La cécidomyie orange du blé, Sitodiplosis mosellana (Géhin): appréhension des risques et gestion intégrée

La cecidomyie orange du ble, Sitodiplosis mosellana (Gehin), est un ravageur commun du froment. Presente sur les trois continents de l’hemisphere Nord, cette espece est connue depuis deux siecles mais son controle reste difficile tant par sa presence discrete que par ses effectifs hautement variables. En Wallonie, les niveaux d’infestations sont globalement faibles mais atteignent localement des seuils inquietants.Au debut des annees 2000, la pheromone sexuelle de S. mosellana a ete identifiee au Canada. Cette decouverte a permis la fabrication de pieges qui ont considerablement ameliore la detection et la mesure des vols de cet insecte minuscule. De 2007 a 2010, les captures de S. mosellana ont ete mesurees quotidiennement dans plusieurs dizaines de champs de Wallonie, aux historiques et aux couverts varies.Les volumes de captures au piege a pheromone sexuelle ont ete tres importants. Il a fallu en etudier la signification, notamment en termes de mesure du risque. En effet, si les mâles sont efficacement captures, seules les femelles constituent un risque de degâts. L’interpretation correcte des captures a l’aide de ce type de piege, a ete rendue possible par l’observation de differences fondamentales concernant la mobilite et la distribution spatiale des mâles et des femelles de S. mosellana. Meme s’ils ne mesurent pas directement l’emergence proprement dite, les pieges a pheromone ont permis, grâce a leur tres grande sensibilite, de preciser les connaissances sur l’emergence des adultes et de reveler que plusieurs vagues d’emergence pouvaient se succeder au cours d’une meme annee.La prevision des emergences de la cecidomyie orange du ble, constitue la cle de voute de la lutte contre ce ravageur dont un controle efficace par des insecticides ne se justifie eventuellement que lorsque la courte saison des pontes coincide avec l’epiaison des froments. Les patrons d’emergence obtenus par les pieges ont ete confrontes aux previsions de differents modeles concus en Europe ou en Amerique du Nord, et appliques aux conditions meteorologiques observees de 2007 a 2010. Aucun de ces modeles n’a prevu correctement les emergences sur l’ensemble des quatre annees.Les donnees d’emergence obtenues a l’aide des pieges a pheromone (effectifs eleves et releves quotidiens) ont fait apparaitre une relation de cause a effet entre, d’une part les vagues d’emergences et, d’autre part les episodes pluvieux observes trois a six semaines plus tot. L’ecart entre une pluie inductrice et la vague d’emergence induite correspondante s’est avere constant en termes d’accumulation de temperature : il equivaut a 160 degres-jours en base 7°C. Partant de ce constat et des acquis des modeles anterieurs, un modele previsionnel original des emergences a ete developpe et valide sur le terrain. Allie a une meilleure connaissance de la biologie du ravageur, il constitue un outil majeur de la lutte integree.Par ailleurs, les travaux menes ont egalement revele l’existence d’un biais frequent dans les essais d’evaluation des varietes, entraine par la concentration des pontes de cecidomyie orange sur les premieres parcelles atteignant le stade epiaison. Dans le systeme d’evaluation en vigueur, notamment pour l’inscription dans les catalogues nationaux, ce biais conduit a une sous-estimation du potentiel de rendement des varietes de ble les plus precoces.Enfin, la decouverte du role inducteur des pluies sur l’emergence des adultes a ete exploitee en conditions controlees pour planifier des emergences echelonnees, et pour disposer, pendant une longue periode, de jeunes adultes prets a pondre. Cette application permet des a present de mesurer en serre le niveau de resistance des varietes exposees de facon homogene a l’insecte, quel que soit leur degre de precocite.De diverses facons, cette etude contribue a une meilleure connaissance de la cecidomyie orange du ble et offre de nouveaux outils pour la lutte integree contre ce ravageur.--------------------------------------------------The orange wheat blossom midge, Sitodiplosis mosellana (Gehin), a common pest of wheat throughout the northern hemisphere, is known for two centuries but remains difficult to control due to its discrete behavior and its highly variable population level.In general, the infestation levels in Wallonia (Belgium) are low, although levels could locally exceed worrying thresholds. In the early 2000s, the sexual pheromone of S. mosellana has been identified in Canada. This discovery has led to the manufacturing of traps which have greatly improved the detection of this tiny insect. From 2007 to 2010 in Wallonia, S. mosellana captures have been daily registered in about 20 fields with different cropping histories and grown with different crops.Insect captures by pheromone traps were numerous. Relation between amount of captures and risk measurement has been studied. As expected, only the males are attracted by the pheromone and the risk of ears infestation is mainly related to the presence of females. The correct interpretation of captures in pheromone traps has been established by the observation of fundamental differences between males and females concerning their mobility and their spatial distribution. Despite the fact that pheromone traps are not real emergence traps, they have led to new information on adult emergence indicating that several emergence waves can be consecutive during the same year. Forecasting the emergence of the adult orange wheat blossom midge is a key element on pest management. Insecticides treatments are sometimes justified when the egg laying period of the insect coincides with ear emergence of wheat. Emergence patterns established from captures of pheromone traps have been compared with the forecast of several models built in Europe or North America. These forecasting models were used with the meteorological data observed from 2007 until 2010. None of the six tested models provided a reliable forecast across the four years of our study. Emergence data from catches in pheromone traps were very accurate because the number of catches were high and were taken each day. This emergence data showed a relation between emergence waves and rainfalls occurring during the preceding 3 to 6 weeks. The lag between inductive rain and emergence wave is constant in terms of temperature accumulation: it is equivalent to 160 degree–days above 7°C. This discovery, combined with experience from previous models, was incorporated into a new forecasting model.In addition, the present work has also revealed the existence of a common bias in variety evaluation trials leading sometime to the concentration of the eggs in the earliest earing variety. In the current evaluation system, this bias leads to a sub-evaluation of the yield for the most precocious varieties. Finally, the discovery of the inductive rain for adult emergence has been used in the screening for resistant varieties to S. mosellana by providing adults during the complete duration of the test. This application of the model allows to measure, under controlled conditions, the level of resistance of all varieties (early and late heading varieties) which are exposed homogeneously to the insect.In total, this study has contributed to a better understanding of the orange wheat blossom midge and provides some new tools in the management of this pest.

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