Density-dependent seed and seedling mortality caused by pests and diseases has been implicated in maintaining high plant diversity in tropical forests To investigate how these Janzen-Connell effects vary with rainfall we monitored seed arrival and seedling recruitment for 18 months at eight sites spanning a natural precipitation gradient across the Isthmus of Panama Seedling plots were treated with either a fungicide, an insecticide or used as a control. Seedling recruitment was negatively density-dependent in control plots, but this density-dependence was greatly reduced in the insecticide-treated plots. Insecticide treatment also largely eliminated the increase in diversity observed in control plots when comparing seedlings to seeds The positive effects of insects on plant diversity increased significantly with rainfall. Our results suggest a mechanistic explanation for well-documented positive relationships between rainfall and (i) the strength of negative density dependence and (ii) overall tropical forest plant diversity.