Exploring the impact of fungicide exposure and nutritional stress on the microbiota and immune response of the Cape honey bee (Apis mellifera capensis)

Abstract

Honey bees (Apis mellifera) harbour a stable core microbial community within their gut, that is suggested to play a role in metabolic functioning, immune regulation, and host homeostasis. This microbiota presents a unique opportunity to observe the effects of stressors on honey bee health. We examined the effects of two common honey bee stressors: indirect fungicide contamination and nutrient limitation. These effects were observed through changes in their hind- and midgut microbiota using Automated Ribosomal Intergenic Spacer Analysis (ARISA), alongside high-throughput amplicon sequencing. Expression of the honey bees' immune response was examined through the expression of three immune-related genes, namely, immune deficiency (imd), proPhenolOxidase (proPO), and spaetzle (spz). Additionally, longevity of the honey bees was monitored through observation of the expression levels of Vitellogenin (Vg). Both treatment groups were compared to a negative control, and a diseased positive control. There was no effect on the hindgut microbiota due to the stressors, while significant changes in the midgut was observed. This was also observed in the expression of the immune-related genes within the treatment groups. The Imd pathway was substantially downregulated, with upregulation in the prophenoloxidase pathway. However, no significant effect was observed in the expression of spz, and only the pollen treatment group showed reduced longevity through a downregulation of Vg. Overall, the effect of these two common stressors indicate a compromise in honey bee immunity, and potential vulnerabilities within the immune defence mechanisms.

Keywords: Fungicide; Honey bee; Immune system; Microbiota; Nutrition.