Des scientifiques de l'Université
de Neuchâtel révèlent de nouvelles opportunités dans la
lutte contre Diabrotica virgifera virgifera. Les larves
de cet insecte dévorent les racines du maïs, faisant des ravages
considérables aux USA et plus récemment en Europe. Mais les
racines se protègent en émettant une substance volatile qui
attire des ennemis de la larve. Réalisé dans le cadre du Pôle de
recherche national (PRN) Survie des plantes, ce
travail* est publié jeudi dans la prestigieuse revue
Nature.
Quoi de plus naturel que
d'appeler à l'aide lorsqu'on est attaqué. C'est exactement ce
que fait le maïs sitôt que des insectes commencent à le dévorer.
La plante diffuse des odeurs pour appâter les ennemis naturels
du ravageur et assurer sa protection. Ces substances sont bien
connues quand les attaques proviennent d'insectes herbivores
vivant au-dessus du sol. Mais que se passe-t-il sous la terre,
là où opère la redoutable larve de Diabrotica virgifera
virgifera? C'est la question à laquelle a répondu
Ted Turlings,
directeur de recherche à l'Université de Neuchâtel, et son
équipe au terme d'une étude impliquant le
Max-Planck-Institut für chemische Ökologie à Jena
(Allemagne) et le
CABI
Bioscience Switzerland Centre à Delémont.
Avec le soutien du PRN
Survie des plantes, Ted Turlings et ses collègues ont
identifié une substance produite par les racines qui appâte des
nématodes. Une bonne nouvelle, car ces vers minuscules infectent
et tuent les larves indésirables, délivrant la plante de son
ravageur. La principale prouesse de l'équipe neuchâteloise fut
d'avoir réussi à isoler une substance volatile dans le sol, une
première mondiale! En congelant les racines dans de l'azote
liquide, puis en les pulvérisant, les molécules volatiles ainsi
libérées ont pu être analysées. De la comparaison entre racines
attaquées et indemnes, le signal responsable de l'attraction des
nématodes a été identifié: il s'agit de molécules à base de
caryophyllène.
Mais ces expériences, qui font
partie de la thèse de doctorat de Sergio Rasmann, ont abouti à
une autre conclusion majeure. Elles ont démontré que les
variétés de maïs cultivées aux USA n'émettent pas ce signal
lorsqu'elles sont attaquées par Diabrotica. "En
Amérique du Nord, la faculté de produire du caryophyllène a été
probablement perdue au cours des processus de sélection,
explique Ted Turlings. Le maïs qu'on cultive aujourd'hui dans
ces régions ne serait donc plus capable de répondre par ce biais
aux attaques de l'insecte ravageur."
Pour vérifier l'importance du
signal comme attracteur des nématodes vers l'insecte ravageur,
des expériences sur le terrain ont été menées en Hongrie, dans
une station de recherche du CABI, où les Diabrotica
comptent parmi les organismes nuisibles majeurs. Diabrotica
est en effet apparue en Bosnie au début des années 1990, en
provenance d'Amérique du Nord, sans doute transportée par avion
au moment de la guerre en ex-Yougoslavie. L'insecte a peu à peu
envahi les Balkans et sa présence est signalée autour des
aéroports européens. En Suisse, seul le Tessin est touché.
Aussi, pour préserver le reste du territoire, des essais en
champs, qui impliquent de relâcher des Diabrotica dans
la nature, demeurent prohibés.
Les expériences hongroises ont
été couronnées de succès. Les conclusions ont de quoi réjouir
définitivement Ted Turlings: "Notre travail ouvre des
perspectives en matière de contrôle biologique des larves
ravageuses. On peut songer à la culture de variétés qui
relâcheraient la molécule attractrice dans des quantités
adéquates de façon à renforcer l'efficacité des nématodes. On
aurait ainsi les bases d'une méthode de lutte qui ne fait pas
intervenir de pesticides comme c'est le cas aujourd'hui." Des
progrès dans ce sens permettraient, on l'espère, de réduire la
facture des agriculteurs. Aux USA, les coûts en pesticides que
représente la lutte contre Diabrotica se montent à plus
d'un milliard de dollars par an!
A
discovery from Neuchâtel published in
Nature: Injured maize roots emit a fragrant call for help
Scientists from the University
of Neuchâtel have revealed a new option in the fight against
Diabrotica virgifera
virgifera (Western corn rootworm or WCR). The WCR devour maize
roots causing devastating damages in the
USA and more recently in Europe. However, the roots protect
themselves by emitting a volatile substance that
attracts the enemies of the larvae. Conducted within the
framework of the National Centre of Competence in
Research (NCCR) Plant Survival, the results are published this
week* in the prestigious journal Nature.
There is nothing more natural
than calling for help in a moment of distress. That’s exactly
what maize does when being devoured by insects. To ensure its
protection, the plant emits odours that attract natural enemies
of the pest. The substances released when attacked by above
ground herbivorous insects are well known, but what happens
underground where the infamous larva of Diabrotica virgifera
virgifera operates? Ted Turlings, Director of Research at the
University of Neuchâtel, and his team answered that question in
a study that included the Max-Planck-Institute for Chemical
Ecology in Jena (Germany) and the CABI Bioscience Switzerland
Centre in Delémont.
With the support of the NCCR
Plant Survival, Ted Turlings and his colleagues have identified
a root-produced
substance that attracts nematodes. This is great news since
these miniscule worms infect and kill the undesirable
larvae and hence rid the plant of the pest. The main feat of the
team from Neuchâtel was to have succeeded in
isolating a volatile substance in the soil, a world first. By
first freezing them in liquid nitrogen, the roots could be
pulverized, after which the volatiles could be analysed. By
comparing the damaged roots to intact ones, the signal used to
attract nematodes was identified: a molecule based on
caryophyllene.
These experiments, which are
part of Sergio Rasmann’s PhD thesis, have brought about another
major conclusion. They have shown that the varieties of maize
grown in the USA do not emit the signal when attacked by
Diabrotica. “In North America, the capacity to produce
caryophyllene was probably lost during the selection process”,
explains Ted Turlings. “This could imply that the maize grown
today in those regions are no longer able to use this defence
mechanism when attacked by the insect pest.”
To confirm the signal’s
importance as an attractant of nematodes towards the pests,
field experiments were carried out in Hungary at a CABI research
station, where the WCR is already a major pest. In fact,
Diabrotica first appeared in Bosnia in the early 1990s, probably
imported by airplanes coming from North America during the war
in ex-Yugoslavia. Little by little the insect invaded the
Balkans and its presence has been reported around European
airports. In Switzerland, only the Tessin has been affected and
in order to avoid its spread to the rest of the country it is
impossible to conduct field trials that involve the release of
Diabrotica in nature.
The Hungarian experiments were
very successful. The conclusions give Ted Turlings much reason
to rejoice: “Our
work has opened up new perspectives in the biological control of
the pest larvae. We can now consider planting
varieties able to release the attractant molecule in sufficient
amounts that would enhance the nematodes’ efficiency. We would
also have the basis for a control method that does not make use
of pesticides such as is the case today.” Hopefully this methods
will also reduce the costs of pest control for farmers. In the
USA, the costs of pesticides used for the control of Diabrotica
alone are more than one billion dollars per year!
Related news item from
SwissInfo via
Checkbiotech
Corn
roots out unwanted pests
By Scott Capper,
SwissInfo
Researchers at Neuchâtel
University have demonstrated that the roots of some varieties of
corn send out a chemical signal to help ward off pests.
Their results could lead to more
efficient pest control and solve a billion-dollar problem faced
by maize growers around the world.
The study focused on how corn plants react to damage inflicted
on them by the larvae of a beetle called the Western Corn
Rootworm.
The female lays eggs beneath the soil near a plant, and when
they hatch, the larvae start feeding on the roots. The damage
they inflict is enough to make a corn stalk collapse, reducing
harvest yields.
The scientists found that some varieties of maize won’t give up
without a fight. Their roots emit a compound that attracts a
tiny parasitic worm – a nematode - that lays its own eggs inside
the corn’s attacker, eventually killing it.
"The rootworm is the most important maize pest in the United
States, and it reached Europe during the Bosnian war in the
1990s," said the head of the research team, Ted Turlings.
The beetle was probably carried to the Balkans as part of a corn
shipment, before spreading further afield, including southern
Switzerland.
Identical mechanism
The discovery comes a few years
after scientists showed an identical mechanism in corn leaves.
"We found at the time that when a caterpillar eats the leaves of
maize, the plant emits a smell that attracts a parasitic wasp,"
Turlings told swissinfo. "We wanted to see if the same thing
happened underground."
Besides determining the actual compound involved in fending off
the rootworm – caryophyllene – the researchers also confirmed
that not all corn varieties act the same way.
"Some types do not produce the compound, and these are usually
the most popular American varieties," added Turlings. "This
seems to be the first example whereby artificial selection to
improve maize plants has wiped out an important property."
This incapacity to produce the compound carries a huge cost for
North American farmers. They spend around $1 billion (SFr1.21
billion) each year just to pay for insecticides and cover for
lost crops.
The problem is spreading to Europe, where not all varieties of
corn are resistant to the rootworm either.
Genetics
Another team of researchers at
Germany’s Max Planck Institute is now looking at the genetic
make-up of the varieties that do produce the chemical.
Reintroducing the gene thought to be responsible for the
compound into types of maize that lack it, although fairly
simple, isn’t on the cards though.
According to Turlings, besides raising issues related to the use
of genetic engineering, it would also be cheaper to consider
varieties that can defend themselves.
The researchers are still unsure whether this form of biological
pest control is cost-efficient. Producers would have to purchase
the nematodes needed to ward off rootworms.
Results from field tests in Hungary seem to indicate some cost
benefit, but more trials will have be carried out to find out if
pest-resistant maize varieties have high enough yields.
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