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

Modeling disruption of Apis mellifera (honey bee) odorant-binding protein function with high-affinity binders

Chemical toxins pose a great threat to honey bee health because they affect memory and cognition, diminish immunity, and increase susceptibility to infection, resulting in decreased colony performance, reproduction, and survival. Although the behavioral effects of sub-lethal chemical exposure on honey bees have been intensively studied, how xenobiotics affect olfaction, at the molecular level, still needs to be elucidated. In the present work, in silico tools, such as molecular docking, binding free energy calculations, and molecular dynamics simulations are used to predict if environmental chemicals have stronger binding affinities to honey bee antennal odorant-binding protein 14 (OBP14) than the representative floral odors citralva, eugenol, and the fluorescent probe 1-N-phenylnaphthylamine. Based on structural analysis, 21 chemicals from crop pesticides, household appliances, cosmetics, food, public health-related products, and other sources, many of which are pervasive in the hive environment, have higher binding affinities than the floral odors. These results suggest that chemical exposures are likely to interfere with the honey bee's sense of smell and this disruptive mechanism may be responsible for the lower associative learning and memory based on olfaction found in bees exposed to pesticides. Moreover, bees mainly rely on olfactory cues to perceive their environment and orient themselves as well as to discriminate and identify their food, predators, nestmates, and diseased individuals that need to be removed with hygienic behavior. In summary, sub-lethal exposure to environmental toxins can contribute to colony collapse in several ways from the disruption of proper olfaction functioning

Honeybee Colony Vibrational Measurements to Highlight the Brood Cycle

Insect pollination is of great importance to crop production worldwide and honey bees are amongst its chief facilitators. Because of the decline of managed colonies, the use of sensor technology is growing in popularity and it is of interest to develop new methods which can more accurately and less invasively assess honey bee colony status. Our approach is to use accelerometers to measure vibrations in order to provide information on colony activity and development. The accelerometers provide amplitude and frequency information which is recorded every three minutes and analysed for night time only. Vibrational data were validated by comparison to visual inspection data, particularly the brood development. We show a strong correlation between vibrational amplitude data and the brood cycle in the vicinity of the sensor. We have further explored the minimum data that is required, when frequency information is also included, to accurately predict the current point in the brood cycle. Such a technique should enable beekeepers to reduce the frequency with which visual inspections are required, reducing the stress this places on the colony and saving the beekeeper time

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

Effects of burial on leaf litter quality, microbial conditioning and palatability to three shredder taxa

1. Heterotrophic microorganisms are crucial for mineralising leaf litter and rendering it more palatable to leaf-shredding invertebrates. A substantial part of leaf litter entering running waters may be buried in the streambed and thus be exposed to the constraining conditions prevailing in the hyporheic zone. The fate of this buried organic matter and particularly the role of microbial conditioning in this habitat remain largely unexplored. 2. The aim of this study was to determine how the location of leaf litter within the streambed (i.e. at the surface or buried), as well as the leaf litter burial history, may affect the leaf-associated aquatic hyphomycete communities and therefore leaf consumption by invertebrate detritivores. We tested the hypotheses that (i) burial of leaf litter would result in lower decomposition rates associated with changes in microbial assemblages compared with leaf litter at the surface and (ii) altered microbial conditioning of buried leaf litter would lead to decreased quality and palatability to their consumers, translating into lower growth rates of detritivores. 3. These hypotheses were tested experimentally in a second-order stream where leaf-associated microbial communities, as well as leaf litter decomposition rates, elemental composition and toughness, were compared across controlled treatments differing by their location within the streambed. We examined the effects of the diverse conditioning treatments on decaying leaf palatability to consumers through feeding trials on three shredder taxa including a freshwater amphipod, of which we also determined the growth rate. 4. Microbial leaf litter decomposition, fungal biomass and sporulation rates were reduced when leaf litter was buried in the hyporheic zone. While the total species richness of fungal assemblages was similar among treatments, the composition of fungal assemblages was affected by leaf litter burial in sediment. 5. Leaf litter burial markedly affected the food quality (especially P content) of leaf material, probably due to the changes in microbial conditioning. Leaf litter palatability to shredders was highest for leaves exposed at the sediment surface and tended to be negatively related to leaf litter toughness and C⁄P ratio. In addition, burial of leaf litter led to lower amphipod growth rates, which were positively correlated with leaf litter P content. 6. These results emphasise the importance of leaf colonisation by aquatic fungi in the hyporheic zone of headwater streams, where fungal conditioning of leaf litter appears particularly critical for nutrient and energy transfer to higher trophic levels

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

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

Early stages of leaf decomposition are mediated by aquatic fungi in the hyporheic zone of woodland streams

1. Leaf litter constitutes the major source of organic matter and energy in woodland stream ecosystems. A substantial part of leaf litter entering running waters may be buried in the streambed as a consequence of flooding and sediment movement. While decomposition of leaf litter in surface waters is relatively well understood, its fate when incorporated into river sediments, as well as the involvement of invertebrate and fungal decomposers in such conditions, remain poorly documented. 2. We tested experimentally the hypotheses that the small interstices of the sediment restrict the access of the largest shredders to buried organic matter without compromising that of aquatic hyphomycetes and that fungal decomposers in the hyporheic zone, at least partly, compensate for the role of invertebrate detritivores in the benthic zone. 3. Alder leaves were introduced in a stream either buried in the sediment (hyporheic), buried after 2 weeks of exposure at the sediment surface (benthic-hyporheic), or exposed at the sediment surface for the entire experiment (benthic). Leaf decomposition was markedly faster on the streambed surface than in the two other treatments (2.1- and 2.8-fold faster than in the benthic-hyporheic and hyporheic treatments, respectively). 4. Fungal assemblages were generally less diverse in the hyporheic habitat with a few species tending to be relatively favoured by such conditions. Both fungal biomass and sporulation rates were reduced in the hyporheic treatment, with the leaves subject to the benthic-hyporheic treatment exhibiting an intermediate pattern. The initial 2-week stage in the benthic habitat shaped the fungal assemblages, even for leaves later subjected to the hyporheic conditions. 5. The abundance and biomass of shredders drastically decreased with burial, except for Leuctra spp., which increased and was by far the most common leaf-associated taxon in the hyporheic zone. Leuctra spp. was one of the rare shredder taxa displaying morphological characteristics that increased performance within the limited space of sediment interstices. 6. The carbon budgets indicated that the relative contributions of the two main decomposers, shredders and fungi, varied considerably depending on the location within the streambed. While the shredder biomass represented almost 50% of the initial carbon transformed after 80 days in the benthic treatment, its contribution was <0.3% in the hyporheic one and 2.0% in the combined benthic-hyporheic treatment. In contrast, mycelial and conidial production in the permanently hyporheic environment accounted for 12% of leaf mass loss, i.e. 2–3 times more than in the two other conditions. These results suggest that the role of fungi is particularly important in the hyporheic zone. 7. Our findings indicate that burial within the substratum reduces the litter breakdown rate by limiting the access of both invertebrate and fungal decomposers to leaves. As a consequence, the hyporheic zone may be an important region of organic matter storage in woodland streams and serve as a fungal inoculum reservoir contributing to further dispersal. Through the temporary retention of litter by burial, the hyporheic zone must play a significant role in the carbon metabolism and overall functioning of headwater stream ecosystems