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[SANET-MG] Honey bees may succumb to friendly fire?
1 May 2007
Prof. Joe Cummins
Parasitic fungi and pesticides interact synergistically to kill insect
pests: Honey bees may succumb to friendly fire?
Honey bees are facing an unparalleled threat from something in the
environment which is causing them to leave their hives but never return.
The condition is called Colony Collapse Disorder (CCD). A number of
ideas have been put forward to explain the catastrophe (1). The leading
areas of concern are systemic insecticides ( the neonicotinoid systemic
pesticides used worldwide to treat seeds and crops) including the
genetically modified (GM) crops (1,2), parasitic fungi (3) and radiation
associated with mobile phones (4). These environmental threats all are
believed to have similar effects on the honey bees.
Parasitic fungi are used extensively as entomopathogenic biocontrol
organisms. Fungal spores are used to deliver the parasites in sprays or
baits. It has been observed that the parasites frequently interact
synergistically with neonicotinoid pesticides, particularly the
insecticide imidacloprid which delivered at a sublethal level t in a
liquid suspension of parasite spores. Significant enhancement to the
insecticidal activity of the fungal spores are often observed. The
spores of the insect killing fungus Beauveria bassinia was used to treat
the brown leafhopper pest of rice , when accompanied by a sublethal dose
of imidacloprid the formulations acted earlier and greater activity than
the spores of the fungus alone (5). The sweet potato whitefly was
controlled using the fungus Lecanicillium muscarium combined with
sublethal levels of imidacloprid gave satisfactory control of the fly
and merited inclusion in integrated control programs (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) Termites were controlled by imidacloprid at sublethal
levels enhancing the fungal parasite Metarhizium anisopliae (8). The
point of this discussion is to show that the presence of neonicotinoid
insecticides in at sublethal levels causes enhanced insect killing by
the fungal parasites. The neonicotinoid insecticides used to treat seeds
are systematic and accumulate in plant parts including flowers .The
presence of the insecticides at sub lethal level seems to interfere with
the insect’s immune system leading to susceptibility to fungal pathogens.
The parasitic fungus, Nosema ceranea, a single celled parasite was 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 , that incorporates rational use of
chemical pesticides with biological options such as the microsporidian
Nosema locustae and the hymenopteran egg parasitoids Scelio spp., has
become a realistic option. (9). Nosema bombycis has been a major pest of
the silkworm but it has been used as a to control Diamondback moth.
Another microporidian , Vairimorpha sp.,isolated from the Diamondback
moth in Malaysia caused 100% 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. Purified Bacillus thuringiensis Cry1Ab toxin was
fed to Nosema infected and uninfected borer larvae. Nosema infection
reduced the lethal dose of Cry!Ab toxin to one third the lethal dose of
the uninfected larvae (11). The 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 honey
bee, rather, I am pointing out that there is a good indication that
exposure to sublethal levels of systemic insecticides used in seed
treatment of both conventional and GM crops and in widespread soil and
foliar applications can effect beneficial insects by reducing their
immunity to parasitic fungi. 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
honey bees has been evaluated using measurements of lethal dose alone ,
ignoring the clear evidence that sublethal doses of the insecticides act
synergistically with fungal parasites of the insects that are beneficial
along with the insect pests. The honey bees may b e falling victim to
“friendly fire” directed to exterminating insect pests. Unfortunately,
regulators around the world have dealt with decline of honey bees
through tunnel vision, ignoring well established pesticide-fungal
parasite interactions. Such regulators must be awakened from their nap.
1.Ho,M-W and Cummins,J. Mystery of disappearing bees Science in Society
2007 34 in press
2.Cummins,J. Requiem forthe honey bee Science in Society 2007 in press
3.Cummins,J. A parasitic fungus Nosema ceranea and honey bee decline in
4. Ho,M-W Mobile phones and vanishing bees Science in Society 2007 , 34
5. Feng MG and Pu XY. Time-concentration-mortality modeling of the
synergistic interaction of Beauveria bassiana and imidacloprid against
Nilaparvata lugens. Pest Manag Sci. 2005 Apr;61(4):363-70.
6. Cuthbertson AG, Walters KF and Deppe C. Compatibility of the
entomopathogenic fungus Lecanicillium muscarium and insecticides for
eradication of sweetpotato whitefly, Bemisia tabaci. Mycopathologia.
7. Santos AV, de Oliveira BL and Samuels RI. Selection of
entomopathogenic fungi for use in combination with sub-lethal doses of
imidacloprid: perspectives for the control of the leaf-cutting ant Atta
sexdens rubropilosa Forel (Hymenoptera: Formicidae). Mycopathologia.
8. R Ramakrishnan, DR Suiter, CH Nakatsu, RA Humber and Bennett,G.
Imidacloprid-Enhanced Reticulitermes flavipes (Isoptera:
Rhinotermitidae) Susceptibility to the Entomopathogen Metarhizium
anisopliae J. Econ. Entomol, 1999,92(5): 1125-32
9. Lomer CJ, Bateman RP, Johnson DL, Langewald J and Thomas M.
Biological control of locusts and grasshoppers. Annu Rev Entomol.
10. Sarfraz, M. Keddie, A and Dosdall, L. Biological control of the
diamondback moth, Plutella xylostella : A review Biocontrol Science and
Technology 2005, 15,763-89
11. Reardon BJ, Hellmich RL, Sumerford DV and Lewis LC. Growth,
development, and survival of Nosema pyrausta-infected European corn
borers (Lepidoptera: Crambidae) reared on meridic diet and Cry1Ab. J
Econ Entomol. 2004 Aug;97(4):1198-201.
12. Pierce CM, Solter LF and Weinzierl RA. Interactions between Nosema
pyrausta (Microsporidia: Nosematidae) and Bacillus thuringiensis subsp.
kurstaki in the European corn borer (Lepidoptera: Pyralidae). J Econ
Entomol. 2001 Dec;94(6):1361-8.
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