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[SANET-MG] The Mysterious Disappearance of Honey Bees



 Prof. Joe Cummins
The Mysterious Disappearance of Honey Bees
In October 2006 United States National Research Council published the report of their Committee on the Status of Pollinators in North America, The report pointed out that an existing decline in honey bee pollinators was devastating North America. The report pointed out the importance of pollinators noting that three quarters of the earth’s flowering plants depended on pollinators for propagation. The extensive report dealt with bureaucratic issues aimed at dealing with the catastrophe and delineated ways that more human and financial resources should be focused on honey bee decline The report did not pin down causes in decline but instead focused on introduced parasites and microbial disease causing organisms such as fungi, bacteria and viruses Other causes included habitat decline, fragmentation and deterioration. The remedies suggested included testing commercial pollinators to insure that they were disease free and similar bureaucratic measures rather than a sharp focus on the primary causes of decline (1). The impact of pesticide on bees the uses and the spread of genetically modified (GM) crops modified for insect resistance and herbicide tolerance were barely discussed in the report leaving the impression that these were not considered important by the committee. The following discussion will deal with those important issues regardless of the views of the NRC Committee. Science Magazine reported on the pollination crisis but emphasized the need to replace the current pollinators with more robust insects (2). The New York Times emphasized the impact of bee decline on farmers and reported a salient observation that bees were flying off from the hive and simply not returning.(3). The Independent commented on the swift colony decline and noted that the problem of a tremendous pathogen load in the remaining members of a colony (4). The cogent point is that the bees in the colonies appear to have lost their immunity to viruses, bacteria and fungal diseases. The loss of resistance to disease may be caused by parasites, virus infections, or pesticides (both applied and present in GM crops).The disappearance of bees may have originated with one thing that diminished the bee’s immunity or by a combination of environmental factors diminishing the immune system ,all hitting the bee colonies at the same time. Parasites reduce bee immunity : Varroa mites (Varroa destructor) were fist observed in the United States in the mid 1980s , they appear to have jumped from native bees to honey bees in the Orient during the mid 1960s. The parasite was first observed in North America in the mid 1980s, since that time they have spread throughout North America. The parasite invades the hives and sucks the hemolympth from the pupae and adult bees infecting them with viruses carried by the parasite. The parasite reduces the immune response of the bees causing them to be prone to infection with virus, bacteria or fungi (5,6). A number of virus diseases are enhanced in the parasite infected bee colony , particularly the deformed wing virus disease that causes crippling deformity in the bees (7). Multiple viruses frequently infect the bees attacked by Varroa parasite. These viruses are spread no only by the parasite but vertically from queen to brood (8,9). The parasite infected colonies are frequently treated with a pyrethroid insecticide , fluvalinate, but this insecticide may influence the behaviour of the honey bee and has well the parasite has grown resistant to the insecticide (10). Honey bee disappearance has not always been associated with parasite infection and it is clear that man made environmental contaminants may influence bee immunity and bee behaviour. The immune pathways of bees have been studied. Honey bees have 17 gene families involved in immunity but they have roughly one third the number of genes involved in immunity as have Drosophila and Anapheles Mosquitoes. Honey bees seem to have limited immune flexibility (11). The immune inflexibility of honey bees may make them sensitive to devastating pathogens. Pesticides may have sublethal effects on bees: Most of the information on pesticides reports the lethal effects on insects or on non-target beneficial organisms. Bees are particularly sensitive to pesticides, whether insecticides, fungicides or herbicides, that effect development, adult longevity, immunity or behavioural effects such as navigation, feeding behaviour, oviposition, or learning. Sublethal effects can prove fatally disruptive to hive communities. Numerous pesticides have been found to disrupt bees following sublethal exposures (12). A wide array of pesticides including insecticides and fungicides including fluvalinate (the chemical used to treat hives to eliminate parasites) disrupted the behaviour of honey bees leading to feeding and navigation problems (13). Bees suffering from sublethal pesticide intoxication resembled the behaviour of bees described by observers of honey bee disappearance phenomenon. Sublethal doses of fipronil (a veterinary insecticide) in honey bee impaired the olfactory memory process (14). Spinosad, a prominent and much used natural insecticide fed to bumble bees in pollen were slower foragers while a higher dose of the insecticide caused colony death within two to four weeks (15). It is difficult to find studies on the impact of herbicides on bee behaviour and such studies are desirable. A study of the effect of glufosinate ( a glutamine synthesis inhibitor) on the bees gut microbes showed that over half of the gut microbes were sensitive to the herbicide (16). Glufosinate had pronounced insecticidal activity towards the caterpillars of skipper butterfly (17) Glutamine is important in sensory nerve transmission in insects. Genetically Modified (GM) crops may have sublethal effects on bees: The possibility that the great distribution of GM crops in North America is contributing to the decline in honey bees was given little consideration by the NRC Committee studying the problem (1) even though the time that the decline appeared was at the time GM crops were first widely deployed. GM crops are either modified to tolerate herbicides including gyphosate or glufosinate or to contain insecticides (the Bt Cry toxins). The insect resistance toxins produced in GM crops are not highly toxic to bees they are toxic to moths and beattles but in some instances the toxins cause bee lethality or behavioural modification. The Bacillus thuringiensis (Bt) toxin Cry1Ab caused reduced foraging activity before and after treatment with syrup containing the toxin. However,the Bt toxin produced less pronounced impacts on bee behaviour than did the chemical pesticides deltamethrin or imidacloprid (18). Bt bacteria caused mortality in bees when fed in broth cultures or sugar solutions (19). A number of purified Bt Cry toxins have been studied in the laboratory to determine their toxicity to honey bees and bumble bees. For the most part, those studies showed little threat from the Cry toxins. However, sublethal effects on the bees were not recorded in the experiments (20). Transgenic glyphosate resistant canola pollen posed no threat to honey bees (21). Organic, conventional and herbicide resistant Canola were compared regarding the pollination by wild bees in Alberta, Canada. The herbicide tolerant canola plots had the greatest pollination deficit , while conventional and organic plots were equally well served by the wild bees (22). Fuller studies are needed to evaluate the impact of GM crops on sublethal effects such as learning and feeding behaviour. The potential consequences of pollinator declines on stability of food crops and on the conservation of biodiversity has been discussed. The loss in food production can be staggering and the impact on biodiversity may be irreversible(23). In conclusion, the decline in honey bees is likely to be far from finished. The most significant factor in this decline may be fundamental it may be the well documented inflexibility in the genes of the bee’s immune system. Immune inflexibility was not discussed in the NRC report on bee decline (1). That factor is , most likely, the primary hurdle in an ultimate reversal of the decline. Using genetic techniques such as microarray analysis and marker assisted selection it should be possible to create honey bees with robust immune systems. In the absence of such robust honey bees it is clear that honey bees cannot be treated like “babies in a bubble” to protect them from toxins and pathogens in the environment. Agents impacting on the honey bee immune system go beyond mites to chemical pesticides and to some degree GM crops. Bees need to be protected from the agents that interfere with their immunity or behaviour. However, the NRC Committee report (1) does little to establish a broadly based approach to identify and control the pesticides and crops causing the decline. The NRC report is narrowly focused and should not be considered a serious effort to deal with the decline of honey bees. The impact of pesticides and GM crops requires fuller attention as does a program to create honey bees with robust immune systems.
References

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