Food Limitation Affects Parasite Load and Survival ofBombus impatiens(Hymenoptera: Apidae) Infected WithCrithidia(Trypanosomatida: Trypanosomatidae)

Abstract

Bumble bees (genus Bombus) are globally important insect pollinators, and several species have experienced marked declines in recent years. Both nutritional limitation and pathogens may have contributed to these declines. While each of these factors may be individually important, there may also be synergisms where nutritional stress could decrease pathogen resistance. Understanding interactions between bumble bees, their parasites, and food availability may provide new insight into the causes of declines. In this study, we examined the combined impacts of pollen and nectar limitation on Crithidia, a common gut parasite in Bombus impatiens Cresson. Individual worker bees were inoculated with Crithidia and then assigned in a factorial design to two levels of pollen availability (pollen or no pollen) and two nectar sugar concentrations (high [30%] or low [15%] sucrose). We found that lack of pollen and low nectar sugar both reduced Crithidia cell counts, with the most dramatic effect from lack of pollen. Both pollen availability and nectar sugar concentration were also important for bee survival. The proportion of bees that died after seven days of infection was ∼25% lower in bees with access to pollen and high nectar sugar concentration than any other treatment. Thus, nectar and pollen availability are both important for bee survival, but may come at a cost of higher parasite loads. Our results illustrate the importance of understanding environmental context, such as resource availability, when examining a host-parasite interaction.

Context-dependent medicinal effects of anabasine and infection-dependent toxicity in bumble bees

“…The effects of dietary chemicals on both hosts and parasites may also be modulated by nutritional environment. For example, dietary protein is necessary for host expression of immune genes [ 37 ], detoxification pathways that protect hosts from phytochemicals [ 38 ], and survival and reproduction [ 39 , 40 ]. However, a well-nourished host may also provide a more suitable environment for parasites [ 41 ], which would explain the higher infection levels found in bees fed ample pollen [ 40 , 42 ].…”

“…For example, dietary protein is necessary for host expression of immune genes [ 37 ], detoxification pathways that protect hosts from phytochemicals [ 38 ], and survival and reproduction [ 39 , 40 ]. However, a well-nourished host may also provide a more suitable environment for parasites [ 41 ], which would explain the higher infection levels found in bees fed ample pollen [ 40 , 42 ]. Because of dietary protein’s importance in induction of detoxification genes [ 38 ], phytochemical tolerance may be higher in well-nourished hosts, such that phytochemicals are more beneficial in nutritionally rich environments.…”

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“…The present study suggests a mechanism by which Nosema infection could contribute to trypanosome infection via negative effects on sugar absorption in bees. Healthy bees and other nectivorous insects have an excellent ability to digest and absorb sugars from nectar (Nicolson, 1998), which may explain why there was no effect of dietary sugar concentration on infection intensity in B. impatiens (Conroy et al, 2016) . However, gut infection by microsporidians can disrupt the midgut epithelium (Higes et al, 2008).…”

“…Pollen, like nectar, is an indispensable source of nutrients for bees and other pollinators that supports insect immunity (Brunner et al, 2014), survival (Conroy et al, 2016), and reproduction (Vaudo et al, 2015). However, pollen may nourish parasites as well as hosts.…”

“…Hence, pollen could be expected to have particularly strong effects on parasites that are susceptible to inhibition by phytochemicals. However, studies in bees have found that, even though pollen consumption increased expression of immune genes (Brunner et al, 2014), dietary pollen increased C. bombi infection intensity (Conroy et al, 2016;Logan et al, 2005) in Bombus terrestris and B.…”