The decline of colonies of honey bees across the world is threatening local plant biodiversity and human food supplies. Neonicotinoid pesticides have been implicated as a major cause of the problem and are banned or suspended in several countries. Other factors could also be lowering the resistance of bees to opportunist infections by, for example, new resistant strains of the Varroa mite and by the parasite "Nosema". Intensive agricultural practices and modern beekeeping practices may also be implicated. A model is proposed that envisages honey bees, weakened by sub-optimal doses of neonicotinoid pesticides, becoming more susceptible to environmental stresses.