PesticidesFurther information: Pesticide toxicity to bees, Imidacloprid effects on bee population, and Bees and toxic chemicals
According to the U.S. Department of Agriculture, pesticides may be contributing to CCD. Scientists have long been concerned that pesticides and possibly some fungicides may have sub-lethal effects on bees, not killing them outright but instead impairing their development and behavior. Of special interest is the class of insecticides called neonicotinoids, which contain the active ingredient imidacloprid, and similar other chemicals, such as clothianidin and thiamethoxam. Honey bees may be affected by such chemicals when they are used as a seed treatment because they are known to work their way through the plant up into the flowers and leave residues in the nectar. Scientists note that the doses taken up by bees are not lethal, but they are concerned about possible chronic problems caused by long-term exposure. Virtually all of the genetically engineered Bt corn grown in the U.S. is treated with neonicoticoids and a 2012 study found high levels of clothianidin in pneumatic planter exhaust. In the study, it was found that the insecticide was present in the soil of unplanted fields nearby those planted with Bt corn and on dandelions growing near those fields. Another 2012 study done in Italy also found clothianidin and imidacloprid in the exhaust of pneumatic seeding equipment.
A 2010 survey reported 98 pesticides and metabolites detected in aggregate concentrations up to 214 ppm in bee pollen – this figure represents over half of the individual pesticide incidences ever reported for apiaries. It was suggested that "while exposure to many of these neurotoxicants elicits acute and sublethal reductions in honey bee fitness, the effects of these materials in combinations and their direct association with CCD or declining bee health remains to be determined."
It is particularly difficult to evaluate pesticide contributions to CCD for several reasons. First, the variety of pesticides in use in the different areas reporting CCD makes it difficult to test for all possible pesticides simultaneously. Second, many commercial beekeeping operations are mobile, transporting hives over large geographic distances over the course of a season, potentially exposing the colonies to different pesticides at each location. Third, the bees themselves place pollen and honey into long-term storage, effectively meaning that there may be a delay of anywhere from days to months before contaminated provisions are fed to the colony, negating any attempts to associate the appearance of symptoms with the actual time at which exposure to pesticides occurred.
Pesticides used on bee forage are far more likely to enter the colony via the pollen stores rather than via nectar (because pollen is carried externally on the bees, while nectar is carried internally, and may kill the bee if too toxic), though not all potentially lethal chemicals, either natural or man-made, affect the adult bees: many primarily affect the brood, but brood die-off does not appear to be happening in CCD. Most significantly, brood are not fed honey, and adult bees consume relatively little pollen; accordingly, the pattern in CCD suggests that if contaminants or toxins from the environment are responsible, it is most likely to be via the honey, as it is the adults that are dying (or leaving), not the brood (though possibly effects of contaminated pollen consumed by juveniles may only show after they have developed into adults).
To date, most of the evaluation of possible roles of pesticides in CCD have relied on the use of surveys submitted by beekeepers, but it seems likely that direct testing of samples from affected colonies will be needed, especially given the possible role of systemic insecticides such as the neonicotinoid imidacloprid (which are applied to the soil and taken up into the plant's tissues, including pollen and nectar), which may be applied to a crop when the beekeeper is not present. The known effects of imidacloprid on insects, including honey bees, are consistent with the symptoms of CCD; for example, the effects of imidacloprid on termites include apparent failure of the immune system, and disorientation.
In Europe, the interaction of the phenomenon of "dying bees" with imidacloprid has been discussed for quite some time now. It was a study from the "Comité Scientifique et Technique (CST)" which was in the center of discussion, which led to a partial ban of imidacloprid in France. The imidacloprid pesticide Gaucho was banned, in 1999 by the French Minister of Agriculture Jean Glavany, primarily due to concern over potential effects on honey bees. Consequently when fipronil, a phenylpyrazole insecticide and in Europe mainly labeled "Regent", was used as a replacement, it was also found to be toxic to bees, and banned partially in France in 2004.
In February 2007, about forty French deputies, led by UMP member Jacques Remiller, requested the creation of a Parliamentary Investigation Commission on Overmortality of Bees, underlining that honey production was decreasing by 1,000 tons a year for a decade. As of August 2007, no investigations were yet opened. Five other insecticides based on fipronil were also accused of killing bees. However, the scientific committees of the European Union are still of the opinion "that the available monitoring studies were mainly performed in France and EU-member-states should consider the relevance of these studies for the circumstances in their country".
Around the same time, French beekeepers succeeded in banning neonicotinoids, the Clinton administration permitted pesticides which were previously banned, including imidacloprid. In 2004, the Bush Administration reduced regulations further and pesticide applications increased.
In 2005, a team of scientists led by the National Institute of Beekeeping in Bologna, Italy, found pollen obtained from seeds dressed with imidacloprid contains significant levels of the insecticide, and suggested the polluted pollen might cause honey bee colony death. Analysis of maize and sunflower crops originating from seeds dressed with imidacloprid suggest large amounts of the insecticide will be carried back to honey bee colonies. Sublethal doses of imidacloprid in sucrose solution have also been documented to affect homing and foraging activity of honey bees. Imidacloprid in sucrose solution fed to bees in the laboratory impaired their communication for a few hours. Sublethal doses of imidacloprid in laboratory and field experiment decreased flight activity and olfactory discrimination, and olfactory learning performance was impaired.
Research, in 2008, by scientists from Pennsylvania State University found high levels of the pesticides fluvalinate and coumaphos in samples of wax from hives, as well as lower levels of 70 other pesticides. These chemicals have been used to try to eradicate varroa mites, a bee pest that itself has been thought to be a cause of CCD. Researchers from Washington State University, under entomology professor Steve Sheppard in 2009, confirmed high levels of pesticide residue in hive wax and found an association between it and significantly reduced bee longevity.
The WSU work also focused on the impact of the microsporidian pathogen Nosema ceranae, the build-up of which was high in the majority of the bees tested, even after large doses of the antibiotic fumagillin. Penn State's Dr. Maryann Frazier said, "Pesticides alone have not shown they are the cause of CCD. We believe that it is a combination of a variety of factors, possibly including mites, viruses and pesticides."
In 2010, Fipronil was blamed for the spread of colony collapse disorder among bees, in a study by the Minutes-Association for Technical Coordination Fund in France, which found that even at very low nonlethal doses, this pesticide still impairs the ability to locate the hive, resulting in large numbers of foragers lost with every pollen-finding expedition, though no mention was made regarding any of the other symptoms of CCD; other studies, however, have shown no acute effect of Fipronil on honey bees. Fipronil is designed to eliminate insects similar to bees, such as yellowjackets (Vespula germanica) and many other colonial pests by a process of toxic baiting, whereby one insect returning to the hive spreads the pesticide among the brood.
In 2012, researchers announced findings that sublethal exposure to imidacloprid rendered honey bees significantly more susceptible to infection by the fungus Nosema, thereby suggesting a potential link to CCD, given that Nosema is increasingly considered to contribute to CCD.
Also, in 2012, researchers in Italy published findings that the pneumatic drilling machines that plant corn seeds coated with clothianidin and imidacloprid release large amounts of the pesticide into the air, causing significant mortality in foraging honey bees. According to the study, "Experimental results show that the environmental release of particles containing neonicotinoids can produce high exposure levels for bees, with lethal effects compatible with colony losses phenomena observed by beekeepers." Commonly used pesticides, such as the neonicotinoid imidacloprid reduce colony growth and new queen production in experimental exposure matched to field levels. Lu et al. (2012) reported they were able to replicate CCD with imidacloprid. Another neonicotinoid thiamethoxam causes navigational homing failure of foraging bees, with high mortality.
A 2012 in situ study provided strong evidence that exposure to sub-lethal levels of imidacloprid in high fructose corn syrup (HFCS) used to feed honey bees when forage is not available causes bees to exhibit symptoms consistent to CCD 23 weeks post imidacloprid dosing. The researchers suggested that "the observed delayed mortality in honey bees caused by imidacloprid in HFCS is a novel and plausible mechanism for CCD, and should be validated in future studies".
In March 2013, two studies were published showing that neonicotinoids affect bee long term and short term memory, suggesting a cause of action resulting in failure to return to the hive. Growth in the use of neonicotinoid pesticides has roughly tracked rising bee deaths.
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