in the experiment in which eight queens were exposed to the insecticide. The highest concentration killed queens, but those exposed to lower concentrati ons survived and were placed back in their nests. Reductions in leaf consum ption, fungus garden volume, and foraging activity were observed in colonie s whose queens were exposed to higher abamectin concentration (50mg AIml(-1 )). Colonies with queens exposed to 50mg AIml(-1) were suppressed after 11 weeks. The dissection of queen ovaries revealed a marked decrease in the nu mber of ovarioles per ovary and oocytes per ovariole with increased insecti cide concentration. It would appear that, as abamectin-exposed queens becam e reproductively impaired, producing little or no progeny, colony members w ere not replaced and, this led ten colony suppression. (C) 2000 Society of Chemical Industry.