We owe a lot to the honeybees. These amazing little creatures are not just producers of the sweet honey they are also responsible for a large part of the food on our plates. The bees pollinate more than 100 different plant species, including apples, rapeseeds, almonds, and many others. And do the bees it all for free! In fact, the estimated global cost of the pollination service that the bees provide is around 215 billion dollars.
However, over the last decade or so, a marked decline in the bee populations around the world has been noticed. In 2006, the first reports of what is now called the Colony Collapse Disorder (CCD) started to emerge. The term CCD is used to refer to a phenomenon, in which the number of worker bees in a hive decreases so dramatically that the bees are no longer able to maintain a viable colony. The cause for the increased global CCD prevalence is not clear and it appears that many factors are contributing to it, including pesticides, climate change, and habitat loss. In addition, studies have now showed that pathogens, such as viruses, may lead to CCD as well. Perhaps even more worrying is the fact that these pathogens can form mutualistic relationships with the parasites of bees, leading to a somewhat unfair two-against-one fight for survival.
By studying changes in the bee colony population over several summer months, scientists in Italy have noticed that the colonies, which were not treated with pesticides that kill mites, gradually became smaller and smaller. Not surprisingly, mites (appropriately called V. destructor) were infesting the beehives. However, in addition to mites, many samples from the tested bee colonies also contained several different virus species. In particular, the collapse of the bee colonies was correlated to an increase in the so-called Deformed wing virus (DWV) numbers. To assess if there is a link between the number of mites and increased DWV numbers, lab experiments were conducted. Scientists artificially infested several honeybee larvae with the mites and indeed observed that DWV replication has increased in the infected larvae. Moreover, the increase was positively correlated with the number of mites used for infestation, suggesting that DWV is benefiting from the presence of mites.
Like all animals insects, including bees, have an immune system to fight off the many pathogens that are constantly attacking them. Insect immune system, however, is not quite like ours, as they lack the adaptive immunity. On the other hand, insects have some innate immunity signalling pathways, which are quite similar to ours. One such pathway, referred to as NF-κB signalling pathway, plays a major role in activating genes, which act to inhibit pathogen replication in various ways. When scientists looked at what genes were activated in the bees infected with DWV they found that many of the gene involved in the NF-κB signalling pathway were suppressed. This suggested that the virus infection leads to immunosuppression in the bees, which makes them even more vulnerable to further infections and potentially environmental stresses too.
Somewhat surprisingly perhaps, the Deformed wing virus infections at low levels are often asymptomatic in bees. That is DWV alone is unlikely to cause the colony collapse. However, the experiments described above suggest that once the mites are added to the equation the balance tilts and the bees can no longer maintain the DWV load. So, what is the relationship between the V. destructor mites and the DWV? Well, because the feeding of mites on the larvae can increase DWV replication, it is clear that virus benefits from the mites. Moreover, mites can act as vectors for the DWV. Once a mite jumps from one bee in a colony to another the virus basically hitches a ride with it. But what is the benefit for the mite itself? It turns out that among those immune genes that were found to be suppressed when the virus infects a bee, was also a gene that typically controls melanotic encapsulation in insects. Melanotic encapsulation is a process, which insects use to isolate any foreign agents (such as a parasite) which enter their bodies. The encapsulation allows the insect to separate the foreign agent from the rest of insect’s tissues and in this way prevent it from spreading. The foreign agent is initially encapsulated by a group of specialised cells, which then secrete melanin or release antimicrobial peptides to kill the invader. Here, in our bee-virus-mite scenario, the invader is a mite, which makes a hole in the bee’s larva and tries to feed through it. In the absence of the gene required for the melanotic encapsulation the larva cannot defend itself from the feeding mite and is unlikely to survive. There also semms to be another benefit for the mite in collaborating with the DWV. The Deformed wing virus, as its name suggest, causes wing deformations in bees, but only when its replication reaches very high numbers (as perhaps happens when the mites infest the bees). It also appears that there’s a positive correlation between the fertility of mites and the deformed wing phenotype in bees, suggesting that the high virus titres may increase the fertility of V. destructor.
It is a sad tale for the honeybees, it seems. Two against one is definitely not a fear fight. But then again nature is not, and has never been, about the fairness, its concern is always the survival of the fittest. I just hope that in future humans can help to even out this fight. After all, we are partially guilty of the many stresses that are contributing to the colony collapse disorder in the honeybee population. Moreover, without the services that the bees provide our plates would be quite boring and perhaps even half-empty.
Di Prisco, Gennaro, et al. “A mutualistic symbiosis between a parasitic mite and a pathogenic virus undermines honey bee immunity and health.”Proceedings of the National Academy of Sciences (2016): 201523515.
Nazzi, Francesco, et al. “Synergistic parasite-pathogen interactions mediated by host immunity can drive the collapse of honeybee colonies.” PLoS Pathog8.6 (2012): e1002735.
Cox-Foster, Diana. “Saving the honeybee.” Scientific American 300.4 (2009): 40-47. (Nice article for some facts, and great picture of how the food on our plates would look without the bees)