We investigated the effects of plant density, plant diversity, and light availability on susceptibility of saplings ofStryphnodendron microstachyumPoeppig&Endlicher to attack by herbivores and pathogens in the lowlands of northeastern Costa Rica. Seedlings ofS. microstachyumwere planted at either low or high density. High density stands were planted in 2 environments with 2 plant diversities within each environment: monocultures in abandoned pastures (=sun) and in 5–yr–old secondary forest (=shade), or polycultures in whichS. microstachyumwas planted with 4 other species also in sun and shade. Low density stands consisted of isolated individuals in 4 types of sites (abandoned pasture, secondary forest understory, primary forest understory, and primary forest light gap). Numbers of galls (Diptera: Cecidomyiidae), larvae (Lepidoptera: Noctuidae:Euclystis), and fungal spots (Melanconiales:Pestalotia) were greater in the high density stand than in low density stands, and greater in the shade than in sun. We found no differences in infection between monocultures and polycultures at high density, nor did we findEuclystislarvae or Cecidomyiidae galls in low density stands. Numbers of fungal spots per leaf and density of galls were greater on larger plants. These data suggest that higher plant densities increase the probability of attack by herbivorous insects and leaf pathogens, and that plantations of tropical native tree species in partial shade, as a means of reforestation, run the risk of outbreaks of normally rare plant enemies. Furthermore, the observed positive relation between plant size and attack suggests that plantations of this tree species will become more susceptible to pest outbreaks as the trees mature.