Energetic stress as a potential cause of risky foraging in honeybees infected with Nosema ceranae

Parasites are dependent on their hosts for energy to reproduce and can exert a significant nutritional stress on them. The microsporidian Nosema ceranae is a relatively new parasite in the honeybee Apis mellifera that is replacing Nosema apis throughout the world. Using Proboscis Extension Response (PER) and feeding experiments, we show that bees infected with N. ceranae have higher hunger and appetite levels that lead to lower survival. We also demonstrate that the survival of infected bees fed ad libitum is not different from that of uninfected bees, suggesting energetic demand is the primary cause of lowered survival. N. ceranae is suspected to have higher virulence because it creates a higher parasite load than N. apis in its new host Apis mellifera. Energetic demand placed on the host is especially high in this case where the parasite-host complex is less co-evolved. Currently, we are quantifying the imposed energetic stress from infection by comparing hemolymph sugar levels of infected and uninfected bees as they starve over time. The significance of energetic stress as a general mechanism by which infectious diseases influence host behavior and physiology will be discussed. We argue that energetic stress can lead to the precocious and risky foraging observed in Nosema infected bees and discuss its relevance to colony collapse syndrome.