Effects of short‐term exposure to naturally occurring thymol concentrations on transmission of a bumble bee parasite

1. Plants produce antimicrobial phytochemicals that can reduce growth and infectivity of parasites in animals. Pollinator parasites are transmitted between hosts that forage on shared flowers. Floral transmission directly exposes parasites to phytochemicals on floral surfaces and in nectar, both at flowers and, post‐ingestion, in the crop. This exposure could directly affect parasite transmission to new hosts. 2. Nectar chemical analyses were combined with field and cell culture experiments to test the effects of the floral phytochemical thymol on the transmission potential of the trypanosomatid gut parasite Crithidia in Bombus impatiens. First, thymol concentrations in Thymus vulgaris nectar were measured. Second, the effect of adding thymol to floral nectaries on parasite transmission to foraging bees was tested. Third, cell cultures were used to determine direct, dose‐dependent effects of short‐term thymol exposure on subsequent in vitro parasite growth. 3. A total of 26.1 ppm thymol was found in T. vulgaris nectar, five‐fold higher than previously documented in this species. However, addition of thymol to flowers of parasite‐inoculated inflorescences of four plant species did not affect acquisition of Crithidia infection during a foraging bout. Cell culture experiments showed that the thymol concentrations needed to reduce subsequent Crithidia growth by 50% (120 ppm) were 4.6‐fold higher than the highest detected nectar concentration. 4. Although thymol exposure can influence Crithidia viability, Crithidia are robust to the duration and magnitude of exposure encountered during floral foraging under natural conditions. These experiments suggest that any effects of thymol alone on Crithidia–host infection dynamics probably reflect indirect, possibly host‐mediated, effects of chronic thymol ingestion.