Impact of three ant species on pest populations in Mediterranean citrus orchards

Abstract: We conducted an ant-exclusion experiment to study the influence of three ant species, the native to the Mediterranean Pheidole pallidula (Nylander) and Lasius grandis (Forel) and the invasive Linepithema humile (Mayr), on the population densities and parasitism rates of three citrus pests: the honeydew producer woolly whitefly Aleurothrixus floccosus (Maskell) and the non-honeydew producers, California red scale Aonidiella aurantii (Maskell) and citrus leafminer Phyllocnistis citrella (Stainton). The ant-exclusion was carried out in three citrus orchards, each one dominated by one ant species, from April 2011 to November 2012. We measured ant activity, California red scale densities on fruits and twigs, whitefly and citrus leafminer population densities as well as the parasitism of these pests in ant-allowed and ant-excluded trees. The most active ant species was L. humile, while P. pallidula and L. grandis showed considerably lower activity levels. On fruits, A. aurantii densities in the ant-excluded trees were on average 27% lower for the three ant species studied than in the ant-allowed trees. On twigs, differences in A. aurantii densities were detected between treatments only for L. grandis. In the case of A. floccosus, differences between ant-excluded and ant-allowed treatments were found for P. pallidula and L. humile. For P. citrella, we found no significant differences in the percent of leaf surface loss. Regarding the parasitism, we only found higher percent parasitism in the case of P. citrella in L. humile dominated orchard. These results show that ant activity may have an effect on the abundance of honeydew and non-honeydew producing pests, yet this effect depends on the ant species involved. This effect is possibly not produced through parasitism since our results show no differences in percent parasitism for the three ant species studied between ant-allowed and ant-excluded treatments.