Indirect interactions between two aphid species in relation to ant attendance

The influence of foraging by the ant, Lasius niger, on the population growth of two aphid species, Lachnus tropicalis and Myzocalalis kuricola, on chestnut trees, Castanea crenata, was examined. The anttending effect was divergent depending on the aphid density per ant: it was positive when there were few aphids per ant, but negative when there were many aphids per ant. In addition, the density of one aphid species also influenced the ant-tending effect on the other aphid. Furthermore, the influences were asymmetrical: an increase in L. tropicalis density per ant reversed the ant's effect on this species and on M. kuricola, while an increase in M, kuricola per ant did not significantly influence the ant's effect on L. tropicalis. Thus, the ant seems to stabilize the L. tropicalis population density and keep this species from extinction, while the ant's effect an M. kuricola depends on the density oft. tropicalis and may lead M. kuricola to extermination. This change in the ant-tending effect corresponds to the previously detected density-dependent change in predation activity of the ants on aphids. In contrast, the density-dependent change in the protection effect of the ants against natural enemies does not explain the results.