Foraging and disease – healthy nutrition for bees
Enough pollen and nectar sources keep bees healthy – simply by providing them all the nutrients they need. But there is also another aspect to this. Flowers may be the place where predators wait (for example crab spiders) and eat them. Which surely is unhealthy for the bee. Or, in a more subtle way, because an ill bee left some pathogens on the flower. The next flower visitor then gets this infection, too. I already wrote about the manmade element in disease transmission between honey bees and other pollinators. But there are several other aspects to the relationship between bee foraging and bee health. A very interesting review summarizes the current knowledge but also the concerning gaps that exist in this field.
The most obvious aspect of the relationship between foraging and and bee health is the nutritional one. Bumblebees that get a protein-rich diet are less susceptible to the gut parasite Crithidia bombi. And diverse pollen diets enhance the immune reactions of honey bees. Though the high nutritional value is also positive for the parasites, bees that have access to a high quality diet are healthier. For instance, well-fed bumblebees had higher loads of C. bombi. However, they survived longer than bees that were starving. The parasite numbers alone, therefore, does not determine the severity of the disease. The nutriotional quality strongly influences the susceptibility or resistance of bees against diseases. This is encouraging and concerning at the same time. Encouraging because it means that we can help bee health by providing diverse food sources. But concerning, because of the “green desertification” of agricultural landscape.
Floral traits for healthy bees
As mentioned above, bee pathogens are transmitted on flowers. Healthy bees can “pick up” diseases and parasites while foraging and also transport it to new flowers. By this, pests and pathogens can spread between individuals of the same species; but, unfortunately, also to new species. This has done some harm especially by introducing alien bees: the honey bee parasite Nosema ceranae has been found in several other bee species. This is partly a double introduction: N. ceranae is originally a parasite of the Asian honey bee Apis cerana and has switched host to A. mellifera. On the American continent, where honey bees are an alien species, they have now transferred this parasite also to other pollinators. European bumblebees are probably responsible for the decline of native species because of their parasites. Between them, the world’s biggest bumblebee, B. dahlbomii. Interestingly, more complex flowers make it less probable that transmission occurs.
Flowers provide the primary nutrients for bees – carbohydrates, proteins and lipids. But they also contain many other substances, like flavonoids (which make flowers colourful), alcaloids (which protect plants against herbivores, like nicotine) and other substances. These are called secondary metabolites, having many functions for the plants. The chemistry of pollen and nectar is still poorly understood. Even less do we understand the role that these secondary metabolites play for bee health. There are some indications that these compounds reduce the parasite load. Bees may also forage explicitly for nectar and pollen with these substances to “cure” themselves, there is some evidence again for bumblebees and C. bombi.
Changed foraging behaviour due to pathogens and parasites
Another interesting aspect may be the evolution of oligolectic foraging to avoid parasites. This may be the case for bees that feed on plants from the daisy family. Sapygid wasps did not enter nests of Osmia species specialized on these flowers. This may be an example for an evolutionary foraging adaptation due to parasites and absolutely has to be studied further. But there are also immediate adaptations in foraging behaviour: both bumblebees and honey bees avoid flowers with bacteria in their nectar. How they detect this is still not clear.
The parasite, on the other hand, may also change the foraging behaviour itself: bumblebees infested with conopid flies, prefer certain flowers to others and collect less pollen for their colonies.
There is still much to study about how nutrition keeps bees healthy. Most knowledge we have is from honey bees and bumblebees, but what about all the other species? We know only little about their diseases and even less about how nutrition affects their health. However, the existence evidence suggests that:
One of the best and most practical methods to improve pollinator health may therefore be to ensure the availability of diverse and health promoting floral resources in urban, agricultural and natural landscapes.
Couldn’t agree more.