Un stress hydrique au stade plantule a l'origine d'une necrose

National audienc

La jaunisse nanisante de l'orge chez le maïs-grain. Aspects pathologiques et efficacité des insecticides

National audienceBYDV-PAV is one of the main viruses responsible for the disease known as barley yellow dwarf virus. In France BYDV-PAV is the most common virus on corn crops and is known to be a primary cause of red discoloration of leaves. If the virus is inoculated at the 2-3 leaf stage, there is little effect on plant development, but the yield can be reduced by up to 20% for the two varieties studied. The most sensitive measurement of the extent of the virus is by counting the number of grains per ear. This reduction in yield occurs if all plants are infected and if the local conditions are favourable to early systemic infection. Where there is a high inoculum level at the beginning of the corn-plant development cycle, insecticides have only a partial effect, as is shown by the experience of seed treatments using imidaclopride. The use of the latter is not really justified in the treatment of BYDV except under exceptional conditions or in cases of parallel control of other bioagressors. The article recommends research into the source of resistance to BYDV-PAV.Le virus BYDV-PAV est l’un des virus responsables de la maladie désignée sous le nom générique de jaunisse nanisante de l’orge (JNO). En France le BYDV-PAV est le virus de la JNO le plus fréquent chez le maïs chez lequel il est à l’origine de rougissements foliaires. Ce virus inoculé au stade 2-3 feuilles affecte peu le développement végétatif des plants de maïs hybrides mais peut réduire le rendement d’environ 20 % chez les deux variétés étudiées. Le nombre de grains par épi est la composante du rendement la plus sensible à la présence du virus. Cette baisse de rendement est observée si toutes les plantes sont infectées et si les conditions du milieu sont favorables à une infection systémique précoce du virus. En cas de pression d’inoculum élevée en début de cycle du maïs les insecticides ne limitent que partiellement le nombre de plantes infectées, comme le montre le traitement des semences par l’imidaclopride. Son utilisation ne se justifie vis-à-vis de la JNO que dans des conditions exceptionnelles ou lorsqu’il s’agit de lutter parallèlement contre d’autres bioagresseurs. La recherche de sources de résistance vis-à-vis du BYDV-PAV est préconisée

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