La résistance aux produits phytopharmaceutiques

SPEGESTADChristophe Délye, chargé de recherche à l'UMR 1347 de l'INRA à Dijon, et Annie Micoud, responsable de l'Unité Résistance aux Produits Phytosanitaires du laboratoire de l'ANSES à Lyon, ont présenté à la section « Alimentation et santé » du CGAAER un exposé sur le phénomène de résistance des bio-agresseurs (ravageur, agent pathogène, adventice…) aux produits phytopharmaceutiques (PPP). L’enjeu est majeur. Peu de substances ayant des modes d’action nouveaux seront commercialisées dans la décennie à venir, alors que des substances anciennes, devenues inefficaces, sont régulièrement retirées du marché. La résistance d'un bio-agresseur aux produits phytopharmaceutiques (PPP) se caractérise par deux facteurs : l’intensité et le spectre. L'intensité de la résistance est le rapport entre la quantité de PPP requise pour avoir le même effet sur un individu résistant que sur un individu sensible. Le spectre de la résistance est la gamme de substances auquel un bio-agresseur sera résistant. Le caractère de résistance est génétiquement transmissible d'une génération de bio-agresseurs à une autre, car il est déterminé par une ou plusieurs modifications du génome. On distingue globalement deux types de résistances, selon la nature du mécanisme en cause : - la résistance liée à la cible du PPP (ex : dans le cas d'un produit larvicide, le gène codant le neurotransmetteur cible du PPP a muté, rendant la larve insensible au produit) ; - la résistance non liée à la cible et qui fait intervenir d’autres mécanismes comme la métabolisation (l'organisme cible acquiert la capacité à métaboliser le PPP et donc à le neutraliser), la séquestration (ex : la cuticule de l'insecte ravageur qui acquiert la capacité de réduire la pénétration du PPP dans son organisme) ou l’afflux accru du PPP au travers des stomates foliaires qui réduit l'exposition du bio-agresseur au PPP. Sous l’effet des applications de PPP, la fréquence des bio-agresseurs résistants augmente dans les populations, jusqu’à causer éventuellement une perte de contrôle. L’enjeu des stratégies anti-résistances est donc de ralentir la sélection d'individus résistants par de nouveaux PPP. Les interventions ont été émaillées de conseils pratiques : « l’idée est de traiter le plus tôt possible et le plus fort possible » ; « il faut aussi veiller à la diversité dans les rotations, à varier les modes d’actions, les matières actives et éviter les faux mélanges de produits ayant le même mode d'action qui ne font qu’augmenter le risque » ; « Une gestion concertée du désherbage avec ses voisins est souhaitable car la résistance se propage aussi par les airs »

Dramatic changes in the genotypic frequencies of target insecticide resistance in French populations of [i]Myzus persicae[/i] (Hemiptera: Aphididae) over the last decade

International audienceIn 2001, although target site resistance against pyrethroids was wide-spread in clonal populations of Myzus persicae on oilseed rape in France (kdr), target site resistance against carbamates was rare. Starting in 2005, control failure by carbamates appeared. The current study was designed to document changes in insecticide-resistant genotype frequencies in France during the last decade. Two resistant genotypes (RGs) were dominant in populations in 2009-2010: RG1 with the kdr 1014F allele (16%), RG2 with the MACE 431F allele and the atypical s-kdr 918L allele (83%). The widespread prevalence of RG2 suggests that a substantial shift has occurred (allele 431F in <2% individuals in 2001) presumably in response to agrichemicals. Analysis of neutral markers revealed that 2009-2010 populations were composed of two differentiated genetic pools (instead of one in 2001) and that RG2 was found in two divergent superclones of M. persicae. Possible explanations for these observed shifts in population structure and especially for the dramatic increase of the double mutant RG2 are discussed

Tavelure du pommier sa résistance aux strobilurines en France - Tests biologiques et moléculaires 2004-2006

National audienceDans la lutte chimique contre la tavelure, il est essentiel de savoir si l'agent de la maladie présente une résistance à certains des fongicides utilisés. D'où la surveillance évoquée dans cet articl

Ostrinia nubilalis parasitism and the field abundance of non-target insects in transgenic Bacillus thuringiensis corn ( Zea mays )

International audienceIn this study, we evaluated in field trials the effects on non-target species, of transgenic corn producing the Cry1Ab toxin of Bacillus thuringiensis (Bt). In 1998, we collected Ostrinia nubilalis (Hübner) larvae from transgenic Bt corn (Novartis Hybrid 176) and non-Bt corn at four geographical sites. We found a significant variation in parasitism by the tachinids Lydella thompsoni (Herting) and Pseudoperichaeta nigrolineata (Walker) among sites, and more parasitism in non-Bt than in Bt fields. The Bt effect did not vary significantly among fields. In 1999, we performed a field experiment at two sites, comparing the temporal abundance of non-target arthropods in Bt corn (Monsanto Hybrid MON810) and non-Bt corn. The non-target insects studied included the aphids Metopolophium dirhodum (Walker), Rhopalosiphum padi (L.) and Sitobion avenae (F.), the bug Orius insidiosus (Say), the syrphid Syrphus corollae (Meigen), the ladybird Coccinella septempunctata (L.), the lacewing Chrysoperla carnea (Stephens), thrips and hymenopteran parasitoids. For all species but one, the number of individuals varied greatly over the season but did not differ between the types of corn. The only exception was thrips which, at one site, was significantly more abundant in Bt corn than in non-Bt corn. However this difference did not remain significant when we took the multiple tests into account. Implications for pest resistance management, population dynamics and risk assessment are discussed

Host plants and insecticides shape the evolution of genetic and clonal diversity in a major aphid crop pest

International audienceUnderstanding the spatiotemporal dynamics of pesticide resistance at the landscape scale is essential to anticipate the evolution and spread of new resistance phenotypes. In crop mosaics, host plant specialization in pest populations is likely to dampen the spread of pesticide resistance between different crops even in mobile pests such as aphids. Here, we assessed the contribution of host-based genetic differentiation to the dynamics of resistance alleles in Myzus persicae, a major aphid pest which displays several insecticide resistance mechanisms. We obtained a representative sample of aphids from a crop mosaic through a suction trap for 7 years and from various crops as a reference collection. We genotyped these aphids at 14 microsatellite markers and four insecticide-resistant loci, analyzed the genetic structure, and assigned host-based genetic groups from field-collected aphids. Four well-defined genetic clusters were found in aerial samples, three of which with strong association with host-plants. The fourth group was exclusive to aerial samples and highly divergent from the others, suggesting mixture with a closely related taxon of M. persicae associated with unsampled plants. We found a sharp differentiation between individuals from peach and herbaceous plants. Individuals from herbaceous hosts were separated into two genetic clusters, one more strongly associated with tobacco. The 4-loci resistance genotypes showed a strong association with the four genetic clusters, indicative of barriers to the spread of resistance. However, we found a small number of clones with resistant alleles on multiple host-plant species, which may spread insecticide resistance between crops. The 7-year survey revealed a rapid turn-over of aphid genotypes as well as the emergence, frequency increase and persistence of clones with resistance to several families of insecticides. This study highlights the importance of considering landscape-scale population structure to identify the risk of emergence and spread of insecticide resistance for a particular crop

Status of resistance towards SDHIs in French populations of Sclerotinia sclerotiorum and characterization of resistant strains

International audiencePopulations of fungal pathogens may be subject to many selective pressures in agricultural environments. Among them, fungicides constitute one of the most powerful determinants of population adaptation acting in a short time span. Here, we investigated whether fungicides sprays applied yearly in the Champagne vineyard to control the grey mold causal agent Botrytis cinerea could shape population structure and evolution. We carried out a 2-year survey (4 collection dates) on three treated/untreated pairs of plots. We found that fungicides treatments had no or little impact on population subdivision at neutral loci, as well as on diversity or reproduction mode. Nevertheless, we found evidence of stronger genetic drift in some treated plots, consistent with the regular application of fungicides. Moreover, we observed spatial structure in resistance frequency for two loci under contemporary selective pressure, as reflected by cline patterns. At last, using a modeling approach, we estimated fitness costs of resistance to fungicides, responsible for resistance frequency decay during winter. Further work is in progress to estimate parameters of positive selection and migration exerted on B. cinerea populations, and disentangle the relative effect of the evolutionary forces at work

Résistance de Sclérotinia sclerotiorum aux fongicides utilisés sur colza : bilan de la surveillance du territoire de 2000 à 2008.

International audienc