Miconia calvescens de Candolle (Melastomataceae) is an invasive tree considered the most serious threat to natural ecosystems of Hawaii and other Pacific islands. The success of M. calvescens as an invasive species is greatly owing to its shade tolerance and the shaded habitat it creates, where many native plant species that are light-demanding cannot survive. Salbia lotanalis Druce (Lepidoptera: Crambidae), a neotropical leaf roller attacking M. calvescens, was evaluated for two mechanisms by which it reduces leaf area of its host plant: feeding (defoliation), which removes leaf tissue, and tying leaf rolls, which reduces exposed area of leaves. These impacts were quantified over a 1-yr period at a field site in Costa Rica, where densities of S. lotanalis larvae attacking M. calvescens peaked at the end of the rainy season and declined in the dry season. Up to 47.5% of leaves were attacked by S. lotanalis, with cumulative defoliation by an undetermined number of larvae removing an average of ≈30% (253 cm2) of each leaf attacked. Defoliation and leaf rolling were compared in a greenhouse experiment in which individual S. lotanalis larvae defoliated an average of 3.7% (17.8 cm2) of each attacked leaf, and reduced exposed leaf area as a result of leaf rolling by an average of 12.8% (66.2 cm2). Our results complement the findings of previous studies of S. lotanalis and confirm its potential as a biological control agent of M. calvescens.