Larvae of Thagona tibialis (Walker) (Lepidoptera: Erebidae) have been reported to defoliate tropical almond plants, Terminalia catappa L. (Combretaceae), in urban areas of Belo Horizonte (2005), Viçosa (2006), Morro Reuter (2007), and Brasília (2012), Brazil. The origin of T. tibialis is uncertain, but it is now dispersed throughout urban areas of all Brazilian states that have T. catappa. This pest not only has the capacity for high infestation and defoliation rates on T. catappa, but it also can invade houses and cause allergies to humans because of larval hairs and adult scales that are easily dislodged. The objectives of this research were to describe the morphological and ecological characteristics of T. tibialis and to evaluate defoliation of T. catappa by T. tibialis larvae in Viçosa. The specimens collected for this study were a variant of T. tibialis that does not have dark dots on the forewings. Larvae were bluish-white, with orange sub-dorsal stripes and black spots throughout the body. Final instars were approximately 40 mm long. Adults exhibit sexual dimorphism. Females were always white, while males varied from white to light or dark brown. Eggs were round, except for the flattened base and micropyle regions, with the greatest diameters being 0.86 mm long and 0.62 mm wide. Larvae of T. tibialis injured from 3.6% to 98.9% of the leaves on T. catappa trees. From 78.2% to 96.9% of mature leaves and from 3.2% to 21.8% of young leaves were injured, indicating T. tibialis larvae prefer mature leaves of T. catappa.