1 September 2017

Like any organism, bees have diseases. In honey bees, there are treatments against the Varroa mite. With other diseases, the beekeeper just can give some help, like chosing the right place for the apiary etc. Non-managed bees, however, have to deal with their parasites and pathogens without human help. This doesn’t mean they’re completely helpless: they may actively search for cures or preventions, which is called self-medication. This means that an individual takes up substances to prevent or cure a disease or parasite infestation.

Plants produce certain compounds, so-called secondary metabolites, to protect themselves against herbivores. Some examples are nicotine, caffeine or solanine (in tobacco, coffee and potatoes, for instance). These substances are toxic for herbivores, but also protect them against parasites in some cases. As nectar often contains secondary metabolites, they may have the same function also for pollinators. In fact, there is some evidence that this may be the case.

Some indications of self-medication in bees

Self-medication is a deliberate behaviour, which means that an animal actively searches for a cure or prevention against parasites and pathogens. It doesn’t happen coincidentally. Therefore, this is a complex behaviour, which many wouldn’t attribute to insects. But, as often, they surprise us, there are actually several examples for it. A good known example are honey bees: they collect resin from plants and transform it into propolis together with some of their secretions. This is known to be a protection for the nest, the workers line the interior of the hive with a thin layer of it. Actually, the name itself indicates the protection: “propolis” means “in front of the city” or “in defence of the city”. It has several properties against bacteria, fungi and even viruses. But the question remains: is this a deliberate behaviour, or just something that produced a coincidental benefit to the colonies?

There is some evidence for the former: Simone-Finstrom and Spivak made experiments challenging colonies with chalkbrood, a fungal pathogen of the brood. In a final stage, the fungus completely covers the larva, forming typical “mummies”. In response to this, the colonies actively collected more plant resins – fulfilling the requirement of deliberate behaviour. They found less “mummies” in propolis-rich colonies than in those with less resin, showing that this behaviour really helps against the pathogen.

But what about the secondary metabolites in the nectar? Do they help bees against their parasites? Yes, there is some evidence for this, too. In laboratory experiments, bumblebees infected with Crithidia bombi (a gut parasite) preferred sugar syrup with nicotine to sugar solutions without this substance. This substance did not clear the infestation, but delayed it. The effect on the parasite was relatively weak, but the authors suggest subtle benefits for the host fitness or their colonies.

Helping the individual adult and the brood

Another group of scientists tested a total of eight nectar chemicals against the same parasite. In the lab, four of these secondary metabolites reduced the load of C. bombi from 61-81%. However, even the most efficient chemical, anabasine, did not mitigate the negative effects of the parasite on the individual. The infested bees also didn’t drink more sugar solution with this chemical. The results therefore aren’t explicit, but there is a hint for self-medication in bumblebees that surely is worth further studies. In addition, these studies stress the importance of nutrition for bee health, as I already discussed in the nutrition series.

You may have noticed an important difference between the honey bee and bumblebee studies: in the honey bee example, the protection was directed to the brood or the colony as a whole. The bumblebees, on the other hand, primarily medicated themselves as an individual. This in a second step may have helped also the colonies, though. But, in honey bees we speak of “social immunity”, because of multiple behaviours and mechanisms to protect the superorganism honey bee colony against parasites and diseases. Bumblebees are social, too, though in a more “primitive” way. I’m wondering if social evolution can be reconstructed also by different levels of social immunity. I’m not aware of any data about this.

Solitary bees – what about about their self-medication?

The other thing which intrigues me: if secondary metabolites help bumblebees, they should also have an effect on solitary bees’ parasites, or even other pollinator parasites. I don’t know anything about this and didn’t find any references. Suggestions are very welcome. But, solitary bees may be a good study object for something else: the relationship between specializing on certain plants and parasitism. As I already discussed in another post, specialization on certain plant species may also be a consequence of avoiding parasitism. Dakota Spear and her co-authors discuss some Osmia species oligolectic on plants of the daisy family. The pollen of these plants has low nutritive value, but may protect the offspring from cleptoparasites.

They are two aspects here: in this case the anti-parasitic properties come from the pollen, not nectar. It would be interesting to know if the pollen of other plant species have the same effect and if this influenced the evolution of other oligolectic species. In general, these facets of bee health are clearly understudied in solitary bees. The other element to consider is that many bee species consume pollen that may have these anti-parasitic properties. Is that bumblebee on the photo above foraging for the colony or medicating itself? Are dandelions so popular to a variety of pollinators not only because of their high amount of nectar and pollen but also for their health benefits? And what role do the different nesting materials of solitary bees (resin, leaves etc.) have for the protection of their offspring? There are many more open questions that, once studied, may change our perception of bee health.

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