Mechanisms of Pathogen and Pesticide Resistance in Honey Bees

Abstract

Bees are the most important insect pollinators of the crops humans grow, and Apis mellifera, the Western honey bee, is the most commonly managed species for this purpose. In addition to providing agricultural services, the complex biology of honey bees has been the subject of scientific study since the 18th century, and the intricate behaviors of honey bees and ants, fellow hymenopterans, inspired much sociobiological inquest. Unfortunately, honey bees are constantly exposed to parasites, pathogens, and xenobiotics, all of which pose threats to their health. Despite our curiosity about and dependence on honey bees, defining the molecular mechanisms underlying their interactions with biotic and abiotic stressors has been challenging. The very aspects of their physiology and behavior that make them so important to agriculture also make them challenging to study, relative to canonical model organisms. However, because we rely on A. mellifera so much for pollination, we must continue our efforts to understand what ails them. Here, we review major advancements in our knowledge of honey bee physiology, focusing on immunity and detoxification, and highlight some challenges that remain.

Keywords: Apis mellifera; Paenibacillus larvae; Varroa destructor; honey bee.

FIGURE 1.

Defense pathways against bacterial pathogens As insects, honey bees have the canonical Toll and Imd pathways, which lead to expression of antimicrobial peptides, lysozyme, and phenoloxidase, all of which can kill bacteria. Some honey bees also exhibit hygienic behavior and related social immunity traits, where larvae or pupae that are diseased or dying are removed from the colony. PRP, peptidoglycan recognition protein; PPO, prophenoloxidase; PPAE, prophenoloxidase processing enzyme; PO, phenoloxidase.

FIGURE 2.

Defense pathways against viruses The RNA interference (RNAi) system is the main antiviral pathway in honey bees. It can be stimulated directly, by detection of double-stranded (ds)RNA, but RNA can also be passed from a worker to a larva, priming its RNAi pathway to be able to respond to the virus without having previously been exposed to it. MRJP, major royal jelly protein; RISC, RNA‐induced silencing complex.

FIGURE 3.

Defense pathways against mites Some honey bees exhibit social mechanisms of Varroa mite resistance. Grooming behavior and several behaviors related to hygienic behavior (but which target mite-infested cells, in particular) all function to keep mite populations under control, either by removing infested brood, disrupting the lifecycle of the mite, or removing mites from their bodies.