Because of their unnatural composition in terms of age and size, colonies used for queen rearing are especially vulnerable to attack by pests and diseases. These include established problems such as the varroa mite, nosema, viruses such as deformed wing virus, black queen cell virus and chronic bee paralysis virus, melanosis, tracheal mites, and American and European foulbroods, as well as emerging exotic threats such as the small hive beetle, the yellow-legged Asian hornet, and Tropilaelaps mites. Chemicals such as agricultural pesticides and compounds used by beekeepers to control pests and diseases can have direct harmful effects on honey bee colonies, but can also have more subtle effects such as reducing the lifespan and fertility of queens and drones, with adverse effects on queen rearing and bee breeding operations. For all of these reasons, scrupulous hygiene is essential in queen rearing operations, including regular replacement of combs, sterilization of equipment, and keeping water and food free of contamination. Finally, the spread of pests and diseases through the local, regional, and international transport of queens, packages, and colonies must be fully taken into account.