Seasonal variations in foraging by ants (Hymenoptera : Formicidae) in two New South Wales citrus orchards

Summary. Pitfall traps were used to monitor the seasonal activity of ants in 2 citrus orchards in the Murrumbidgee Irrigation Area of south-western New South Wales for 22–24 months (November 1992–August/October 1994). Twenty-four species were recorded (22 species at Yanco and 18 species at Cudgel), with Iridomyrmex rufoniger group species being numerically dominant at both sites (63 and 59% of total captures at Yanco and Cudgel respectively). Meat ants, Iridomyrmex purpureus (F.Smith), were a significant (9%) component of trap captures at Yanco where regular soil cultivation was practised, but were less common (<1% of captures) at Cudgel. Sample species richness was generally greater at the Yanco site (mean 9.3 species per trapping interval compared with 6.6 at Cudgel). Total ant captures and sample species richness showed a positive correlation with mean daily temperatures over trapping intervals. Variations in seasonal activity were shown by the numerically dominant species, with 2 distinct patterns being evident. Whilst all taxa were less active during winter, some species [I. rufoniger group sp. 2, Pheidole sp., Rhytidoponera metallica (F.Smith)] continued to forage, albeit at reduced levels. Other species of Iridomyrmex including I. rufoniger group sp. 1 and I. purpureus ceased foraging almost entirely during June–September. Iridomyrmex species are known to interfere with the biological control of honeydew-producing insects in citrus orchards, and our results indicate that ant control programs in southern New South Wales should be initiated in late August–early September to achieve optimal results.