Forests are complex ecosystems that provide a variety of valuable products, such as timber, fuel wood, fiber and non-wood forest products and they also contribute to socio economic development. Pest problems have a considerable influence on the health of planted and natural woodlands of this industry. Members of the family Cossidae are often of serious concern to forestry and horticulture due to their aggressive, often gregarious wood-boring behaviour. Some species of this family (e.g Coryphodema tristis and Chilecomadia valdiviana) is known to be highly polyphagous and members occupy diverse ecological zones. Coryphodema tristis and Chilecomadia valdiviana are examples of cossid species that have recently expanded their host range to include non-native species. Coryphodema tristis was first reported infesting the non-native plantation species, Eucalyptus nitens, in South Africa in 2004, and C. valdiviana was first recorded infesting the same species in Chile in 1992. These insects have been reported to cause substantial damage to E. nitens. Both species are associated with a number of native trees in their respective countries, as well as non-native fruit trees and ornamentals. Despite what appears to be extreme polyphagy in C. tristis, many trees, including many Eucalyptus species and even hybrids of E. nitens and E. grandis planted in the province of Mpumalanga, are apparently free of any cossid moth infestation. These novel host range expansions raise interesting questions regarding the factors that influence new host associations. This review examines Cossidae as pests in native and non-native forest systems. Possible factors contributing to success of Cossidae in forestry and control strategies employed to manage the group are discussed.