Pest insects cause production losses of billions of $ every year, both in agriculture but also in the forestry sector. The overall objective of this PhD thesis was to investigate Cryphalus piceae (Ratzeburg, 1837) (Coleoptera: Curculionidae) and Adelges nordmannianae (Eckstein, 1890) (Hemiptera: Adelgidae), both considered problematic pests in Danish Christmas tree and greenery production at the start of the thesis. The goal of the thesis was to provide research and knowledge assisting producers and researchers in eventually reaching a situation, where the two studied organisms can be controlled in a sustainable way. In paper I and II we provide knowledge on C. piceae and related Cryphalus species, relevant to the scientific community. Paper II also provide tools ensuring correct identification, necessary for growers or forest entomologists. In III and IV we explore the life cycle and damaging capabilities of C. piceae, and discuss relevant control methods and eventually question the need for control of C. piceae. Appendix I provide the necessary knowledge on the species-complex of the studied Adelgid species, relevant for a successful biocontrol program. Following previous work on biocontrol of Ad. nordmannianae, Paper V and VI explore aspects of the predatory fly Leucopis hennigrata (Diptera: Chamaemyiidae), relevant for evaluating it as a classical biocontrol agent against Ad. nordmannianae.