Testing Dose-Dependent Effects of the Nectar Alkaloid Anabasine on Trypanosome Parasite Loads in Adult Bumble Bees

Abstract

The impact of consuming biologically active compounds is often dose-dependent, where small quantities can be medicinal while larger doses are toxic. The consumption of plant secondary compounds can be toxic to herbivores in large doses, but can also improve survival in parasitized herbivores. In addition, recent studies have found that consuming nectar secondary compounds may decrease parasite loads in pollinators. However, the effect of compound dose on bee survival and parasite loads has not been assessed. To determine how secondary compound consumption affects survival and pathogen load in Bombus impatiens, we manipulated the presence of a common gut parasite, Crithidia bombi, and dietary concentration of anabasine, a nectar alkaloid produced by Nicotiana spp. using four concentrations naturally observed in floral nectar. We hypothesized that increased consumption of secondary compounds at concentrations found in nature would decrease survival of uninfected bees, but improve survival and ameliorate parasite loads in infected bees. We found medicinal effects of anabasine in infected bees; the high-anabasine diet decreased parasite loads and increased the probability of clearing the infection entirely. However, survival time was not affected by any level of anabasine concentration, or by interactive effects of anabasine concentration and infection. Crithidia infection reduced survival time by more than two days, but this effect was not significant. Our results support a medicinal role for anabasine at the highest concentration; moreover, we found no evidence for a survival-related cost of anabasine consumption across the concentration range found in nectar. Our results suggest that consuming anabasine at the higher levels of the natural range could reduce or clear pathogen loads without incurring costs for healthy bees.

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

“…This idea is supported by the significant variation in anabasine’s medicinal effects across colonies. It could be that prior experiments that found medicinal effects of anabasine [ 16 , 100 ] used colonies that happened to be exceptionally sensitive to this compound, harbored microbiota with low rates of anabasine catabolism, or converted anabasine into compounds with stronger antiparasitic effects.…”

“…[ 16 ] and Anthony et al . [ 100 ] starved bees overnight, which may have resulted in high anabasine consumption immediately following infection as bees attempted to refeed and rehydrate. Similarly, another trial that showed medicinal effects of anabasine [ 18 ] was conducted under hot and variable conditions that may have raised carbohydrate and water requirements and prompted higher consumption of anabasine-containing solutions when this was the only liquid available.…”

Mitigating the Effects of Habitat Loss on Solitary Bees in Agricultural Ecosystems

“…of plants. Anabasine reduced levels of the parasite, Crithidia bombi, by 81% in one study [69], and in another was shown to have no negative effect on the health of unparasitized bees [70]. Solitary bees can also be infected by a variety of viral and fungal pathogens and can be affected by nest parasites, including species of blister beetles (Coleoptera: Meloidae) and cuckoo wasps (Hymenoptera: Chrysididae) [41,71].…”

Uptake and impact of natural diet-derived small RNA in invertebrates: Implications for ecology and agriculture

“…For example, bumble bees self-medicate by ingesting more nectar containing the anti-parasitic compound Anabasine when parasitized by a specific trypanosome. 167 Provision of such compounds has positive effects on plant fitness 168 and could therefore provide an evolutionary rationale for the production of sRNAs with cross-kingdom effects by plant species.…”