Jun 13, 2013 | Atlanta, GA
One of the most common questions we hear is, “Aren’t the bees in danger?” While bee die-offs are certainly in the news a lot, there is more and more known about how to protect bees every day. Meredith Greene, a junior biology major at Georgia Tech wrote the article below to help answer your questions. This type of article is known as a literature review or a review article. Meredith used the database Web of Knowledge to find peer-reviewed research articles to reference in her piece. Before peer-reviewed articles are published, several scientists in the field read them to ensure that the research in the article was done well and that the conclusions make sense. If you are interested in reading the original research papers, the references are listed at the end.
What is Colony-Collapse Disorder? by Meredith Greene
Beginning in late 2006, beekeepers in Europe and North America noticed the sudden disappearance of adult worker honey bees (Apis mellifera) from up to 40 percent of their bee hives during the winter. After investigating, beekeepers did not find bodies of bees and they rarely noted previous observations of obvious disease, pests or parasites around the hive. Often there was still sufficient food left in the hive, and immature bees were left untouched. This phenomenon has been named colony-collapse disorder (CCD).
So what is causing these mysterious disappearances? Research suggests that the sudden increase in incidences of CCD is multifactorial, and thorough examinations of honeybee ecology and physiology are necessary to fully understand the disorder.
One of the most consistently posited causes of CCD involves "extraordinary stress" to the colony, commonly involving malnutrition. Migratory beekeeping practices are often cited as a major source of stressors to the bees. Hives can be transported cross-country to pollinate crops and on the way are subject to confinement, fluctuations in temperature, and drastic geographic changes from the areas they usually navigate. Large numbers of colonies can intermingle once reaching their destination, which can increase the transmission rate of novel pathogens.
The bees may be limited in their food choice when used to pollinate monocultures in the summer or when fed high-fructose corn syrup (HFCS) in the winter. Bees that are fed pollen from a variety of species have a stronger immune system than those fed only from one species.
Beekeepers that use HFCS as a honey replacement in the winter may be putting their colonies at risk, as a recent study found that a compound in honey but absent in HCFS may be important to the honey bee immune system. P-coumaric acid up-regulates detoxification genes and antimicrobial peptide genes, and its absence may reduce the bees' ability to handle pesticides and pathogens. Beekeepers must reevaluate conditions subject to their control if they want to lessen the effects of outside stressors like disease and the widespread use of pesticides.
A possible factor contributing to CCD in some apiaries is the presence of Varroa mites, parasites that can quickly destroy a colony after a severe infestation. Varroa mites carry pathogens like Deformed Wing Virus (DWV) that are harmful to bees, with an increased level of mite infestation corresponding to an increased transmission of the virus in the population and subsequent reduced colony survivorship . Varroa infestation of colonies was shown to cause the collapse of colonies in Ontario.
This explanation only applies to some cases of CCD, because members of afflicted hives generally don’t show the signs of distress that hallmark parasite infestation. But, infestations of Varroa don’t always result in deformity in infected bees. Foragers parasitized by Varroa show significant cognitive impairment by way of decreased learning capability. Infestation by Varroa destructor is lower in foragers that return to their colony compared to those that leave the colony. Heavily infested bees are either slow to return to their hive, or never return. Infested bees were shown to have a harder time orienting themselves to the hive entrance. This could be an adaptation of the bees to remove members of the colony that pose a threat of transmitting parasites or pathogens .
Bees infected by DWV show down regulation of a gene responsible for dopamine synthesis, which leads to impaired neural development, cognition and learning. Incidences of "mad bee disease" after Varroa infestation could account for the disappearance but not immediate death of foraging adults.
Insecticides are tested for their effects on non-target organisms, but may cause unexpected toxicity when bees are exposed to high concentrations of insecticides in pollen or in their nectar. Imidacloprid has been used since the early 1990’s as an insecticide treatment for sunflower and corn seeds because of its high effectiveness in controlling sap-sucking insects. When exposed to even very low amounts of imidacloprid, bees show decreased performance in memory and associative learning tests. A foraging bee exposed to a high enough level of this compound may become lost and be unable to return to the hive.
Bees should have little interaction with this compound, but research has shown that seeds treated with imadacloprid can produce pollen that contains the compound in unexpectedly high levels . Levels of this and other nicotine-like insecticides can be high enough in the secretions of treated corn plants to kill honey bees within a few minutes . Bees could also be suffering from increased levels of exposure to imidacloprid by ingesting high-fructose corn syrup (HFCS) containing high levels of imidacloprid and other neonicotinoids. A recent in situ study showed that honey bee hives fed HFCS with treated with imidacloprid showed the same extent of hive death as reported in incidences of colony-collapse disorder (CCD) . This could be linked to the inability of an exposed foraging bee to find their hive. Starting in 2004, companies began treating corn seeds with five times the usual dosage of neonicotinoids compared to their earlier usage . This relatively recent and drastic increase could account for the safety of HFCS throughout the 1990s until 2006-2007.
Imidacloprid and other neonicotinoids are the pesticides garnering the most attention, but research has shown that other common pesticides can decrease learning performances in honeybees . Other compounds in pesticides assumed to be inert have been shown to decrease the olfactory learning ability of honey bees . These effects are not as obvious to a beekeeper as bee mortality and may go unnoticed until too many bees have left the hive. A rise in the concentration of specific classes of insecticides roughly correlates with the first incidences of CCD, a large clue that it may have a significant impact.
After several years of losses, the 2011-2012 winter showed a 10 percent increase in survivorship in U.S. colonies. In response to the losses of previous years, the U.S. Department of Agriculture designed new guidelines to curb malnutrition during times of stress. These guidelines along with possible seasonal effects and cycling of any contributing disease may account for the change.
Since 2006 there has been an estimated $2 billion replacement cost incurred by beekeepers to replace their hives . Pollination by honey bees contributes $20-30 billion to agricultural production, and consistent losses to honey bee populations cannot be ignored. Active research is still required to find the causes of previous collapses and identify potential factors that could cause future collapses.
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