The extraordinarily high tree α-diversity of tropical lowland forests has been suggested to be shaped and maintained by specialist insect herbivores that preferentially attack the most abundant tree species in a distance- and density-dependent manner, giving rare species an advantage Insect populations and herbivory might further be enhanced by more stable and favorable climates of more humid tropical forests, driving an increase of tree species richness with rainfall. Conversely, intensified top-down control in wetter, aseasonal forests could reduce the impact of herbivore imposed control on tree species Studies measuring insect herbivory in relation to host plant abundance, and its regulation by predators across gradients of tree species richness are lacking We hypothesized herbivory to be higher on more abundant species, and increase with rainfall. Insect herbivory was quantified in six forest sites across a Neotropical rainfall and tree species richness gradient, in relation to local abundances of 42 focal tree species Predation pressure on insect herbivores was also measured More abundant tree species experienced a higher risk of herbivory, but the amount of damage caused by herbivores decreased with tree abundance Overall, insect herbivory strongly decreased with increasing rainfall In contrast, predation pressure was higher at the wetter site Insect herbivores might thus contribute to local tree species coexistence, but seem unlikely to drive the increase in tree species richness with rainfall The unexpected and contrasting patterns of herbivory and predation support the need for studies on multi-trophic interactions across environmental gradients to understand the processes contributing to tree diversity and ecosystem functioning, predict ecosystem responses to future climate change, and develop effective conservation strategies.