Actualité scientifique
Scientific news
Actualidad científica
N° 438
September 2013
African caterpillars resistant to GM maize
(© IRD / B. Le Ru) Corn fields in Kenya near Kilimanjaro (in the background).
Like many other transgenic crops, Bt maize synthesises its own pesticide: a toxic protein produced
in its leaves and stems, which kills pests in a matter of days. Perfect… Except when insect populations develop resistance to the toxin! To date, management strategies implemented to delay the
evolution of resistance have been successful. Notwithstanding the success of these strategies, IRD
scientists and their South African partners have now revealed that a major pest of maize, the moth
Busseola fusca, has developed an unusual defense mechanism against Bt toxin in South Africa. By
contrast with the usual expectations, this resistance is inherited as a dominant trait, a characteristic
that may have contributed to its rapid geographical expansion. This result recently published in the
journal PLoS ONE, suggests that insect resistance management should be more finely tuned to
local pests and should go beyond the simple implementation of refuges for Bt-susceptible moths.
Worth knowing
The damage caused by Busseola fusca larvae in East and Southern Africa is considerable: an average 10 to 15% loss in (non-GM) maize
crops, and up to 30% in some areas. In South Africa, the first African country that introduced Bt maize in the late 1990’s, these losses
have, in some areas, returned to their initial level (about 10%), due to the speed with which the moths have developed resistance.
Glossary
GMO: genetically modified or transgenic organism.
Protein : molecules coded by genes, which perform different functions – structural, hormonal, gene expression, etc. – within the cell or
organism.
Resistance : adaptation through which organisms succeed in surviving in the presence of an agent that is usually hostile to them.
Genome : all of an organism’s genetic information, encoded in its DNA.
Non-Bt maize fields are protecting Bt maize fields
The refuge strategy consists of planting a
small proportion of land with non-Bt maize;
the aim being to maintain pockets of insects
that remain susceptible to the toxin. In line
with other known cases of Bt-resistance,
resistance in Busseola fusca was expected
to involve modification of the cells in the gut
wall, which prevents the toxin from binding.
Crucially, this type of adaptation is inherited
recessively: both parents must be resistant
to produce fully resistant offspring. Since the
probability of resistant individuals arising in
the field is low, any resistant insects surviving
on Bt maize will mate with one of the many
Bt-susceptible individuals originating from the
refuge area and their progeny will not survive
in the Bt-maize field. This tactic has been successful, especially in North America where the
first Bt maize has been planted since 1995
with resistance yet to develop among lepidopteran pests.
The exception to the rule
However, about seven years after Bt maize was
introduced to South Africa in the late 1990’s,
scientists observed resistant Busseola fusca
caterpillars and, more importantly, these resistant insects seemed to reproduce and spread
rapidly. To explain this phenomenon, scientists in South Africa, together with IRD researchers, crossed resistant South African moths
with susceptible moths imported from Kenya,
where Bt maize is not yet commercialized. The
offspring developed perfectly on Bt maize and
were as resistant as the South African resistant parents. Unlike everything known so far,
this resistance evolved in the field was inherited as a dominant trait.
A likely new resistance mechanism
This result shows for the first time that resistance to Bt maize can be inherited in a dominant rather than recessive way. It also explains
why resistance has spread rapidly. The moth
does not seem to have followed the expected pattern of adaptation. At this stage, there
are several hypotheses as to the nature of the
mechanism, but it is very likely that Busseola
fusca has developed an unconventional resistance mechanism yet to be identified.
CONTACTS
Coordination
Gaëlle Courcoux
Information and Culture
Department
Implications
In South Africa, most farmers are still cultivating single-toxin Bt maize. In many cases
they need to apply at least one pesticide spray,
which makes planting of Bt varieties less attractive. As a result of the study, the planting
of refuges needs to be reconsidered in South
Africa, and a possibility exists that the refuge
strategy may totally change in the future.
However these are very short term solutions.
In the medium term, single-toxin Bt maize
is being progressively replaced by a stacked
variety producing two different toxins but, in
a worst case scenario, one cannot exclude
that Busseola fusca could also quickly adapt
to varieties expressing more than one toxin. In
the long term, new Insect Resistance Management strategies, likely more complex, should
be developed against Busseola fusca. Such
perspectives could include a more diverse
array of toxins for the control of pest populations, possibly supplemented with a biological
component such as pathogenic fungi or parasitic wasps.
Tel: +33 (0)4 91 99 94 90
Fax: +33 (0)4 91 99 92 28
fichesactu@ird.fr
www.ird.fr/la-mediatheque
Media Contact
Cristelle DUOS
Tel: +33 (0)4 91 99 94 87
presse@ird.fr
Partners
North-West University in South Africa, International Centre for Insect Physiology and Ecology (Icipe) in Kenya.
References
Campagne Pascal, Kruger M., Pasquet Rémy,
Le Ru Bruno, Van den Berg J. Dominant Inheritance of Field-Evolved Resistance to Bt Corn
in Busseola fusca. PLoS ONE, 2013, 8(7):
e69675. doi:10.1371/journal.pone.0069675
Contacts
Rémy Pasquet, researcher at IRD
Tel.: +254 (0) 721 491 430
remy.pasquet@ird.fr
Johnnie Van den Berg, researcher at NorthWest University, South Africa
Tel.: +27-18-299 2376
Johnnie.vandenberg@nwu.ac.za
RU for Biodiversity and evolution plant/insectpest/antagonist complexes – BEI associated
with the Laboratory for evolution, genomes
and speciation (LEGS) at Paris-Sud University.
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Indigo, IRD Photo Library
Daina Rechner
Tel: +33 (0)4 91 99 94 81
indigo@ird.fr
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© IRD/DIC, Juin 2013 - Conception et réalisation graphique : L. CORSINI
Bt maize and resistance development
Genetically engineered maize is created by
introducing a gene into the plant genome that
expresses a toxic protein from a bacterium, i.e.
Bacillus thuringiensis (Bt). Both the leaves and
stems of Bt maize produce this toxin which
destroys the gut of any moth larvae eating
the plant. The technique is effective and unlike
wide spectrum pesticides, it only targets larvae
of moths. However, sooner or later, insect species may be able to develop a mechanism of
resistance against any pesticides. Bt maize is
not fundamentally different in this regard and
in order to delay the evolution of resistance in
pest populations, the concept of maintaining
refuges for Bt susceptible moths was developed.