In recent years, the bagworm Oiketicus kirbyi (Guilding) has become a pest of increasing importance in Peruvian avocado orchards. A study conducted in six commercial orchards evaluated the ecology of, and feeding damage caused by, bagworms on avocado on the southern coast of Peru. In March 2009, the population of live individuals at different locations consisted of second to sixth larval instars. The sex-ratio in the previous generation of bagworms (the parents of larvae) was female-biased, and the mortality rate was higher for females than males. The percentage of mated females varied between 76.7 and 99.1% and declined when the proportion of males was low. The damage caused by bagworm larvae usually results from defoliation, but in the case of O. kirbyi on avocado, the presence of larvae on the fruits was associated with blemishes that limit the marketability of fruits; thus bagworms cause direct economic losses for avocado growers. Future studies are needed to assess the timing of larval emergence with a view to preventing widespread infestation by dispersing neonate bagworms. © 2010 Taylor & Francis.