Attractiveness of bait matrices and matrix/toxicant combinations to the citrus pests Iridomyrmex purpureus (F.Smith) and Iridomyrmex rufoniger gp sp (Hym., Formicidae)

Six bait matrices were tested for palatability to the ants Iridomyrmex purpureus and I. rufoniger gp sp., which are important pests of citrus in south-eastern Australia. Granulated citrus pulp and rice bran impregnated with brown sugar were not accepted by either species, but ground dry dog food and ground insects ( with or without added sugar) were highly attractive to both ant species. The ground dog food was impregnated with various toxicants at concentrations of up to 1% (w/w) and tested for palatability to both ant species. Technical grade pyriproxyfen, fipronil, and fenoxycarb were incorporated using organic solvents, whereas formulated fenoxycarb (Insegar(R)) was incorporated as an aqueous suspension prior to drying and granulation. Relative to the blank samples ( carrier solvents only), bait acceptance by I. purpureus was not significantly reduced (P > 0.05) by any of the treatments except 1% technical fenoxycarb. Bait acceptance by I. rufoniger gp sp. was significantly suppressed by 0.2% fipronil, but not by higher concentrations. When water was used as the carrier solvent the blank sample was significantly (P < 0.05) less attractive to I. purpureus than the untreated control, suggesting that wetting of dog food-based baits in field situations may irreversibly reduce their attractiveness. Laboratory colonies were used in choice and no-choice tests to assess the efficacy of dog food baits containing fipronil (0.05-0.2% w/w). The corrected mortality for I. purpureus ranged from 88 to 100% and from 90 to 97% in the no-choice and choice tests, respectively. In no-choice tests I. rufoniger gp sp. corrected mortality varied from 52 to 60%, but in choice tests the corrected mortality did not exceed 26%. Ground dog food containing fipronil (0.05-0.2% w/w) has the potential to provide effective control of I. purpureus in citrus orchards, but alternative toxicants or lower fipronil concentrations may be required for control of I. rufoniger gp sp.