Juanse writes: > but still I haven't been able to find one study of feeding propolis to bees

How about this:

In their efforts to avoid losses and produce colonies suitable for pollination services, beekeepers have greatly increased their investment per colony, including heavy feeding of pollen substitutes, often mixed with putative health products (DeGrandi-Hoffman and Chen 2015). These “health products,” such as essential oils, are frequently included in bee diet formulations (Imdorf et al. 1999), though it is difficult to objectively determine whether their use actually improves bee health and is worth the investment.

Based on their understanding that propolis in the hive improves bee health, some beekeepers add it to their pollen substitute formulations, though data is lacking concerning whether it is beneficial when fed in this way. To better understand how propolis impacts on honey bee health and how this material could be exploited, we examined its effect on the immune system when fed orally to bees challenged with bacteria.

In our investigation, adding propolis to the food greatly increased immune gene expression in bees injected with E. coli, compared to the uninfected bees (same diets without propolis). This brings new insight into a possible role of propolis in reducing colony susceptibility to pathogens. It is unknown whether bees would normally ingest propolis; however, our findings lead us to hypothesize that they do so when challenged with pathogens.

Turcatto, A.P., Lourenço, A.P. & De Jong, D. Apidologie (2017). https://doi.org/10.1007/s13592-017-0553-z

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