Pesticide may be responsible
for the decline of bees
From an agricultural perspective, 1996 will be remembered for two reasons: the first use of neonicotinoid pesticide and the widespread emergence of honey bee Colony Collapse Disorder (CCD).
Neonicotinoids were developed in the 1980’s and 90’s and were the first new class of insecticide developed in nearly 50 years. Neonicotinoids are derived from nicotine and affect the central nervous systems of insects and, to a lesser extent, mammals. They work by trans-locating in treated plants and causing paralysis and death in the insects feeding on them.
There are numerous neonicotinoid formulations in use. Imidacloprid (Bayer) is the widest selling pesticide in the world. It is available under 22 different brand names and is licensed for a wide range of food crops, seed treatment, landscape and turf grass, household pest control, fruit trees, greenhouse, golf course, and family pets (Advantage for dogs and cats). Some neonicotinoid compounds are persistent in the environment with residues in some instances detectable for several years.
A link between neonicotinoids and bee health was suggested before any products were licensed and they have been widely suspected to be a contributing factor in CCD ever since beekeepers found the first abandoned hives in the fall of 1996.
There have been several other theories advanced to explain CCD – microwaves, parasitic mites, climate change, and viral and fungal infections – but evidence from two recent European studies moves neonicotinoid from possible suspect to probable culprit.
Both papers have been published in the March 30 issue of Science. The first study is from a group at Stirling University led by Penelope Whitehorn and David Goulson. The group raised 75 bumblebee colonies in their laboratory. Some of the colonies were exposed to a high (though sub-lethal) dose of Imidacloprid; some were exposed to a low dose, and some were not exposed at all. After two weeks of exposure, the colonies were relocated outdoors for a further six weeks.
The high-dose and low-dose colonies both grew more slowly (approximately 10 per cent) than the unexposed colonies. More telling, however, was the dramatic difference in the production of queens.
Bumblebee colonies die off each winter and must be re-established the following spring by a hibernating queen. The unexposed colonies produced an average of 13.7 queens; the low-dose colonies produced two, and the high-dose colonies only 1.4.
The low level of queens produced by the exposed colonies severely diminishes the odds that they will be able to re-establish the following year but even these levels might exceed those in fully wild colonies. The researchers found rape crops with levels of Imidacloprid seven times higher than the high-dose administered in their lab.
The second study is from the French National Institute for Agricultural Research. Led by Mickaël Henry, the French group conducted research on wild honeybee hives to test previous laboratory research that raised the possibility that sub-lethal doses of neonicotinoid impaired the honeybee’s memory, foraging ability and ability to find its way back to the hive.
Some hives were exposed to realistic doses of thiamethoxam and others were left alone. The researchers glued tiny radio transmitters to the thoraxes of worker bees from exposed and unexposed hives.
The bees essentially “punched in” each time they returned from a foraging trip. The non-return rate for the exposed hives was twice as high as the unexposed hives which computer models indicate could be sufficient to cause the treated hives to collapse.
The precise manner in which neonicitoids affect the bees is still not fully understood. The diminished ability of exposed bees to forage effectively and find their way back to the hive ties in with the disruptive neuro-active effect of neonicotinoid pesticides. The fact that neonicotinoids have been found to translocate to pollen and nectar could mean that larval bees are being fed sub-lethal doses right in the hive, emerging in an impaired state to simply fly away.
There is another school of thought that suggests that neonicotinoids in themselves don’t cause CCD but simply weaken hives to the point that they are unable to ward off naturally occurring threats.
Given the manufacturer’s admission of bee toxicity (the label for the Bayer Crop Science thiacloprid-based product Provado states, “This product is highly toxic to bees exposed to direct treatment or residues on blooming crops or weeds...”) France, Germany, and Slovenia have adopted a cautionary principle and restricted neonicotinoid use.
That caution has not been widely embraced but the new British and French research should be sufficient reason for closer scrutiny and a more cautious approach to neonicotinoid use.
Surely we owe the bees at least that much.
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