We measured changes in insect herbivore intensity and guild structure in two tropical forests of Costa Rica over a three-year period. We estimated herbivory levels using discrete leaf litter samples to account for factors that influence the types of leaves most likely to be preserved in the fossil record. We found that leaf area removed by insect herbivores did not vary significantly between years within the two sampled forests, but differed across years between forests. The lack of variation in herbivore intensity within forests suggests that estimates of herbivore damage from fossil assemblages may reflect levels of herbivore intensity typically experienced by a given forest, at least over a short ecological timescale. The differences in herbivore intensity between similarly categorized forests suggest that simple one-to-one comparisons between fossil and modern sites can be problematic. The distribution of functional feeding groups also differed on an interannual basis both within and between forests. As guild structure can be highly variable, caution should be taken when comparing modern herbivore guild structures with those from fossil assemblages. Studies of herbivory in fossil assemblages have yielded much lower levels of damage than those found in this study as well as those measured from other modern forests. Leaf fossil preservation, environmental differences, and changes in plant–insect associations through time may account for these differences.