The mysterious cause of Colony Collapse Disorder

By Heiloi Yip

Colony Collapse Disorder (CCD) is a phenomenon that affects honey bee hives, severely crippling the health of the colony. A large proportion of the colony’s worker bees mysteriously disappear, leaving the queen and larvae behind to starve or freeze to death. This phenomenon can occur rapidly, with collapse happening within a few weeks. CCD mainly affects domesticated colonies of the European honey bee (Apis mellifera), although it has also been observed in other domesticated honey bee species (van Engelsdorp et al., 2017). CCD is believed to be a recent phenomenon, having its first reported instance in the winter of 2006. Since then, beekeepers have seen between 30% to 90% of their hives lost to CCD in the United States (Suryanarayanan & Kleinman, 2013; Watanabe, 2008).

For humans, honey bees hold vast cultural and socioeconomical values. They produce food and material products such as honey and wax, which may contribute to the local economy in the form of exported goods. More significantly, as efficient pollinators, honey bees hold ecological and agricultural importance. Some plants are extremely dependent on pollination for a variety of reasons, such as the production of seeds and the propagation of their species. Ecosystems may rely on the pollination of native plants to maintain a healthy level of biodiversity, while many crops such as almonds are dependent on pollination to produce harvests at economically feasible quantities. The loss of a major pollinator would be very damaging to local ecosystems and food production systems (Artz et al., 2013). 

The exact cause of CCD is still unknown, although there have been multiple proposals. The initial proposal for the cause was the effects of pesticides and other toxins. Indeed, honey bees can be affected by insecticides, which accumulate in their bodies beyond lethal doses. However, the evidence is too weak to support this claim, with little correlation between CCD cases and pesticide usage (Suryanarayanan & Kleinman, 2013). Climate change was next to be blamed for disrupting the bee’s natural life cycle, but that claim also lacked concrete evidence. Due to the tendency of CCD to spread from infected colonies to nearby healthy colonies, it is believed that the most likely culprit is some sort of pathogen or parasite. Microarray analysis showed that CCD-affected worker bees possess higher quantities of viral rRNA in their body than healthy bees, suggesting that a virus is responsible for causing the disorder. This would explain the phenomenon of apparent CCD outbreaks, as beekeepers in the US tend to transport beehives across states for ‘rental’, thus potentially spreading pathogens harboured in infected hives (Johnson et al., 2009; van Engelsdorp et al., 2009).

When attempting to identify the responsible strain however, different CCD-afflicted colonies returned different virus strains, sometimes containing multiple strains in one hive. As such, the cause cannot be pinned on a single strain of virus, instead seemingly involving a complex set of interactions between the pathogens and the host immune system. Another explanation is that rather than the viruses being the cause, CCD is the result of compromised immune systems of the bees. Bees are known to have weakened immune systems when overstressed, which could arise from improper hive temperature or from overworking. Stress on the larvae also negatively impacts the development of worker bees, resulting in reduced lifespans and greater likelihood of dying while foraging outside the hive, which would explain the lack of bee corpses around CCD-affected hives (Zeaiter & Myerscough, 2020). There are certainly many factors that seem to play into a colony becoming inflicted with CCD. Perhaps the cause may not even be attributable to any single factor, instead requiring a complex set of conditions to be fulfilled. The exact combination of conditions for triggering CCD is yet to be replicated (van Engelsdorp et al., 2009).

While the cause of CCD is not fully understood, measures can be taken to minimize the impact of CCD. Following the standard procedure of dealing with a CCD-affected colony, the hive and all related equipment are immediately put into quarantine and disinfected, while preventing bees from healthy colonies reaching the contaminated regions. This is only a short-term solution however due to the incubation period of CCD of around 3 weeks. Looking more on the long term, some groups are developing breeds of honey bees with resistance to multiple types of viruses and parasites, in a bid to produce colonies with reduced chances of CCD infection. While this would certainly lead to healthier bees, it is not a guaranteed immunity from CCD (van Engelsdorp et al., 2009). For farms and orchards that rely on honey bees to pollinate crops, it would be advisable to not over-rely on honey bees as the sole pollinators, and instead invest in housing alternative pollinators such as bumble bees or mason bees. Even if all the colonies in an area has been afflicted with CCD, a diverse backup force of pollinators are still present to maintain the health of the plants. 

References:

Artz, D., Allan, M., Wardell, G. and Pitts-Singer, T., 2013. Nesting site density and distribution affectOsmia lignaria(Hymenoptera: Megachilidae) reproductive success and almond yield in a commercial orchard. Insect Conservation and Diversity, 6(6), pp.715-724. DOI: 10.1111/icad.12026

Johnson, R., Evans, J., Robinson, G. and Berenbaum, M., 2009. Changes in transcript abundance relating to colony collapse disorder in honey bees (Apis mellifera). Proceedings of the National Academy of Sciences, 106(35), pp.14790-14795.

Suryanarayanan, S. and Kleinman, D., 2013. Be(e)coming experts: The controversy over insecticides in the honey bee colony collapse disorder. Social Studies of Science, 43(2), pp.215-240. DOI: 10.1177/0306312712466186

van Engelsdorp, D., Evans, J., Saegerman, C., Mullin, C., Haubruge, E., Nguyen, B., Frazier, M., Frazier, J., Cox-Foster, D., Chen, Y., Underwood, R., Tarpy, D. and Pettis, J., 2009. Colony Collapse Disorder: A Descriptive Study. PLoS ONE, 4(8), p.e6481. DOI: 10.1371/journal.pone.0006481

van Engelsdorp, D., Traynor, K., Andree, M., Lichtenberg, E., Chen, Y., Saegerman, C. and Cox-Foster, D., 2017. Colony Collapse Disorder (CCD) and bee age impact honey bee pathophysiology. PLOS ONE, 12(7), p.e0179535. DOI: 10.1371/journal.pone.0179535

Watanabe, M., 2008. Colony Collapse Disorder: Many Suspects, No Smoking Gun. BioScience, 58(5), pp.384-388. DOI: 10.1641/B580503

Zeaiter, Z. and Myerscough, M., 2020. Poor hive thermoregulation produces an Allee effect and leads to colony collapse. Journal of Theoretical Biology, 503, p.110361. DOI: 10.1016/j.jtbi.2020.110361

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