Strategies of disease resistance in the superorganism: investigations into the effects of diet on the immunocompetence and behaviour of honey bees.

Abstract

As superorganisms, eusocial insect colonies possess both individual and social strategies for epidemic control. Both the physiological immune system within individuals and an array of social behaviours, such as self–quarantine, collectively comprise the colony’s immunocompetence. Diet is a modulator of immunocompetence in insects and furthermore insects can self-medicate by ingesting nutrients that promote immunocompetence. However, how diet impacts multiple strategies of epidemic control both physiologically and through social behavioural processes, within a superorganism is not known. Therefore, the central aim of this thesis is to elucidate the role of diet for immunocompetence in the eusocial European honey bee (Apis mellifera). In the first data chapter (Chapter 2), I set the framework for measuring honey bee immunocompetence by describing a time course for the expression of two key components of the physiological immune system after challenge; the phenoloxidase pathway and antimicrobial peptides. I establish that only antimicrobial activity is elicited by an experimental pseudo-bacterial challenge and I identify appropriate time points to assess the impact of diet on immunocompetence. I demonstrate that short-term pollen starvation has no impact on physiological immunocompetence. In chapter 3 I show that a pseudo-bacterial challenge causes honey bees to adopt a diet that reduces their intake of pollen, whilst maintaining their intake of carbohydrates and I demonstrate that immunologically challenged honey bees forage more intensively. Based on these two findings, I therefore propose that a dietary mechanism underlies increased foraging intensity, which is adaptive as a form of nutritional targeting for self-removal to reduce colony infection. In chapter 4, I demonstrate that a sustained lack of essential amino acids both promotes antimicrobial peptide activity and reduces longevity. Furthermore, I show that, like the trend observed with pollen consumption, a pseudo-bacterial challenge causes honey bees to reduce their intake of essential amino acids, Taken together, these results provide new support for the proposition that through dietary modulation, honey bees nutritionally self-medicate at the level of the superorganism in order to reduce in-hive rates of pathogen transmission by increased physiological immunocompetence, self-removal, and mortality in infected individuals.