The willow sawfly, Nematus oligospilus Förster, has recently been recorded from Colombia. Natural populations were searched for in Bogotá City, where one of the host plants, Salix humboldtiana, grows as an ornamental tree. Larvae and cocoons were found, which thrive on the leaves of this plant. Parthenogenetic reproduction of this sawfly was confirmed. Fecundity of every female was estimated through counting the number of eggs laid on the leaves of the host. In addition, some variations in the color pattern of the adult are described and several of its morphological traits measured. For the larval stage, the body length of each instar was measured and their stadium (intermoult period or instar duration) calculated. Six larval instars were observed regularly; only one case of seven instars occurred.