Abstract
The influence of foraging by the ant,Lasius niger, on the population growth of two aphid species,Lachnus tropicalisandMyzocallis kuricola, on chestnut trees,Castanea crenata, was examined. The ant‐tending 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 inL. tropicalisdensity per ant reversed the ant’s effect on this species and onM. kuricola, while an increase inM. kuricolaper ant did not significantly influence the ant’s effect onL. tropicalis. Thus, the ant seems to stabilize theL. tropicalispopulation density and keep this species from extinction, while the ant’s effect onM. kuricoladepends on the density ofL. tropicalisand may leadM. kuricolato 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.
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