Behavioral alteration in the honeybee due to parasite-induced energetic stress

2012 Spring.Includes bibliographical references.Parasites are dependent on their hosts for energy and honeybee foragers with their high metabolic demand due to flight are especially prone to an energetic stress when they are infected. The microsporidian gut parasite Nosema ceranae is relatively new to the honeybee, Apis mellifera and because it is less co-evolved with its new host the virulence from infection can be particularly high. Using a series of feeding and survival experiments, I found that bees infected with N. ceranae have a higher appetite and hunger level, and the survival of infected bees is compromised when they are fed with a limited amount of food. However, if fed ad libitum the survival of infected individuals is not different from that of uninfected bees, demonstrating that energetic stress is the primary cause of the shortened lifespan observed in infected bees. I then developed a high throughput colorimetric assay to analyze hemolymph sugar levels of individual bees to demonstrate that the parasite mediated energetic stress is expressed as lower trehalose levels in free-flying bees, which suggests that infected bees are not only likely to have a reduced flight capacity but they are also unable to compensate for their lower energetic state. One of the ways in which the changing energetic state of an individual is predicted to impact its behavior is its sensitivity to risk although this has never been convincingly demonstrated. According to the energy budget rule of Risk Sensitivity Theory, it is adaptive for an animal to be risk averse when it is on a positive energy budget and be risk prone when it is on a negative budget because the utility of a potential large reward is much higher in the latter case. By constructing an empirical utility curve and conducting choice tests using a Proboscis Extension Response assay in bees that have been variously manipulated with respect to their energy budgets, I comprehensively demonstrated that bees shift between risk averse to risk prone behavior in accordance with the energy budge rule. Even more importantly, I showed that this shift is contingent upon a change in the energy budget as bees maintained on constant high or low energy budgets were found to be risk indifferent. Given that Nosema infected bees have been seen to forage precociously and inclement weather, my results suggest that such risky foraging might be a consequence of the lower energetic state of infected foragers. As these previous results suggest that parasitism, by lowering their energetic state could have a significant influence on how infected bees forage, I decided to test if the energetic state of an individual can regulate its foraging independent of the colony level regulation of foraging. I uncoupled the energetic state of the individual from that of the colony by feeding individual bees with the non-metabolizable sugar sorbose, thereby creating hungry bees in a satiated colony. I found that these energy depleted bees initially compensate for their lower energetic state by being less active within the colony and taking fewer foraging trips, but not by feeding more within the colony. However, with further depletion in their energetic state, these bees increase their foraging frequency showing that foraging is still partly regulated at the individual level even in a eusocial animal such as the honeybee. My research therefore shows that the energetic stress from a parasite could be a general mechanism that leads to significant behavioral alterations in infected individuals. Since the energetic state of an animal is a fundamental driver of its behavior, such a mechanism underlying behavioral alterations could have a significant impact on the life history of the host and transmission dynamics of a disease. More specifically, these results also suggest that a parasitic infection leading to energy depleted bees going out to forage in a risky manner also provides a plausible mechanism that explains the recent observations of bees disappearing from their colonies

LD50 values may be misleading predictors of neonicotinoid toxicity across different bee species

The importance of not only honey bees (Apis mellifera) but also other non-managed bee species and their pollination services has come to light with their recently reported declines. One contributing factor in these declines is thought to be sub-lethal exposure to neonicotinoid insecticides such as thiacloprid. However, current government regulatory agencies do not require the assessment of insecticide toxicity on bee species other than the honey bee, even though previous studies have demonstrated that sensitivity to insecticides is not likely to be generalizable from honey bees to non-managed bee species. Replicating standardized protocols and testing five different doses of thiacloprid on individual caged bees, we assessed the acute contact toxicity by calculating mortality and the lethal dose (LD50) value for three bee species with different life history traits: Apis mellifera, Bombus terrestris, and Osmia bicornis. We found that Apis mellifera and Osmia bicornis had significantly higher mortality in comparison to Bombus terrestris, but there was no dose-dependent response for any of the three bee species. Bee size and sex were also not useful predictors of thiacloprid toxicity. These results suggest that solely relying on LD50 values, especially when they do not produce a dose-dependent response, may be misleading when assessing insecticide toxicity risk for honey bees and other non-managed bee species

Hive minded: like neurons, honeybees collectively integrate inhibitory stop signals to efficiently regulate forager recruitment

Social insects, such as ants and honeybees, and neurons in the brain, display collective decision-making, whereby groups of individuals or neurons collectively determine which stimuli to respond to and how to respond, yet crucially, no individual is aware that a decision is taking place. Honey bees rely on collective decision making when allocating foragers to the most profitable food source, based on positive reinforcement of the waggle dance. Here we examined whether negative feedback, via the audible stop signal, is being used at a collective level to more efficiently allocate of foragers to profitable food sources. We recorded feeder visits, waggle dances, waggle dance pheromones, and stop signals for bees that were marked and trained to a high 2.5 M sucrose solution feeder that was then switched to a poor food quality of 0.75 M sucrose solution. There was a burst of stop signals directed towards waggle dancing bees mostly from untrained individuals (contra signaling) right after the feeder switch, which then quickly returned back to baseline levels. Following this burst, waggle dancing (recruitment) and waggle dance pheromones continually decreased, but surprisingly previously trained bees visited the poor quality feeder more frequently. We then adjusted a neuron firing rate model with a sigmoidal gain-curve in order to model the dynamics of foragers waggle dancing for one of two potential food sites. We found that the addition of a brief spike of stop signals was enough to reproduce the dynamics of the focal population shown in the experiment. Our results suggest that: (1) honey bees can regulate foraging recruitment on a collective level independent from foraging frequency that appears to be regulated at an individual level and the two are not necessarily correlated, and (2) neural networks and honeybee colonies utilize similar network properties when making a decision

Nosema spp. infections cause no energetic stress in tolerant honeybees

Host-pathogen coevolution leads to reciprocal adaptations, allowing pathogens to increase host exploitation or hosts to minimise costs of infection. As pathogen resistance is often associated with considerable costs, tolerance may be an evolutionary alternative. Here, we examined the effect of two closely related and highly host dependent intracellular gut pathogens, Nosema apis and Nosema ceranae, on the energetic state in Nosema tolerant and sensitive honeybees facing the infection. We quantified the three major haemolymph carbohydrates fructose, glucose, and trehalose using high-performance liquid chromatography (HPLC) as a measure for host energetic state. Trehalose levels in the haemolymph were negatively associated with N. apis infection intensity and with N. ceranae infection regardless of the infection intensity in sensitive honeybees. Nevertheless, there was no such association in Nosema spp. infected tolerant honeybees. These findings suggest that energy availability in tolerant honeybees was not compromised by the infection. This result obtained at the individual level may also have implications at the colony level where workers in spite of a Nosema infection can still perform as well as healthy bees, maintaining colony efficiency and productivity.The Deutsche Forschungsgemeinschaft DFG priority programme SPP 1399 ―Host-parasite co-evolution‖ (grant number MO373/26- 2).http://link.springer.com/journal/4362017-06-30hb2016Zoology and Entomolog

Honey bee (Apis mellifera) exposomes and dysregulated metabolic pathways associated with Nosema ceranae infection

Honey bee (Apis mellifera) health has been severely impacted by multiple environmental stressors including parasitic infection, pesticide exposure, and poor nutrition. The decline in bee health is therefore a complex multifactorial problem which requires a holistic investigative approach. Within the exposome paradigm, the combined exposure to the environment, drugs, food, and individuals' internal biochemistry affects health in positive and negative ways. In the context of the exposome, honey bee hive infection with parasites such as Nosema ceranae is also a form of environmental exposure. In this study, we hypothesized that exposure to xenobiotic pesticides and other environmental chemicals increases susceptibility to N. ceranae infection upon incidental exposure to the parasite. We further queried whether these exposures could be linked to changes in conserved metabolic biological pathways. From 30 hives sampled across 10 sites, a total of 2,352 chemical features were found via gas chromatography-time of flight mass spectrometry (GC-TOF) in extracts of honey bees collected from each hive. Of these, 20 pesticides were identified and annotated, and found to be significantly associated with N. ceranae infection. We further determined that infected hives were linked to a greater number of xenobiotic exposures, and the relative concentration of the exposures were not linked to the presence of a N. ceranae infection. In the exposome profiles of the bees, we also found chemicals inherent to known biological metabolic pathways of Apis mellifera and identified 9 dysregulated pathways. These findings have led us to posit that for hives exposed to similar chemicals, those that incur multiple, simultaneous xenobiotic stressors have a greater incidence of infection with N. ceranae. Mechanistically, our results suggests the overwhelming nature of these exposures negatively affects the biological functioning of the bee, and could explain how the decline in bee populations is associated with pesticide exposures

The microsporidian parasites Nosema ceranae and Nosema apis are widespread in honeybee (Apis mellifera) colonies across Scotland

Nosema ceranae is spreading into areas where Nosema apis already exists. N. ceranae has been reported to cause an asymptomatic infection that may lead, ultimately, to colony collapse. It is thought that there may be a temperature barrier to its infiltration into countries in colder climates. In this study, 71 colonies from Scottish Beekeeper’s Association members have been screened for the presence of N. apis and N. ceranae across Scotland. We find that only 11 of the 71 colonies tested positive for spores by microscopy. However, 70.4 % of colonies screened by PCR revealed the presence of both N. ceranae and N. apis, with only 4.2 or 7 % having either strain alone and 18.3 % being Nosema free. A range of geographically separated colonies testing positive for N. ceranae were sequenced to confirm their identity. All nine sequences confirmed the presence of N. ceranae and indicated the presence of a single new variant. Furthermore, two of the spore-containing colonies had only N. ceranae present, and these exhibited the presence of smaller spores that could be distinguished from N. apis by the analysis of average spore size. Differential quantification of the PCR product revealed N. ceranae to be the dominant species in all seven samples tested. In conclusion, N. ceranae is widespread in Scotland where it exists in combination with the endemic N. apis. A single variant, identical to that found in France (DQ374655) except for the addition of a single nucleotide polymorphism, is present in Scotland