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[SANET-MG] Parasitic Fungi and Pesticides Act Synergistically to Kill Honeybees?
ISIS Press Release 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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website. http://www.i-sis.org.uk/index.php
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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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