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Sensibilité des larves deSpodoptera littoralis [Lep.: Noctuidae] aux hyphomycètes entomopathogènesNomuraea rileyi etPaecilomyces fumoso-roseus
Authors:J Fargues  Dora Rodriguez-Rueda
Institution:(1) Station de Recherches de Lutte Biologique, I.N.R.A., la Minière, 78280 Guyancourt, France
Abstract:Résumé L'étude de la sensibilité des chenilles deSpodoptera littoralis Boisd. à des doses croissantes de spores deNomuraea rileyi (F.)Samson, montre que le 6e et dernier stade est plus résistant que le 1er et surtout les 4e et 5e stades larvaires. La virulence du pathotypeN. rileyi no 5 à l'égard des larves de cette noctuelle est élevée puisque le temps léthal 50 % (TL 50) est de 6 j en moyenne. Pendant l'incubation de la maladie les chenilles continuent de s'alimenter mais l'infection peut réduire jusqu'à 60 % la prise alimentaire par rapport à la consommation des larves des lots témoins. Toutes conditions égales par ailleurs, la mortalité provoquée parN. rileyi no 5 après traitement des larves nouvelles-nées est supérieure lorsque les insectes sont maintenus à 25°C par rapport à celle constatée à 20°C. Cependant pour une dose d'inoculum élevée, les conditions thermiques (20°, 25° ou 28°C), ne modifient pas sensiblement la réponse à l'infection parN. rileyi no 5 alors qu'elles limitent l'efficacité d'un pathotype moins virulent:Paecilomyces fumoso-roseus Wize noo 39.
Summary Laboratory studies were conducted to determine the susceptibility of various larval instars of the cotton leafworm,Spodoptera littoralis Boisd. to different spore doses ofNomuraea rileyi (F.)Samson to investigate the influence of temperature on the infection by this fungus and byPaecilomyces fumosoroseus Wize. Contaminations were obtained by direct spraying on larvae in tower apparatus or by feeding of larvae on treated pieces of leaf during 48 h or 72 h. The influence of cryptogamic infection on food consumption was studied by measuring surfaces of standard cabbage leaf disces submitted to treated and control larvae. Angular values mortality rates were submitted to the 2- or 3-way analysis of variance and comparisons of means were made by theDuncan's test. In some cases we have also considered the time-mortality and the dose-mortality curves. The 6th instar was more resistant than all other tested instars. TL50 were found to be 6 days in most cases. During incubation of the disease, larvae continued to feed, but food consumption could be reduced at 40 % of controls. Larval mortality due toN. rileyi No 5, recorded after 8 days of incubation, was higher at 25°C than at 20°C. Nevertheless, at high dosage, efficiency ofN. rileyi No 5 was not affected by temperature at 20°, 25° and 28°C. The other pathotype,P. fumoso-roseus No 39 was more effective at 20°C than at 25° and 28°. At 32°C, the temperature, unfavourable to fungal growth, limited mortality at non significant rates.


Avec la collaboration technique deH. Vermeil de Conchard.
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