Plant secondary compounds in floral nectar reduce bumble bee parasite infection
Herbivory and pollination are often placed at opposite ends of a continuum from antagonistic to cooperative behavior in plant-animal interactions, but are similar in that both are consumer-resource interactions that may be mediated by plant secondary compounds (PSM), including alkaloids, terpenoids, and phenolics. The role of these compounds as foliar anti-herbivore defenses is well known, but their functional significance in nectar and pollen remains obscure. The majority of studies on nectar PSM have focused on effects on plant reproduction. Yet, as consumers of nectar and pollen, pollinators are also exposed to these compounds. Recent studies have highlighted that PSM can reduce parasite loads in herbivores, and by extension, these same compounds may also affect parasite loads in pollinators. Bee pollinators are attacked by an array of parasites, some of which have been implicated in recent bee declines. In lab experiments, we tested the prediction that consumption of each of eight compounds found in floral nectar reduces the growth of the bumble bee intestinal parasite Crithidia bombi. We inoculated Bombus impatiens workers with C. bombi, then fed them either an artificial nectar solution containing a PSM or a nectar solution control, and compared gut parasite load after seven days.
Bees consuming the alkaloids nicotine and anabasine, the iridoid glycoside catalpol and the terpenoid thymol had significantly lower numbers of C. bombi cells than those fed the control. No significant difference was detected in trials with the alkaloid caffeine, the iridoid glycoside aucubin, the phenolic compound gallic acid and the cyanogenic glycoside amygdalin. None of the compounds reduced nectar consumption or increased bee mortality at the concentration given. These results demonstrate that some PSM found in nectar can slow the growth of an intestinal parasite known to reduce bee fitness. We are building on this work with lab and field experiments to study trade-offs between costs and benefits for bees that consume these compounds.