Adult female cossid moths lay eggs on the bark of trees, usually in a sheltered place such as cracks in the bark. Upon emergence, larvae bore through bark and feed on the cambium. As the larvae grow, they bore into the wood, where they cause extensive tunneling. Pupation occurs in pupal cocoons constructed inside the larval tunnels. Just prior to adult emergence, the pupae cut themselves out of the cocoons and wriggle towards the tunnel openings until their bodies project halfway out the tree. In this position the adults emerge from the pupal cases, resulting in the shed pupal cases protruding half way out the tree or falling to the ground. The cossid moth is reported to have a two-year life cycle in the Western Cape, but the duration of the life cycle in the summer rainfall area is not yet known.
Larvae feeding in the cambium and the extensive tunneling in the sapwood and heartwood results in severe damage to trees, which often also die. Both the main trunks and branches are attacked. Trees from five to fourteen years old have been infested, but it is likely that the cossid will infest both younger and older trees, provided the diameters of the trunks/branches are sufficient to enable the larvae to feed. Associations with fungal pathogens seem likely given the black discolouration of the stems and members of the Tree Protection Cooperative Programme at FABI are currently studying this matter.