Possible Causes
The mechanisms of CCD are still unknown, but many causes have been proposed as causative agents: malnutrition, pathogens, immunodeficiencies, mites, fungus, pesticides, beekeeping practices (such as the use of antibiotics, or long-distance transportation of beehives) and electromagnetic radiation. Whether any single factor or a combination of factors (acting independently in different areas affected by CCD, or acting in tandem) is responsible is still unknown; however most recent information suggests a combination of factors is most likely. It is likewise still uncertain whether CCD is a genuinely new phenomenon as opposed to a known phenomenon that previously only had a minor impact.
At present, the primary source of information, and the presumed "lead" group investigating the phenomenon, is the Colony Collapse Disorder Working Group, based primarily at Pennsylvania State University. Their preliminary report pointed out some patterns but drew no strong conclusions. A survey of beekeepers early in 2007 indicated that most hobbyist beekeepers believed that starvation was the leading cause of death in their colonies while commercial beekeepers overwhelmingly believed that invertebrate pests (Varroa mites, honey bee tracheal mites, and/or small hive beetles) were the leading cause of colony mortality. A scholarly review in June 2007 similarly addressed numerous theories and possible contributing factor, but left the issue unresolved.
In July 2007, the United States Department of Agriculture (USDA) released its "CCD Action Plan", which outlined a strategy for addressing CCD consisting of four main components:
- survey and data collection;
- analysis of samples;
- hypothesis-driven research; and
- mitigation and preventive action.
In July 2009, the first annual report of the U.S. Colony Collapse Disorder Steering Committee was published. It suggested that colony collapse may be caused by the interaction of many agents in combination.
Similarly, in 2009 the CCD Working Group published a comprehensive descriptive study that concluded: "Of the 61 variables quantified (including adult bee physiology, pathogen loads, and pesticide levels), no single factor was found with enough consistency to suggest one causal agent. Bees in CCD colonies had higher pathogen loads and were co-infected with more pathogens than control populations, suggesting either greater pathogen exposure or reduced defenses in CCD bees."
The second annual Steering Committee report was released in November 2010. The group reported that although many associations, including pesticides, parasites, and pathogens have been identified throughout the course of research, "it is becoming increasingly clear that no single factor alone is responsible for ". Their findings indicated an absence of damaging levels of the parasite Nosema or parasitic Varroa mites at the time of collapse.
They did find an association of sub-lethal effects of some pesticides with CCD, including two common miticides in particular, coumaphos and fluvalinate, which are pesticides registered for use by beekeepers to control varroa mites. It was reported that studies also identified sub-lethal effects of neo-nicotinoids and fungicides, pesticides that may impair the bee's immune system. It is hypothesized that these pesticides impair the bee's immune system, which leaves the bee more susceptible to bee viruses.
A large 2010 survey of healthy and CCD-affected colonies also revealed elevated levels of pesticides in wax and pollen, but the amounts of pesticides were similar in both failing and healthy hives. They also confirmed suspected links between CCD and poor colony health, inadequate diet, and long-distance transportation. Studies continue to show very high levels of pathogens in CCD-affected samples and lower pathogen levels in non-affected samples, consistent with the empirical observation that healthy honey bee colonies normally fend off pathogens. These observations have led to the hypothesis that bee declines are resulting from immune suppression.
In the 29 March 2012, issue of the journal Science, two separate studies found that neonicotinoids (insecticides) may interfere with bee's natural homing abilities, causing them to become disoriented and preventing them from finding their way back to the hive.
Read more about this topic: Colony Collapse Disorder