Evaluation de l’efficacité de beauveria bassiana, des extraits des graines de thevetia peruviana et d’azadirachta indica contre hypothenemus hampei

Abstract

Coffee production continues to decline considerably despite several government programs stimulus packages and their importance in Cameroon's exports. Phytosanitary constraints and their management strategies are among the major causes of this production reduction. In view of this, this study aimed to assess the effect of the entomopathogenic fungus Beauveria bassiana, aqueous extracts of Thevetia peruviana seeds (AETP), aqueous extracts (AEAI) and oil extracts (OEAI) of Azadirachta indica seeds against Hypothenemus hampei Ferrari(Coleoptera: Scolytidae). Surveys and observations were carried out with sixty-three farmers and their coffee farms in Doumé, Bamendjou and Melong respectively. Subsequently, laboratory tests were carried out by rearing populations of bark beetles collected from infested berries in the field and by isolating B. bassiana (two isolates) from soil samples from the IRAD Nkoémvone (BbIRAD.Nkoe on Robusta coffee) and Foumbot (Bb-IRAD.Fbt on Arabica coffee) timber yards. Efficacy tests were carried out on unsexed adult bark beetles, using five concentrations of bassiana (3×102 (C1), 3×104 (C2), 3×106 (C3), 3×108 (C4), 3×1010 (C5) spores/ml) and four doses (12.5 (C1), 25 (C2), 50 (C3), and 100 (C4) mg/ml for the aqueous extracts and in µl/ml for the oil) of extracts of T. peruviana and A. indica seeds. The same extracts and doses of the plants were used for the compatibility tests with B. bassiana. The effects of the natural substances were compared to the reference controls, including the chemical insecticide Chlorpyriphos-ethyl and the chemical fungicide Chlorothalonil + Dimethomorph. Finally, in the field, in a randomised complete block design, the efficacy of the best laboratory doses of the different treatments (B. bassiana, AETP, AEAI and OEAI) was evaluated against H. hampei at 0-1 m and 1-2 m height of the coffee trees, during the years 2019 and 2020. The results of the surveys and observations in the coffee farms showed that insects were the most formidable phytosanitary constraints in Melong and Doumé. In Bamendjou, Colletotrichum kahawae (the causal agent of anthracnose) was the most formidable attack in 100% of coffee farms. Clearing and pruning of coffee trees was applied by 100% of the farmers surveyed. However, the application of insecticides, fungicides and herbicides depended on the incidence, severity and timing of the pests. Thus, fungicide application was more intense in Bamendjou (80% of growers), while in Melong and Doumé it was herbicide application (90% of growers) and insecticide application (80% of growers) respectively. The two B. bassiana isolates (Bb-IRAD.Fbt and Bb-IRAD.Nkoe) differed in both their developmental parameters and virulence. They proved to be very virulent at high concentrations causing 50-90% mortality of bark beetles, whether applied directly to the beetles or to the berries. OEAI and AETP were more effective than AEAI against H. hampei. Average mortality rates of 78.7-100% bark beetles were recorded with OEAI, whereas with AETP, mortality ranging from 0% (C2) to 94.9% bark beetles (C4) was recorded. These effects were better after direct treatment of bark beetles than after treatment of berries. Compatibility assessment showed that high concentrations of AETP, OEAI, AEAI and Chlorpyriphos-ethyl were incompatible by inhibiting (50-90%) all developmental parameters (mycelial growth, spore production and germination) of both B. bassiana isolates. Sterile distilled water and Chlorothalonil + Dimethomorph were compatible with all growth parameters of both isolates. Field tests showed that B. bassiana isolates, OEAI and Chlorpyriphos-ethyl were significantly more protective of berries at all coffee plant heights than AEAI and AETP. Berry attack rates were reduced by 30-40% after application of the treatments. Between the two years of experimentation, a 1-2% reduction in berry attack rates was recorded. However, environmental conditions increased the virulence of the pests and reduced the effectiveness of the treatments applied. Dry berry yields were better in Doumé with a 60% increase in yield observed with Bb-IRAD.Fbt, Bb-IRAD.Nkoe and Chlorpyriphos-ethyl, than in Melong where the yield increase varied between 33.3% (Chlorpyriphos-ethyl), 34.8% (Bb-IRAD.Nkoe) and 43% (Bb-IRAD.Fbt). In Bamendjou, the improvement rates in dry berry yields were about 70% with the plant extracts. The results of this study showed that it would be necessary for B. bassiana, T. peruviana and A. indica to be integrated into H. hampei control programmes. It would also be essential that a law on organic or sustainable agriculture be put in place to promote and encourage the production, development and use of these natural substances