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ISIS Report 26/04/07
The Mystery of Disappearing Honeybees
Mystery of Disappearing Honeybees
For some time now, honeybees have been disappearing from farmers’ hives
without a trace. Dr. Mae-Wan Ho
and Prof. Joe Cummins on
the trail of possible culprits …
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Honeybees vanishing worldwide
The first alarm was sounded in autumn 2006. Honeybees are disappearing
across the United States, with half of the States affected and beekeeper losing
30 to 90 percent of colonies; one beekeeper with 1 200 colonies expected 9
to survive the winter [1]. The problem began more than two years ago and has
intensified in recent months [1-5]. The bees simply vanish relatively suddenly,
with little or no dead adults in or near the colonies, leaving behind the
queen and a few young. In cases where the colony appears to be actively collapsing
the workforce seems to be made up of young adult bees, insufficient to feed
the brood, but are reluctant to consume provided feed [5]. This “colony collapse
disorder” (CCD) is particularly devastating for growers of fruits and vegetables,
as they depend on insect pollinators.
Since then, CCD has been reported from Germany, Switzerland, Spain, Portugal, Italy, Greece, and the UK [6], where one of the biggest beekeepers lost 23 of his 40
hives. But the Department of the Environment, Food and Rural Affairs (DEFRA)
said that “there is absolutely no evidence” of CCD in the UK.
CCD has baffled scientists, because no one knows what
causes it [5], and ongoing efforts are being made to identify possible pathogens
in the bees and chemical residues in pollen, honey and bees. Viruses, fungal
diseases, parasitic mites, pesticides, or chemical designed to control mites
have considered by the authorities [7], as have GM crops [8-9], and mobile
phones [10] (Mobile
Phones and Vanishing Bees, this series). So how good is
the evidence for the different suspects?
Extent and causes of decline both unknown
The United States National Research Council Committee on the Status
of Pollinators in North America published its report [7] in October 2006.
But the report was rather thin on data and information as to the precise extent
of the decline in honeybees or its causes.
The report discussed introduced parasitic mites, and the bacterial
pathogen that causes foul brood disease in detail, as there
is extensive scientific literature. But it barely touched on pesticides or
GM crops, and did not mention mobile phones at all.
Mites infestations
The introduced parasitic
mites, Varroa destructor and
Acarapis woodi, began to cause
infestation since the late 1980s, and mite infestation became established
in the US within a decade. Varroa destructor,
an external parasite of the honeybee, has caused dramatic declines in honeybees
in North America and throughout the world. During the winter of 1995-1996,
northern US beekeepers experienced their largest losses in history; some states
lost 30 to 80 percent of their colonies. These losses have occurred despite
heavy used of pesticides to control mite populations. Pesticide resistance
has become widespread and many beekeepers are no longer able to use the few
registered pesticides for controlling Varroa.
The tracheal
mite Acarapis woodi is an internal
parasite of the honeybee. It was first detected in the US in 1984, and initially
caused serious damage to colonies, but there appears to be heritable resistance
to the mite.
Parasitic
mites cannot explain colony collapse disorder as there is no evidence that
mite infestation is directly involved, although it may contribute indirectly
by reducing the immunity of the bees to infections by viruses, bacteria and
fungi (see below).
Foul brood disease
Paenibacillus
larvae is the most serious pathogen of honeybees. It causes
American Foul Brood disease (AFB), a disease of the honeybee larvae. It is
highly virulent and easily spread among colonies, and generally fatal if untreated.
During the first half of the last century, AFB was the most serious threat
to beekeeping, and caused tremendous loss of colonies. The incidence of AFB
was reduced dramatically by the introduction of antibiotics, and by state
inspection programme that required the burning of infected hives. However
AFB spores are refractory to antibiotics and can persist on contaminated equipment
for more than 80 years. Treatment of colonies with active cases of AFB eliminates
disease symptoms, but withdrawal of antibiotics is generally followed by disease
recurrence. Resistance to antibiotics has also become widespread since 1994.
As in
the case of parasitic mites, foul brood disease is not associated with colony
collapse disorder.
Pesticides
The use of pesticides,
especially insecticides on crops, is known to kill or weaken thousands of
honeybee colonies in the US each year, and local bee kills have occurred sporadically
for decades. However, the NAS report considered it unlikely that this has
“contributed significantly” to the recent decline. The report stated [7, p.
79]: “Most pesticide-caused honey bee kills are the result of accidents, careless
application, or failure to adhere to label recommendations and warnings.”
It has obviously ignored sub-lethal effects, particularly of new pesticides
that may turn out to be one of the most significant single factors contributing
to the current honeybee decline (see later).
Parasites reduce bee immunity
Varroa
mites infestation reduces the immune response of the bees, causing them to
be prone to infection with virus, bacteria or fungi [11, 12]. A number of
viral diseases are enhanced in the parasite-infested bee colony, particularly
the deformed wing virus disease that causes crippling deformity in the bees
[13]. Multiple viruses frequently infect bees attacked by Varroa parasite. These viruses are spread not only by the parasite,
but also vertically from queen to brood [14, 15]. The parasite-infested colonies
are frequently treated with a pyrethroid insecticide, fluvalinate, but the
parasite has grown resistant to the insecticide [16], and the insecticide
may adversely influence the behaviour of the honeybee (see below). Honeybees
have 17 gene families involved in immunity [17], roughly one-third the number
of immunity genes in Drosophila and Anopheles mosquitoes. Honeybees seem to have limited immune
flexibility, which may make them more sensitive to devastating pathogens.
Pesticides disrupts bee behaviour at sublethal levels
Numerous pesticides have been found to disrupt bee behaviour
following sub-lethal exposures [18]. A wide array of pesticides including
fluvalinate (the chemical used to treat hives to eliminate parasites) disrupted
the behaviour of honeybees leading to feeding and navigation problems [19].
Bees suffering from sub-lethal pesticide intoxication resembled the behaviour
of bees described by observers of the colony collapse disorder. Sub-lethal
doses of fipronil (a veterinary insecticide) impaired the olfactory memory
process of honeybees [20]. Spinosad, a prominent and much used natural insecticide
fed to bumble bees in pollen slowed down their foraging behaviour while a
higher dose of the insecticide caused colony death within two to four weeks
[21]. See Requiem
for the Honeybee [22] for more evidence that sub-lethal
effects of pesticides may be the single most important factor contributing
to disappearing honeybees.
Genetically modified (GM) crops may have sub-lethal effects on bees
The possibility that
GM crops in North America is contributing to the decline in honeybees was
given little consideration by the NRC Committee [7] even though the timing
of the honeybee decline appears to coincide with the widespread deployment
of GM crops. GM crops are engineered to tolerate herbicides, especially gyphosate,
or to contain biopesticides (the Bt Cry toxins from Bacillus thuringiensis), or both. The biopesticide toxins produced
in Bt crops are not highly or acutely toxic to bees, but are toxic to butterflies,
moths and beetles. Nevertheless, in some instances, the toxins can kill bees
or modify their behaviour.
The Bt toxin Cry1Ab caused reduced foraging activity in bees after they were
fed with syrup containing the toxin. However, the Bt toxin produced less pronounced
impacts on bee behaviour than the chemical pesticides deltamethrin or imidacloprid
[23]. Bt bacteria caused mortality in bees when fed in broth cultures or sugar
solutions [24]. 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. But sub-lethal
effects on the bees were not recorded in those experiments [25].
In a series
of experiments in Jena, Germany, bees were found not to be affected when fed
on a diet of pollen doped with 100 times the concentration of toxin found
in the Bt maize pollen; and feeding trials on larvae also showed no effects.
In the field, bee colonies in flight tents were fed with Bt maize pollen to
which a 10-fold concentration of Bt toxin had been added. Again, no negative
effects were detected. But a chance infestation by the parasite microsporidia
resulted in significantly more damage to the Bt-fed colonies compared with
controls [26]. Another limitation of the experiments so far is that they were
carried out with toxins derived from bacteria, not transgenic toxin derived
from the Bt crops, which are known to have very different properties, as we
have pointed out repeatedly, most recently in GM Maize 59122 Not Safe [27] (SiS
34).
Transgenic glyphosate-tolerant canola pollen was reported
to pose no threat to honeybees [28]. However, when organic, conventional,
and herbicide-tolerant canola were compared with regard to 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 [29].
Clearly, the existing evidence calls for much fuller
investigations on the sub-lethal impacts of GM crops on bees, such as learning
and feeding behaviour, and immunity to disease. The potential consequences
of pollinator decline on food crops can be staggering, and the impact on biodiversity
may be irreversible [30].
Mobile phones and bee decline
There has been widespread
report in the mainstream media that mobile phones may be responsible for the
decline of honeybees [for example, 6, 31]. The results are indeed startling.
For details see Mobile
Phones and Vanishing Bees [10], and should be considered
in the context of the increasingly clear evidence that weak radiation from
mobile phones and base stations do have harmful effects on the health of human
beings and wildlife [32] (Drowning
in the Sea of Microwaves,
SiS 34).
The mystery remains
The mystery of disappearing
honeybees is far from solved. The greatest suspects so far are pesticides
and radiation from mobile phone base stations. However, it is likely that
sub-lethal effects due to GM crops, mites infestations and other factors which
alter the bees’ behaviour, affect their memory and learning process or compromise
their health and immunity will all have a role to play.
Honeybees
may be our most sensitive indicator species for all the environmental pollution
and dangerous technologies we perpetrate. When honeybees disappear, we too,
shall follow shortly.
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