Successful reforestation of tropical pastures can be facilitated or impeded depending on the interactions between native timber trees and arthropod communities. We assessed arthropod abundance on Tabebuia rosea (Bignoniaceae), a Central American extrafloral nectary-bearing timber tree, in a plantation of native trees on former pasture in Panama, and we excluded ants from branches of T. rosea to determine ant impact on arthropod assemblages and on leaf damage. The trees' ant fauna was dominated by ground-nesting species and ants were scarce in tree crowns, constituting only 2% of all arthropods collected by branch beating. Leaf damage was principally caused by caterpillars (mainly the pyralid Eulepte gastralis) and by beetles (mainly the chrysomelid Walterianella inscripta). Sugar baits at tree trunks revealed that rewarding food resources recruited high ant numbers to some tree individuals. Activity of the dominant ant, Ectatomma ruidum, at baits was negatively related to leaf damage by E. gastralis, suggesting that trees might be better protected from this herbivore when ant abundance is high. Exclusion of ants resulted in increased beetle diversity. The key herbivore W. inscripta was significantly more abundant on ant-exclusion branches at the beginning, but not towards the end of the rainy season, when beetle density increased. Overall leaf damage and arthropod abundance were not significantly affected by ant exclusion. In conclusion, ant abundance on native timber trees in plantations recently established on former pasture can be low, and ant assemblages may be dominated by ground-nesting generalist species, as shown in this study. This may explain the lacking protection of the timber trees from insect herbivory by ants. Increased protection of establishing timber trees by ants probably requires plantation management measures that increase the abundance of more specialized tree-visiting and tree-nesting ants.