Collapsing Colonies and Crithidia bombi

By Matthew Lowe

Over the past decade there has been a growing concern over the phenomenon known as Colony Collapse Disorder (CCD) among commercially raised and wild bees. CCD was initially characterized in 2006 by David Hackenberg, a prolific apiarist with multiple hives in Florida and Pennsylvania¹. Upon investigation of lagging production, it was discovered that despite there being no adult worker bees in 20-30% of his hives the queens and brood appeared healthy¹. The loss of worker bees and production from the hive while still maintaining a seemingly healthy brood and queen has now become the definitive end result of CCD.

C. bombi

Since 2006, there have been numerous investigations into potential causes but so far no single cause has been definitively linked to CCD. Some of the more extreme claims include cell phone radiation and commercial use of insecticides, however there is no strong evidence to support that either is the cause of CCD¹.  Of the potential rational causes for CCD, the most likely is an increased prevalence of parasites and viruses within the bee population¹. Crithidia bombi, a recently implicated parasite, is one of the more interesting because of its ability to infect a large variety of different bee species, including those found within the pyrobombus, thoracobombus and bombus sensu stricto subgenera².  Of the North American species infected, Occidentalis and Pensylvanicus are currently undergoing massive population decline due to CCD². C. bombi infection is also 6 fold higher in CCD colonies as opposed to hives not experiencing CCD³.

Figure 1 from here

C. bombi is a pathogenic unicellular eukaryote with two distinct life phases; the flagellated choanomastigote and the anchored amastigote cells⁴. The amastigote cells upon ingestion will extend their flagella and swim as a choanomastigote until they can attach to the bee’s intestinal wall and once again become an amastigote⁴. Upon attachment the cell can siphon off nutrients that pass by and will divide into new amastigote cells⁴. Some of these cells will be excreted in feces and can be transmitted from bee to bee through the ingestion of infectious cells to begin the cycle anew^{2, 4, 5}. The amastigote cells can either be ingested within the hive, leading to a high level of infection amongst the workers and queen, or at flowers allowing for the parasitic colonization of new hives⁵. Once within a hive, infection spreads rapidly until about 80% of the colony is infected⁶. Due to C. bombi having a genotype-genotype model of infection, in which the unique genetic profile of the host and the parasite both play a role in infection success, the highly related individuals within a hive are much more frequently infected⁷. However, once a cross hive infection is established it will rapidly spread within the new colony⁷. While infection has not been linked to any significant lethality in otherwise healthy bees, an infected bee experiencing starvation has a 50% increase in mortality⁵. Upon infection the bee will have a harder time distinguishing between the flowers that are the most rewarding based on color, as shown in figure 1⁸. This suboptimal foraging will lead to less food collection for the hive as a whole and a potential minor starvation event that would cause increased mortality in the infected bees⁸. If a queen is infected she will have reduced ovarian capacity leading to a decreased worker population⁶. An infected queen also experiences a decrease in the ability to store energy as fat for the hibernation over winter greatly decreasing her chances of survival⁶. If the queen manages to survive the winter, she will produce fewer offspring that will also become infected⁶. This generational transmission pattern is particularly vexing for apiarists because an infected queen not show symptoms until the following year when she establishes a colony.  During the time between infection and diagnosis, C. bombi is also being spread to nearby flowers and potentially other hives which could account for the high infection rate in commercial colonies every year.

Figure 2 from here

This continual transmission and difficulty of removing C. bombi begs the question, why should we care if all of the bees die? The economic impact of the insect pollination industry is valued at over 150 billion euros⁹. As shown in figure 2, this is roughly equivalent to 10% the total value of global agricultural production⁹. The use of bees makes up over 70% of the insect pollination industry and the value of crops that are been pollinated is roughly 5 times higher than those that are not⁹. This is not even considering that the apiary industry as a whole, including all of the production facilities to manufacture bee related goods, employs millions of workers worldwide. A shrinking of the bee population will also lead to an increase in the cost of pollination services which will get passed on to the consumer as a wide variety of fruits, nuts and vegetables increase in price to compensate.

While the apiary industry is one of the main driving forces for CCD research, the disorder is not limited to just commercial hives. Commercial bees are commonly raised in greenhouses with a crop to pollinate and it has been shown that a few bees escape and carry infection into the wild¹⁰. The close confines of the greenhouse and the fact that the bees are likely more related due to being commercially raised, infection spreads rapidly between hives leading to a higher parasite load than in wild bees^{7, 10}. Any bee that escapes can spread infectious cells out of the greenhouse and to wild bees through the shared use of nearby flowers. Due to C. bombi’s ability to infect a wide range of bee species, this could lead to collapse of wild bee colonies throughout many different regions of the world. Even more worrisome, commercial colonies are commonly transferred across the globe for pollination purposes as well as to start up new colonies¹. Therefore any infection from a single colony has the potential to spread globally and infect numerous native colonies. While it has been shown that a single bee proof mesh placed over the air duct will greatly diminish the chances of escape, Commercial bee keeping still exists as a potential method of transferring infectious agents to native bee populations across the globe¹⁰. If the native population of bees were to die, thousands of native plant species would lose a prime pollinator which could lead to an inability to efficiently reproduce devastating animal populations that rely on them for food.

Though it is likely that there is no single cause to CCD, the research into it has turned up several interesting parasitic species that all are likely to play a role in colony collapse. Crithidia bombi is but one of the more interesting due to its ability to infect a wide range of bees beyond the commercial species and infection has been correlated with CCD^{2, 3}. This in no way means that C. bombi is the sole cause of CCD but that it likely contributes to the decline of affected hives. Further research into the topic is definitely needed to determine other contributory factors that when combined will lead to CCD. Until a large scale prevention method is identified there are a few things that an individual can do to help maintain local bee populations. Becoming an apiarist and setting up a hive or two in your backyard can help provide a home for local varieties of bees and provide a strong pollination source for nearby gardens¹¹. Planting a variety of species in gardens can also lead to more diverse food sources and increase a colony’s overall health¹¹. By supporting further research into CCD and following through on few basic prevention methods we can hopefully save the bee population as a whole!

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2.     Cordes N, et al. 2012. Interspecific Geographic Distribution and Variation of the Pathogens Nosema bombi and Crithidia Species in United States Bumble Bee Populations

3.     Cornman RS, Tarpy DR, Chen Y, Jeffreys L, Lopez D, et al. (2012) Pathogen Webs in Collapsing Honey Bee Colonies. PLoS ONE 7(8): e43562. doi:10.1371/journal.pone.0043562

4.     Olsen OW. 1974. Animal Parasites: Their Life Cycles and Ecology. Courier Dover Publications.

5.     Deshwai S, Mallon EB. 2014. Antimicrobial Peptides Play a Functional Role in Bumblebee Anti-trypanosome Defense. BioRxiv

6.     Erler S, Popp M, Wolf S, Lattorff HMG. 2012. Sex, horizontal transmission, and multiple hosts prevent local adaptation of Crithidia bombi, a parasite of bumblebees (Bombus spp.). Ecol Evol 2:930–940.

7.     Riddell Carolyn, et al. 2014. Insect Immune Specificity in a Host-parasite model. BioRxiv

8.     Gegear RJ, Otterstatter MC, Thomson JD. 2006. Bumble-bee foragers infected by a gut parasite have an impaired ability to utilize floral information. Proc Biol Sci 273:1073–1078.

9.     Savrieno A. 2012. Colony Collapse Disorder As Part Of An Acquisition Strategy. Seeking Alpha. http://seekingalpha.com/article/590411-colony-collapse-disorder-as-part-of-an-acquisition-strategy

10.  Otterstatter MC, Thomson JD. 2008. Does Pathogen Spillover from Commercially Reared Bumble Bees Threaten Wild Pollinators? PLoS ONE 3:e2771.

11.  What you can do about Colony Collapse Disorder - Honeybees - Silence of the Bees | Nature | PBS. http://www.pbs.org/wnet/nature/episodes/silence-of-the-bees/how-can-you-help-the-bees/36/