Watch out, honey fans, honey bee numbers are declining, and new research suggests the decline may be impacted by bees’ social health.

Honey bees play a vital role in natural ecosystems as pollinators; it is estimated that a single bee can collect pollen from 2000 flowers in a day. Agriculturally, bees are important for pollinating many crops, contributing to more than $15 billion dollars in agricultural value each year. Concerns about the plight of the honey bee arose during the winter of 2006-7, when beekeepers in the U.S. began reporting dramatic and unprecedented losses of 30-90% of their hives. From 2006 to 2011, bee populations have continued to decline roughly 33% each year. It is still unclear if the decline is something that threatens bee populations worldwide, partly because losses have not been as high in recent years (e.g., 2012 and 2014). However, many environmental activists stress that honey bees are facing a real crisis and that we need more measures to protect this important species, like by limiting the use of pesticides.

Sudden Death of a Society—Colony Collapse Disorder

Healthy colonies can decline very suddenly before a beekeeper has any chance to intervene. This phenomenon is known as colony collapse disorder (CCD). CCD occurs rapidly: in just 2-4 weeks, a healthy colony can be completely wiped out (Figure 1). While some explanations for CCD have been proposed (e.g., parasites, chemical pollutants, and environmental stress), no single factor fully explains the rate of decline. Additionally, these factors cannot explain why food stores and developing bees remain while adult bees and dead bees are practically nonexistent in and around the hive following collapse. These observations are contrary to what is expected if food supply had limited the growth of the colony or if environmental stress had caused elevated death rates.

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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. Credit: Kristen Seim, Department of Chemistry and Chemical Biology, Harvard University.

The changes in the social composition of bee colonies during CCD prompted researchers to examine how young bees might be affected by the absence of older bees in a colony. Results showed that young bees took on functional roles in the colony (e.g., foraging) earlier in life when adult bees were absent and, as a result, were less effective in their roles. Therefore, the success of the individual bees depends on the age they take on their roles. When age 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 the social dynamics of bee colonies affect the ability of the whole colony to withstand environmental stress.

Social Dynamics of Busy Bees

Honey bees are just one of many social insects (other examples include ants, termites, and wasps). Social insects live together in large groups or colonies, within which individual insects take on different roles: only a few individuals reproduce while all others are less fertile—or even sterile—and carry out support roles.

In bee colonies, the queen is the one fertile female who lays all the eggs, and several fertile males, or drones, could mate with the queen. All other females in the same colony become sterile worker bees, who travel outside the hive to collect nectar and pollen from flowers, defend the hive, and perform numerous tasks within the hive such as 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.

“Forager bees” are the insects who travel outside and collect resources from flowers starting at around 2 to 3 weeks old. However, having too many foragers means fewer bees would be left behind to take care of tasks within the hive. Bees avoid these situations by social inhibition: active forager bees release chemicals known as pheromones that delay younger bees from foraging. These cues help keep the number of active foragers at the numbers needed for the overall health of the colony. In the absence of pheromones, young bees venture outside the hive earlier.

Tracking Baby Bees’ First Flights

In “Rapid behavioral maturation accelerates failure of stressed honey bee colonies,” Clint Perry et al. studied how bees make decisions and communicate. To do this, the researchers glued miniature radio frequency identification tags onto the bodies of individual bees, which tracked each bee. Using this set-up, the researchers examined foraging dynamics in normal hives with bees of different ages (normal-worker demography, NDC) and in hives with only bees of similar ages (single-cohort colony, SCC). Bees for both NDCs and SCCs were taken from the same original colonies to minimize differences between the two set-ups and increase the likelihood that any changes observed were due to the artificially established difference in age. After the hives were set up, younger bees were tagged with radio labels and their activities were monitored for 40 days.

The behavior of bees in SCCs was different compared to that of NDCs. In the SCCs, more bees started foraging before they were 14 days old. 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. Bees, in other words, do not benefit if they start foraging too early in life.

What Can a Mathematical Model Tell Us About Social Insect Biology?

Does the unproductivity of individual bees affect the colony as a whole? To address this question, the authors used their newly found result that age affects foraging efficiency and survival to create a mathematical model of hive dynamics.

The number of bees in a colony is dependent on many variables, such as the preexisting amount of food and number of bees, the death rate of bees, and the amount of food bees bring back to the hive. The authors found that the latter two variables are influenced by the age bees start to forage. The researchers then created new equations predicting death rate, the number of bees, and amount of food present in a hive over time. When death rate was increased, the model predicted a sudden decline in the number of adult bees, a trend that resembled CCD. Previous models did not account for the age of bees, and instead showed a slow decline in the number of bees.

Overall, these results show that younger forager bees can contribute to colony collapse due to their worsened performance, which could explain why no single disease pathogen or type of environmental stress has been shown to be responsible for CCD. In other words, multiple stressors can cause CCD by creating social imbalances in the colony.

Of course, several questions remain. Will the model represent what happens in an actual bee colony? Exposing an SCC to stress and looking for signs of CCD would be informative and provide a system to test out different ways of stopping CCD. Additionally, there may be aspects of CCD that the author’s model didn’t account for. For example, hives undergoing CCD often have food remaining, whereas the young foragers depleted all the food in the theoretical model. To account for the different observations, the authors suggest that the food left behind may not provide enough nutrition for the entire hive, meaning more work should be done to see what food is actually left behind after CCD.

Currently, it is difficult to predict when a bee colony will collapse, giving beekeepers very little opportunity to intervene. The research described here suggests that, rather than looking at the number of developing bees or honey stores, determining how many foragers are present or the age at which bees start to forage might be better ways to predict the onset of CCD. Though the phenomenon of CCD is still somewhat mysterious, it is clear that the social dynamics of a honey bee hive can influence a colony’s health.

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Image Credit:  Jerry Kirkhart via flickr