Possible Synergistic Effects of Thymol and Nicotine Against Crithidia Bombi Parasitism in Bumble Bees

Floral nectar contains secondary compounds with antimicrobial properties that can affect not only plant-pollinator interactions, but also interactions between pollinators and their parasites. Although recent work has shown that consumption of plant secondary compounds can reduce pollinator parasite loads, little is known about the effects of dosage or compound combinations. We used the generalist pollinator Bombus impatiens and its obligate gut parasite Crithidia bombi to study the effects of nectar chemistry on host-parasite interactions. In two experiments we tested (1) whether the secondary compounds thymol and nicotine act synergistically to reduce parasitism, and (2) whether dietary thymol concentration affects parasite resistance. In both experiments, uninfected Bombus impatiens were inoculated with Crithidia and then fed particular diet treatments for 7 days, after which infection levels were assessed. In the synergism experiment, thymol and nicotine alone and in combination did not significantly affect parasite load or host mortality. However, the thymol-nicotine combination treatment reduced log-transformed parasite counts by 30% relative to the control group (P = 0.08). For the experiment in which we manipulated thymol concentration, we found no significant effect of any thymol concentration on Crithidia load, but moderate (2 ppm) thymol concentrations incurred a near-significant increase in mortality (P = 0.054). Our results tentatively suggest the value of a mixed diet for host immunity, yet contrast with research on the antimicrobial activity of dietary thymol and nicotine in vertebrate and other invertebrate systems. We suggest that future research evaluate genetic variation in Crithidia virulence, multi-strain competition, and Crithidia interactions with the gut microbe community that may mediate antimicrobial activities of secondary compounds

Possible Synergistic Effects of Thymol and Nicotine against <i>Crithidia bombi</i> Parasitism in Bumble Bees

<div><p>Floral nectar contains secondary compounds with antimicrobial properties that can affect not only plant-pollinator interactions, but also interactions between pollinators and their parasites. Although recent work has shown that consumption of plant secondary compounds can reduce pollinator parasite loads, little is known about the effects of dosage or compound combinations. We used the generalist pollinator <i>Bombus impatiens</i> and its obligate gut parasite <i>Crithidia bombi</i> to study the effects of nectar chemistry on host-parasite interactions. In two experiments we tested (1) whether the secondary compounds thymol and nicotine act synergistically to reduce parasitism, and (2) whether dietary thymol concentration affects parasite resistance. In both experiments, uninfected <i>Bombus impatiens</i> were inoculated with <i>Crithidia</i> and then fed particular diet treatments for 7 days, after which infection levels were assessed. In the synergism experiment, thymol and nicotine alone and in combination did not significantly affect parasite load or host mortality. However, the thymol-nicotine combination treatment reduced log-transformed parasite counts by 30% relative to the control group (<i>P</i> = 0.08). For the experiment in which we manipulated thymol concentration, we found no significant effect of any thymol concentration on <i>Crithidia</i> load, but moderate (2 ppm) thymol concentrations incurred a near-significant increase in mortality (<i>P</i> = 0.054). Our results tentatively suggest the value of a mixed diet for host immunity, yet contrast with research on the antimicrobial activity of dietary thymol and nicotine in vertebrate and other invertebrate systems. We suggest that future research evaluate genetic variation in <i>Crithidia</i> virulence, multi-strain competition, and <i>Crithidia</i> interactions with the gut microbe community that may mediate antimicrobial activities of secondary compounds.</p></div

The effect of dietary thymol concentration over three orders of magnitude on <i>C</i>. <i>bombi</i> parasitism of <i>B</i>. <i>impatiens</i>.

<p>Points indicate marginal means for each diet treatment; error bars show ± 1 standard error of the mean. Dietary treatments had no significant effect on parasite loads compared to the control (0 ppm) treatment.</p

The effects of dietary thymol, nicotine and a thymol-nicotine combination on <i>C</i>. <i>bombi</i> parasitism of <i>B</i>. <i>impatiens</i>.

<p>Points indicate marginal means for each diet treatment; error bars show ± 1 standard error of the mean. Treatment had a marginally significant effect on <i>C</i>. <i>bombi</i> parasite load (<i>P</i> = 0.06), with pairwise comparisons showing a trend of reduced parasite counts in the nicotine + thymol group (<i>P</i> = 0.08).</p

<i>B</i>. <i>impatiens</i> survival on different concentrations of dietary thymol.

<p>There was a marginally significant effect of thymol concentration on <i>B</i>. <i>impatiens</i> mortality. Pairwise comparisons showed that bees fed the 2 ppm treatment were more likely to die over the course of seven days compared to bees fed the control (0 ppm) treatment (<i>P</i> = 0.054).</p