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

Abstract

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.

Extrafloral nectaries indirectly protect small aphid colonies via ant-mediated interactions

“…Thus, we consider that when the aphid colony grows enough to attract ants, the existence of EFNs has no effect on aphids. In contrast, Suzuki et al (2004) reported that when aphids of A. craccivora effectively attracted ants on the EFNbearing vetch V. angustifolia, the efficiency of herbivore exclusion by ants increased. This also does not support the ant distraction hypothesis, but rather indicates that there are indirect benefits to plants with EFNs from aphid parasitization; therefore, we expect that an indirect mutualistic relationship can potentially occur between plants with EFNs and ant-tended aphids by ant protection according to the aphid colony size.…”

“…We expect that EFNs of V. faba do not induce workers of both L. japonicus and T. tsushimae to prey on aphids. Sakata and Hashimoto (2000), Katayama and Suzuki (2003a), and Suzuki et al (2004) reported that EFNs could not attract ants when aphid colony size increased. Thus, we consider that when the aphid colony grows enough to attract ants, the existence of EFNs has no effect on aphids.…”

“…With increased aphid density, Vicia plants (V. faba L. and V. angustifolia L.) show decreased ability to attract ants via their EFNs (Katayama and Suzuki, 2003a;Suzuki et al, 2004) because the amount of aphid honeydew is much greater than that of extrafloral nectar (Katayama, unpublished data). In this case, although aphids directly damage plants by sap feeding, the plants gain "indirect benefits" under a range of aphid densities, as the benefit of herbivore exclusion by ants surpasses the cost of aphid injury because the aphids attract more ants, which exclude herbivores, to the plants than EFNs , On the other hand, small aphid colonies cannot effectively attract ants due to their lower supply of honeydew (Sakata and Hashimoto, 2000;Suzuki, 2002, 2003a).…”

“…Ant-tended aphids frequently parasitize plants with EFNs (Buckley, 1983;Gaume et al, 1998;Offenberg, 2000;Engel et al, 2001;Katayama and Suzuki, 2003a;Suzuki et al, 2004), and ant-mediated interactions between aphids and plants are important for plant fitness . With increased aphid density, Vicia plants (V. faba L. and V. angustifolia L.) show decreased ability to attract ants via their EFNs (Katayama and Suzuki, 2003a;Suzuki et al, 2004) because the amount of aphid honeydew is much greater than that of extrafloral nectar (Katayama, unpublished data).…”

Changes in Monoterpene Emission Rates of Quercus ilex Infested by Aphids Tended by Native or Invasive Lasius Ant Species

“…Honeydew, a sugar solution excreted by aphids while feeding on sap plants, is the main food of several ant species (Carroll and Janzen 1973). Through aphid attendance, ants may exert indirect effects on plants, such as a decrease in seed production (Rico-Gray and Castro 1996), plant herbivory (Suzuki et al 2004), and pollination (Lach 2007). The degree of these indirect effects varies among native ants, primarily as a result of differences among species in aggressiveness, territoriality, colony density, worker abundance, and the strength and persistence of the interactions between particular ant species (Styrsky and Eubanks 2007).…”