Ant interactions with plants bearing extrafloral nectaries (efns) are often assumed to be beneficial to both partners; ants consume the nectar and benefit the plant by deterring herbivores. Invasive ants are thought to be especially adept at harvesting carbohydrate-rich resources and therefore may be more attentive to the availability of extrafloral nectar than native ants. We compared benefits conferred upon and received from the native Western Australian EFN plant Acacia saligna, among invasive Argentine ants (Linepithema humile), big-headed ants (Pheidole megacephala), and native ants. In winter 2006, we planted twelve pairs of six month old plants (one to which ants were excluded and one to which ants were allowed to visit) in each of eight urban bushland sites: two invaded by Argentine ants, two invaded by big-headed ants, and a matched nearby uninvaded site for each of the four invaded sites. We observed plants biweekly for ants and other arthropods, and assessed growth and herbivory after five months. In laboratory experiments, we compared worker survival of Argentine, big-headed, and native Iridomyrmex chasei workers exposed to herbivorized or non-herbivorized A. saligna or controls.Results/Conclusions

In the field experiment, in the first five months of observations Argentine ants were observed on 83-92 %, big-headed ants on 30-80%, and native Iridomyrmex spp. on 58-92% of plants in their respective sites. Trophobionts (sap-sucking psyllids and lycaenid butterfly larvae) were only observed in sites with native ants. At all sites, ant-allowed trees had more chewing herbivory than ant-excluded trees, but there were no differences between treatments for leaf mining damage. In sites with Argentine and big-headed ants, ant-allowed trees grew more phyllodes and branches than their paired ant-excluded trees, but these differences were not seen in sites with native ants. In the laboratory experiments, ants given herbivorized plants had 7-11 times greater worker survival relative to unfed ants, but there were no differences in survival among the three ant species exposed to the same treatments. Ants with access to non-herbivorized plants had the same survival as unfed ants. Results suggest that the invasive Argentine and big-headed ants appear to be more beneficial to A. saligna growth than native Iridomyrmex, possibly due to the native ants’ mutualistic relationships with trophobionts on the plants; however, all three taxa can benefit from A. saligna extrafloral nectar.