4.1. Rearing HBTM in colonies
Tracheal mite prevalence (the proportion of infested bees in a sample; Margolis et al., 1982) generally decreases during periods of the year when large numbers of young bees are emerging (due to extensive brood rearing and relatively short-lived adult bees). Prevalence increases during fall (when brood rearing is declining) or periods of confinement (e.g. rainy weather), presumably because there is extensive contact between older infested bees and young susceptible bees (Bailey & Ball, 1991). In cold temperate climates, mite prevalence usually increases rapidly from late summer until early winter (Otis et al., 1988; de Guzman et al., 2002). However, HBTM prevalence in individual colonies in summer is not correlated with mite prevalence in late fall; only when bee brood has largely disappeared can mite prevalence in wintering colonies be predicted with any assurance (r = ~0.8, Dawicke et al., 1989). Due to variable bee mortality during the winter as well as brood production in infested colonies in early spring, mite prevalence in the fall is uncorrelated with mite prevalence the following spring (Dawicke et al., 1989). This makes experimentation difficult because heavily infested colonies cannot be identified until shortly before they are needed for experiments.
Once the wintering population of bees has developed (Mattila et al., 2001), mite prevalence tends to remain relatively constant over the winter months in the absence of newly emerging worker bees (Bailey, 1958); however, in some situations mite prevalence has increased over winter (Otis et al., 1988; McMullan, 2011). It is possible that mite emigration from tracheae is stimulated by high or increasing titers of juvenile hormone (JH). This speculation would explain observations that although mite abundance (i.e. the mean number of mites per bee) continues to increase in bee tracheae during the winter months due to continuing mite reproduction, mite prevalence (percent of bees infested of the total number of bees in the samples) generally does not. J McMullan (pers. comm.) indicated that when there is little or no brood present in autumn and winter, it is difficult to influence mite infestations experimentally.