Efficiency of herbivore exclusion by ants attracted to aphids on the vetch Vicia angustifolia L. (Leguminosae)

The efficiency of herbivore exclusion by ants on the vetch Vicia angustifolia L. (Leguminosae) with extrafloral nectary, mediated by ant attraction to aphids was investigated in a field census and laboratory experiments. In the field, workers of Lasius japonicus Santschi and Tetramorium tsushimae Emery frequently visited plants of the vetch parasitized by aphids of Aphis craccivora Koch, but only a few workers visited plants without aphids. An increase in the number of ants visiting a plant with increasing numbers of aphids caused a decrease in the number of larvae of the weevil, Hypera postica Gyllenhal. Therefore, the efficiency of herbivore exclusion by ants was higher on plants parasitized by Ap. craccivora aphids than that on plants unparasitized by aphids. In the laboratory experiments, L. japonicus workers frequently patrolled not only shoots with Ap. craccivora aphids but also shoots without them. However, T. tsushimae workers visited mainly shoots with Ap. craccivora aphids but less frequently on shoots without aphids. Therefore, L. japonicus workers excluded herbivores more efficiently on plants of the vetch than T. tsushimae workers. Consequently, the efficiency of herbivore exclusion by ants on the vetch can be influenced directly by differences in ant species and indirectly by the presence of aphids on plants. The present study highlights the significance of indirect interactions between ants and plants with extrafloral nectary, mediated by ant attraction to aphids for herbivore exclusion of plants.