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Caffeine is a widely occurring plant defense chemical 1 , 2 that occurs in the nectar of some plants, e.g., Coffea or Citrus spp., where it may influence pollinator behavior to enhance pollination. 3 , 4 Honey bees fed caffeine form longer lasting olfactory memory associations, 5 which could give plants with caffeinated nectar an adaptive advantage by inducing more visits to flowers. Caffeinated free-flying bees show enhanced learning performance 6 and are more likely to revisit a caffeinated target feeder or artificial flower, 7-9 although it is not clear whether improved memory of the target cues or the perception of caffeine as a reward is the cause. Here, we show that inexperienced bumble bees (Bombus terrestris) locate new food sources emitting a learned floral odor more consistently if they have been fed caffeine. In laboratory arena tests, we fed bees a caffeinated food alongside a floral odor blend (priming) and then used robotic experimental flowers 10 to disentangle the effects of caffeine improving memory for learned food-associated cues versus caffeine as a reward. Inexperienced bees primed with caffeine made more initial visits to target robotic flowers emitting the target odor compared to control bees or those primed with odor alone. Caffeine-primed bees tended to improve their floral handling time faster. Although the effects of caffeine were short lived, we show that food-locating behaviors in free-flying bumble bees can be enhanced by caffeine provided in the nest. Consequently, there is potential to redesign commercial colonies to enhance bees' forage focus or even bias bees to forage on a specific crop., Competing Interests: Declaration of interests F.L.W. is an employee of Biobest NV. F.L.W. was involved in the study design and interpretation, but the funding bodies themselves were not involved in the design, data collection, analysis, or decision to publish. At time of submission, the authors held no patents related to this work., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Plant phytochemicals can act as natural "medicines" for animals against parasites [1-3]. Some nectar metabolites, for example, reduce parasite infections in bees [4-7]. Declining plant diversity through anthropogenic landscape change [8-11] could reduce the availability of medicinal nectar plants for pollinators, exacerbating their decline [12]. Existing studies are, however, limited by (1) a lack of mechanistic insights into how phytochemicals affect pollinator diseases and (2) the restriction to few, commercially available chemicals, thereby potentially neglecting plants with the biggest antiparasitic effects. To rapidly identify plants with the greatest potential as natural bee medicines, we developed a bioactivity-directed fractionation assay for nectar metabolites. We evaluated 17 important nectar plants against the bumblebee pathogen Crithidia bombi (Trypanosomatidae) [13-17]. The most bioactive species was heather (Calluna vulgaris), the second most productive UK nectar plant [10]. We identified 4-(3-oxobut-1-enylidene)-3,5,5-trimethylcyclohex-2-en-1-one (callunene) from heather nectar as a potent inhibitor of C. bombi. Wild bumblebees (Bombus terrestris) foraging on heather ingest callunene at concentrations causing complete C. bombi inhibition. Feeding on callunene was prophylactic against infections. We show that C. bombi establishes infections by flagellar anchoring to the ileum epithelium. Short-term callunene exposure induced flagellum loss in C. bombi choanomastigotes, resulting in a loss of infectivity. We conclude that plant secondary metabolites can disrupt parasite flagellum attachment, revealing a mechanism behind their prophylactic effects. The decline of heathlands [18-21] reduces the availability of natural bee "medicine" and could exacerbate the contribution of diseases to pollinator declines. VIDEO ABSTRACT., (Copyright © 2019 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Toxic nectar is an ecological paradox [1, 2]. Plants divert substantial resources to produce nectar that attracts pollinators [3], but toxins in this reward could disrupt the mutualism and reduce plant fitness [4]. Alternatively, such compounds could protect nectar from robbers [2], provided that they do not significantly alter pollinator visitation to the detriment of plant fitness [1, 5-8]. Indeed, very few studies have investigated the role of plant toxins in nectar for defense against nectar robbers [4, 9, 10]. Here, we compared two Aconitum species (A. napellus and A. lycoctonum) that have flowers specialized for long-tongued bumblebee pollinators (Bombus hortorum) but are occasionally robbed by short-tongued bumblebees (B. terrestris) [6, 11-13]. Pollinator visits to flowers were much more frequent than by robbers, but visits correlated negatively with nectar alkaloid concentration and declined sharply between 200 and 380 ppm. However, alkaloid concentrations of >20 ppm were deterrent to B. terrestris, suggesting that robbers were less tolerant of nectar alkaloids. Nectar of both plant species contained similar concentrations of carbohydrates and toxic alkaloids, but A. lycoctonum was more likely to secrete nectar in each flower and was also visited more frequently by pollinators and robbers. We conclude that alkaloids in Aconitum spp. nectar affect rates of both pollinator visitation and robbery but may have co-evolved with nectar availability to maintain the fitness benefits of specialized plant-pollinator relationships. Chemical defense of nectar is, however, ultimately constrained by pollinator gustatory sensitivity., (Copyright © 2017 Elsevier Ltd. All rights reserved.)