The Janzen-Connell Hypothesis (JCH) predicts that attack by specialist enemies on seedlings increases with conspecific seedling density, but studies have rarely experimentally tested for the contingency of such effects on tree community context (e.g., diversity, composition) and measured responses by different enemies (e.g. herbivores, pathogens). We conducted a field study in a large-scale system evaluating tree species diversity and conspecific density effects on leaf damage on mahogany (Swietenia macrophylla) seedlings. We established quadrats of eight levels of seedling density across mahogany tree monocultures and species polycultures including mahogany, and recorded percent leaf damage by insects and percent leaf necrosis by a pathogenic fungus on mahogany seedlings. We found contrasting effects of tree species diversity on insects and pathogens. Whereas diversity did not affect leaf damage by insects, it had a significant negative effect on leaf necrosis by pathogens. On average, percent leaf necrosis on mahogany seedlings in polyculture was half of that observed in monoculture. We discuss the potential influences of changes in mahogany tree density vs. frequency (relative to other tree species) driving this diversity effect. Unexpectedly, we found no effect of seedling conspecific density on either leaf insect or pathogen damage (i.e., density-independent attack). Likewise, we found no significant tree diversity by seedling density interaction, indicating that attackers were consistently unresponsive to variation in seedling density across two levels of tree diversity. Overall, this study provides a unique test of the JCH by experimentally evaluating seedling density and tree diversity effects on contrasting plant enemies, shedding light on enemy controls over plant recruitment.