Unless you’ve lived under a rock for the past 5 or 6 years (if you have, all the power to you) you have probably heard about how all the honey bees in the U.S have been mysteriously disappearing. Well my partner and I decided to investigate this phenomenon know as Colony Collapse Disorder and try to synthesize the current state of the research surrounding this strange occurrence.
First let’s start with the importance of the Western Honey Bee. It is estimated that 30% of the human diet is credited to the pollination services provided by honey bees. Additionally, in the U.S alone, managed honey bee colonies are the center of a 15 billion dollar economy! And lastly, the honey bee is widely regarded as a general indicator of ecosystem and environmental health.
However, in the past 60 years, the U.S has experienced a steady decline in pollinator populations, with losses of over 3 million colonies (more than 50% of the total population). In the past 5 or 6 years, beekeepers and scientists have begun to see particularly heavy losses in the winter, with colonies virtually disappearing over night. Colony collapse disorder, as it is now known, is made up of the following symptoms:
- An almost complete loss of adult worker bees, with no dead bees found near the hive
- A healthy queen bee and a a disproportionately young workforce
- The remaining bees reluctance to consume ample food stores
- Upon total collapse, a reluctance of neighboring colonies to raid food stores
We investigated the potential for multiple factors to be working together to create these massive die-offs, as the research has been unable to find a clear culprit. The first factor discussed is the usage of systemic pesticides. Systemic pesticides spread throughout all the tissues of a plant, including the nectar and pollen. This means that adult forager bees are receiving direct exposure to the pesticides, and that entire colonies are experiencing indirect exposure when the foragers return. Systemic pesticides are known as neonicotinoids, which have been shown to have significant effects on the central nervous system.
A study by Pettis et al. demonstrated that honey bees exposed to a systemic pesticide known as imidacloprid were significantly more susceptible to infection from the gut pathogen Nosema (figure 1). A second study by Henry et al. showed that exposure to systemic pesticides decreased foraging success in honey bees. The bees were fitted with radar tagging devices to track their position (figure 2). The bees experienced significant “homing failure,” with up to 31% of bees exposed to pesticides unable to find their way back to hive after foraging. Mortality due to homing failure was even higher when the bees were unfamiliar with their foraging area, as one would expect. Here we can see how just 1 factor, pesticides, is able to have multiple effects on bee health and how these factors could interact to weaken colonies.
The missing pieces, as we saw it, came from 2 studies by Mayack and Naug. The first study demonstrated that bees infected with the parasite Nosema (from the 1st experiment) are experiencing elevated hunger levels as a result of energetic stress (figure 3). The bees suffer from energetic stress for two reasons. First, Nosema is parasitic; it robs the bees of carbohydrates, proteins, and other nutrients that it would normally receive while foraging. Second, the bee is now forced to launch an immunological attack, which uses more energy. In their experiment, Mayack and Naug show that infected honey bees experience higher hunger levels and higher rates of mortality due to starvation. Honey bees are clearly trying to compensate for this nutritional stress. A key fact here is that in this experiment the bees were not required to fly in order to reach their food, and were kept at an ideal temperature. This means that neither the high energy cost of flying, nor the cost of thermoregulation were taken into account here. This signifies that in a real world situation the energetic stress experience by bees is probably much greater. It is also important to note here that the foraging behavior of bees is determined by the nutritional stress of the individual and not of the hive, so an infected bee would feel the urge to forage even if the hive has plenty of food.
The last part of this puzzle comes from what could be considered more of a series of observations than an experiment conducted by Naug. Essentially, Naug shows that natural bee habitat has been generally declining while urban development and monocropping have been increasing over the past 40 years, matching the decline in pollinators (figure 4). The consequences of these land changes are threefold: bees must travel further to reach foraging areas, the suitable foraging areas are now smaller, and therefore there is higher competition for resources.
When we look at all of these factors working together, the end result looks something like this: energy-deficient honey bees are travelling further and facing more risks and higher levels of competition in order to satisfy unnatural levels of hunger. This in turn wastes more energy, further weakening the nutrition-starved bees who are now having difficulty finding their way home.
If you ask me, that sounds like a pretty good way to have all your worker bees vanish without a trace.
Note: this is a very condensed, simplified version of a 40 minute presentation. If you would like more information and/or a works cited, please contact me.