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ISIS Report 07/06/07
Parasitic Fungi and Pesticides Act Synergistically to Kill Honeybees?
Prof. Joe Cummins presents
evidence that parasitic fungi can kill insects when low, otherwise non-lethal
concentrations of pesticides are present
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Co-operating culprits
Honeybees are facing an unparalleled threat from something that’s causing them
to leave their hives, never to return. Scientists call it “colony collapse disorder”
(CCD) [1] (Mystery of Disappearing
Honeybees, SiS 34). The major suspects in the murder of honeybees
appear to be systemic insecticides (the neonicotinoid systemic pesticides used
worldwide to treat seeds and crops), including genetically modified (GM) crops
[1, 2] (Requiem for the Honeybee,
SiS 34), parasitic fungi [3] (Parasitic
Fungus and Honeybee Decline SiS 35), and radiation associated with
wireless phones [4] (Mobile Phones and
Vanishing Bees, SiS 34).
It is unlikely, however, that the suspects act independently of one another,
and there is evidence suggesting that parasitic fungi and pesticides interact
synergistically in killing honeybees.
Parasitic fungi for biocontrol enhanced by sub-lethal levels of neonicotinoid
pesticide
Parasitic fungi are used extensively as biocontrol agents. Fungal spores are
applied in sprays or baits, and it has been observed that the parasites frequently
interact synergistically with neonicotinoid pesticides, particularly imidacloprid,
in killing insects. When the spores are delivered as a suspension together with
low, non-lethal levels of the pesticide, the insect-killing activity of the
fungal spores is significantly enhanced. The spores of Beauveria bassinia
used to treat the brown leafhopper rice pest, when accompanied by a sublethal
dose of imidacloprid, killed the pest earlier and in larger numbers [5]. The
fungus Lecanicillium muscarium in sublethal levels of imidacloprid gave
satisfactory control of the sweet potato whitefly, and merited inclusion in
integrated control programmes [6]. Beauveria bassinia spores combined
with imidacloprid at a level one tenth the lethal dose was found to significantly
enhance control of the leaf cutting ant [7]. Similarly, termites were controlled
by imidacloprid at sub-lethal levels that enhanced the killing activity of the
fungal parasite Metarhizium anisopliae [8]. The presence of the insecticides
at sub-lethal level appears to interfere with the insect’s immune system, making
the insect more susceptible to fungal pathogens.
Bees become exposed to sub-lethal levels of pesticide and biocontrol parasitic
fungi
The neonicotinoid insecticides used to dress seeds are systematic, and accumulate
in plant parts including the flowers. Hence honeybees collecting pollen will
become exposed to the pesticide, and become more susceptible to fungal pathogens.
The parasitic fungus, Nosema ceranae, a single celled parasite was indeed
found in CCD-affected bee hives from around the USA [3].
Nosema locustae has been a commercial biocontrol fungus to control locusts
and grasshoppers. An integrated pest management strategy with an emphasis on
the use of Metarhizium, an ascosporic fungus, incorporates low levels
chemical pesticides with additional biological options such as the microsporidian
Nosema locustae and the hymenopteran egg parasitoids Scelio spp.
[9]. Nosema bombycis has been a major pest of the silkworm but it has
been used to control Diamondback moth. Another microporidian, Vairimorpha
sp., isolated from the Diamondback moth in Malaysia caused 100 percent mortality
when applied to moth larvae at 1500 spores per larva [10]. Nosema pyrausta
infects the European corn borer and can be used in biocontrol of the pest.
Parasitic fungi increases the killing power of Bt biopesticide
Evidence implicating Bt biopesticides from GM crops has also emerged. Purified
Bacillus thuringiensis Cry1Ab toxin was fed to Nosema infected
and uninfected borer larvae. Nosema infection reduced the lethal dose
of Cry1Ab toxin to one third the lethal dose of the uninfected larvae [11].
When Bacillus thuringiensis kurstaki (Dipel) formulations were used to
treat Nosema pyrausta infected and uninfected corn borer larvae. The
infected larvae had a lethal dose 45 times lower than the uninfected larvae
[12].
I am not suggesting that biocontrol agents pose a threat to the honeybee, rather,
the exposure to sub-lethal levels of systemic insecticides used in seed treatment
of both conventional and GM crops and in widespread soil and foliar applications
can affect beneficial insects by reducing their immunity to parasitic fungi.
Furthermore, bees that otherwise are unaffected by exposure to Bt toxins in
GM crops may succumb much more readily when they are infected with parasitic
fungi, as reported in an experiment carried out at the University of Jena, Germany
[13].
Tests have been carried out on one agent at a time
Regulators have allowed extensive deployment of systemic insecticides for seed
treatment and they have allowed extensive use of foliar sprays of the systemic
insecticides on a wide array of food and feed crops. The impact of such pesticides
on honeybees has been evaluated using measurements of lethal dose of the pesticides
alone, ignoring the clear evidence that sub-lethal doses of the insecticides
act synergistically with fungal parasites of the insects. The honeybees may
be falling victim to “friendly fire” directed to exterminating insect pests.
Unfortunately, regulators around the world have dealt with decline of honeybees
through tunnel vision, ignoring well-established pesticide-fungal parasite interactions.
It is time for the regulators to wake up and impose a ban on the systemic pesticides
before more bees succumb.
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