Honey bees are hard-working insects. Their pollination services are in such demand, humans tow hundreds of hives carrying millions of bees around in the back of semitrucks to bring honey bees to various locations such as California almond groves. Humans are also quite partial to the bee colony winter energy storage also known as honey. So while honey bees work hard to collect pollen and nectar from blooming plants and trees and store honey for the winter, humans insist on robbing the colony’s store of delicious sweetener for their own uses. Recent reports of high mortality in honey bee colonies has caused concern in many beekeepers who manage European honey bee apiaries for honey production and pollination services. These severe depletion of honey bee colonies have been attributed to the parasitic mite Varroa destructor in the colony, not only feeding off the larvae and pupae brooding in the colony but also transmitting viruses carried by the mite. Bee nutrition is important for the pollinators especially when overwintering in the hive. Without adequate nutrition, a colony may become weak and succumb to parasite or disease pressure, unable to survive until nectar and pollen are available in the spring. A study was recently published in PLOS ONE that examined how the landscape around Midwestern honeybee hives affected the ability of bees to overwinter and assessed their health by measuring levels of Varroa mites and honey bee viruses.
Honey bees need to find sources of nectar and pollen to feed their growing colony during the spring and summer. This is the time when the colony is expanding, raising new workers who then build more wax comb for more eggs and forage for more nectar and pollen. When there is an excess of nectar, the bees evaporate the water and store it as honey. The stored honey and pollen feed the colony during the winter. One concern is highly cultivated areas are low in pollen and nectar may decrease the ability of the honey bee colony to survive the winter. Soybeans and corn do not need pollinators to reproduce and occupy hundreds of thousands of acres of land. Dolezal et al. recruited Iowa beekeepers with 2–50 hives to study how proximity to cultivated land affected the health of honey bees going into winter. The beehives were categorized as highly cultivated (next to acres of corn or soybean fields) or low cultivation (e.g., woody, grassy or marshy lands not used for agriculture) for up to five miles away from bee hives as honey bees can seek out pollen and honey sources miles from the hive.
Samples of 100ml of bees from 50 hives were collected by beekeepers during last week of October or the first week of November for assessing the disease and nutritional state of the bees after a season of collecting and storing pollen and nectar. This would assess the health of bees that would be overwintering in the hive. At least one hive from each apiary had Varroa mites with an average of 6 mites per 100 bees (range of 1–16 mites per 100 bees). Note that 5–6 mites per 100 bees is the level beekeepers are advised to treat the beehives with miticides. However, there seemed to be no correlation between Varroa and the cultivation of the landscape.
When the lipid levels of honey bees were examined, the ones with the presence of Varroa mite had lower levels of lipids than ones that did not. Landscape alone did not have a correlation to lipid levels. Interestingly, when mites were absent, the bees from hives in highly cultivated landscapes had significantly higher lipid levels than those who collected pollen and nectar from land not used for agriculture; when mites were present, the honey bees in agriculture landscapes had significantly lower levels of lipid compared to those bees without mites. There was no difference in lipid levels for bees with or without mites for the more diverse landscapes. Lipid levels was the only measure that showed an interaction between landscape differences and Varroa presence. Protein analysis and body weight showed no such differences.
The honey bees were also tested for the presence of various viruses that can compromise bee health: black queen cell virus (BQCV), Israeli acute paralysis virus (IAPV), deformed wing virus (DWV) and sacbrood virus (SBV). For three of the viruses (BQCV, IAPV and SBV), there was little effect of landscape diversity, mite presence or both on the level of viruses. However, for deformed wing virus, there was no change in bees with an agricultural or nonagriculture landscape, but when bees had mites, there was a significant increase in DWV titer.
Dolezal et al. followed up with beekeepers to assess how the honey bees overwintered. Not all beekeepers responded and each person varied in how he or she may have treated for mites. However, the self-reporting showed 50% of the hives survived with a trend line, while not significant, that suggested that higher Varroa mite levels corresponded to decreased hive winter survival.
This study investigated if nutritional availability for honey bees in intensively cultivated areas would be significantly different from those bees in more diverse landscapes, potentially affecting the viability of the bee colony over winter. While landscape alone did have some effect, the presence of Varroa mite changed the lipid profiles more significantly and the DWV titers were higher in honey bees with mites. With only 50 hives studied in one state with a great many variables, the data suggests there is an effect of nutrition, mites and viruses on bee health. Does this mean there will be greater colony loss over winter? Possibly, but would require more data with more colonies and potentially controlling for location and hive management to assess the significance of nutrition on bee health and their ability to successfully overwinter.
Dolezal, A.G., Carrillo-Tripp, J., Miller, W.A., Bonning, B.C. and Toth, A.L. (2016) Intensively cultivated landscape and Varroa mite infestation are associated with reduced honey bee nutritional state. PLOS ONE 11, e0153531. doi: 10.1371/journal.pone.0153531