Watch out, honey fans – populations of the honey bee, Apis mellifera, may be in decline, and we don’t really know why. Honey bees play a vital role in natural ecosystems as pollinators; it is estimated that a single bee can visit 2000 flowers in a day . Agriculturally, bees are important for much more than honey, being required for the pollination of many other vegetable, fruit and nut species and generating more than $15 billion dollars in crop value each year . Recent concerns over the plight of the honey bee began during the winter of 2006-7, when beekeepers in the U.S. began reporting dramatic and unprecedented losses of 30-90% of their hives, and from 2006 to 2011 average annual losses were estimated to be 33% . It is still unclear whether or not the decline many beekeepers have observed is something that threatens bee populations worldwide, partly because more recent years have not shown as sharp a decline [2,3]. However, many groups stress that honey bees are facing a real crisis and that we need more measures in place to protect this important species, such as limitation of the use of agricultural pesticides [4,5].
Figure 1 ~ Colony collapse disorder (CCD) is a phenomenon where a healthy bee colony very rapidly changes social structure and dies. After CCD no adult bees remain in the hive. Many factors have been identified as possible causes of CCD including pesticides (in particular, neonicotinoid pesticides), pathogens (including the Varroa mite, gut parasites, and viruses) and nutritional stress such as reduced floral diversity or GMOs.
Sudden death of a society — colony collapse disorder
Regardless of whether or not honey bee decline is a worldwide concern, the fact remains that apparently healthy honey bee colonies can decline very suddenly before a beekeeper has any chance to intervene, a phenomenon known as colony collapse disorder (CCD) . CCD is a rapid process in which a colony can go from apparent health to complete population loss in 2-4 weeks (Figure 1). While many causes for CCD have been proposed, including parasitism, chemical pollutants, and environmental stress, none of these have fully explained the speed of the decline . Additionally, these factors cannot explain the fact that after CCD, there are often remaining food stores and bees still undergoing development in the pupal stage, but no adult bees and few to no dead bees in and around the hive , contrary to what one might expect if food supply had limited the growth of the colony or if some environmental stress had caused elevated death rates. The changes in the social composition of bee colonies during CCD prompted a group of researchers from Australia, the U.K. and the U.S. to examine how young bees might be affected by the absence of older bees in a colony. Their results, published last month in the Proceedings of the National Academy of Sciences (PNAS), showed that young bees took on functional roles in the colony earlier in life when adult bees were absent, and as a result were less effective in their roles . When age-dependent performance was included in mathematical models of bee colony dynamics, the responses of these model colonies to stress closely resembled the rapid declines observed in CCD. Their results suggest that social dynamics of bee colonies affect the robustness of the whole society to stressful conditions in their environment.
Social dynamics of busy bees
Honey bees are just one example of social insects, a group which also includes many species of ants, termites and wasps . Individuals of social insect species live together in large groups or colonies, within which individual insects take on different roles. This division of labor is so strict that that only a few individuals engage in reproduction, while all others are less fertile or sterile and carry out support roles. In bee colonies, there is one fertile female, the queen, who lays all the eggs in that colony, and several fertile males, or drones, whose only function is to mate with the queen . All other females in the same colony develop sterile, and they become worker bees, whose roles include travelling outside the hive to collect nectar and pollen from flowers, defending the hive, and numerous tasks within the hive including producing wax and honey, attending to the queen, and feeding young bees. Such self-organization is thought to be vital for the efficiency and overall success of a colony. In a bee colony, some of the key players are the “forager bees,” those who travel outside to collect resources from flowers (a role they usually begin at 2 to 3 weeks of age). However, one could also imagine that having too many foragers would be disadvantageous, leaving behind too few bees to take care of other tasks in the hive. Such a situation is avoided by mechanisms of social inhibition. For example, active forager bees release chemicals known as pheromones that delay younger bees from starting to forage [6,10]. These social cues help keep the number of active foragers at the level needed for the health of the hive. In the absence of these cues from adult foragers, young bees venture outside the hive earlier. The authors of the above mentioned PNAS study set out to determine the consequences of such “rapid behavioral maturation” both for individual bees and for the health of the colony.
Tracking baby bees’ first flights
The authors of this study, led by Perry, Sovik et al., were initially studying how bees make decisions . To do this, they glued miniature radio frequency identification tags onto the bodies of individual bees, which allowed the bees to be uniquely identified and their motions to be tracked when they travelled past a radio antenna. The authors realized that this system could also be used to investigate the foraging activities of bees, by placing antennae at the entrance and exit points of a hive. Using this type of set-up, they examined foraging dynamics in normal hives containing bees of different ages (normal-worker demography, NDC), and hives they established to have an unusual dynamic using bees collected within 24 hours of their emergence as adults (single-cohort colony, SCC). The SCC is therefore a colony of bees around the same age, and initially, with no mature adults present. Bees for both NDCs and SCCs were taken from the same original colonies, to minimize differences between the two set-ups and increase chances that any changes observed were due to the artificially established difference in age. After the hives were set up in this way, new young bees were introduced with radio tags, and their activities were monitored for 40 days.
As expected, bees in SCCs showed altered behavior compared to those in NDCs. There were more bees that started foraging before being 14 days old in SCCs. Additionally, these young foragers also turned out to be less effective – they spent less time outside the hive, performed fewer trips, and were less likely to survive their trips and return to the hive. Therefore, starting to forage too early in life has significant costs to the individual bee. What about the cost to the colony as a whole? To measure this, the authors used their newly found result that foraging efficiency and survival are dependent on age in a mathematical model of hive dynamics.
What can a mathematical model tell us about social insect biology?
The number of bees in a colony at any given time is a function of different variables, including the amount of food and number of bees present to begin with, the death rate of bees, and the efficiency with which bees bring back food to the hive. The work described above suggests that the latter two variables are influenced by the age at which foraging begins and hence the ratio of adult foragers to young bees. With these findings, the authors created new equations predicting the number of bees and amount of food present in a hive over time, and examined how these outputs changed as one variable, death rate, was increased. Increased death rate was used as a proxy for the response of bees to stress, be it a disease or some other kind of environmental pressure. When death rate was increased, the model predicted a sudden decline in the number of adult bees, a trend which looked like CCD and which was not seen in previous models where age-dependent foraging efficiency was not taken into account .
Overall, these results show that when you consider the effect of age of first flight on the productivity of foraging behavior of bees, you can explain why a colony that starts to experience stress/elevated death, while initially being able to handle the stress, eventually goes into a rapid and irreversible decline as more and more younger bees inefficiently forage leading to more and more stress. This could be why no one disease pathogen or type of environment has been shown to be responsible for CCD; the study described here suggests that any number of different kinds of stress can cause CCD if they generate social imbalances in the colony. Of course, several questions remain. Will the model’s predictions hold true for an actual bee colony? Exposing an SCC to stress and looking for signs of CCD would both be informative and provide a system in which to test out different ways of stopping CCD . Additionally, what about aspects of CCD not explained by the authors’ model? For example, hives suffering from CCD often have food stores remaining, while in the theoretical model, the low efficiency of young foragers would reduce the amount of food in the hive and be a key factor in the decline of the hive. The authors suggest that the food stores left behind may not have all the nutritional variety required for well-being of the hive (for example, both proteins and carbohydrates) and call for a closer examination of what food is actually left behind following CCD .
Currently, it is very difficult to predict when a colony might be about to undergo CCD, giving beekeepers very little opportunity to intervene. The work done here suggests that, rather than looking at the brood size or honey stores, determining how many foragers are present or the age at which young bees start to forage might be better ways to predict CCD . Overall, though the phenomenon of CCD is still somewhat mysterious, it is clear that the social dynamics of a honey bee hive can influence its health, and that we must pay attention to these for the sake of this ecologically and agriculturally important creature.
Niroshi Senaratne is a graduate student in the Biological and Biomedical Sciences program at Harvard Medical School. Special thanks to Kristen Seim, a postdoctoral fellow in the Department of Chemistry and Chemical Biology at Harvard University, for figure design.
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