If you are looking for stories involving bees or literal poisonings, these popular episodes from the series match those themes:
: Finbar claims to have found God and wants to start a new life, but his return rattles the spa owners and staff. [S9E9] Poison in the Honey
"Father Brown" The Wrong Shape (TV Episode 2013) - Plot - IMDb If you are looking for stories involving bees
: Finbar Finch, a former spa employee who had recently been released from prison after serving time for a brutal attack on the spa's owner. [S9E9] Poison in the Honey