Characterization and efficacy of Beauveria bassiana and Metarhizium anisopliae, two candidates entomopathogenic fungi for the development of a biopesticide for microbial control of banana weevil, Cosmopolites sordidus

Abstract

The production of banana/plantains, a staple food of millions of people in the world is facing a severe parasitic pressure of pests and diseases. Among pests, the banana weevil Cosmopolites sordidus is the most damaging insect causing yield loss of up to 100 %. The management of C. sordidus is still a challenge. In the effort of managing this pest, several strategies have been used, but the wide spreading of insecticides is still the mainstay. However, public increasing concerns about risk associated with chemicals have led to the use of safe and eco-friendly products such as biopesticides. The objective of this work was to develop a biopesticide formulation from high virulent isolates with wide tolerance to environmental factors against banana weevil C. sordidus. Pathogenicity tests of six Cameroonian isolates of Beauvaeria bassiana and Metharizium anisolpliae were performed on eggs, larvae and pupae of C. sordidus as well as endophyte colonization test. The epizootic potential of thermotolerant isolates against C. sordidus population was evaluated using pathogenicity test at proportion 10. 30, and 50% vector ratio.Germination, mycelial growth and conidia production was also determine after culture entomopathogenic fungi under divers environmental conditions. Theselection of quality control, compatibility and photoprotection tests for 12 local ingredients.. The mycoinsecticide formulations developed were applied alone or combined with aggregation pheromone Cosmolure, in the laboratory, greenhouse, and natural environment, and mortality and infections rates were determine. The results of insect life stages susceptibility to entomopathogenic fungal isolates showed that the infection rates depended on the insect’s life stage and isolates which varied from 12 - 96 %, 60 - 100 %, and 12.50 - 79.44 % respectively for eggs, 5-instar larvae, and pupae. Apart from being pathogenic and virulent to multiple stages, these isolates transmitted disease from one life stage to the next. The thermotolerant Beauveria isolates, BIITAC6.2.2 and BIITAC10.3.3, Metarhizium isolates MIITAC6.2.2 and MIITAC11.3.4 caused horizontally of up to 52 % mortality of noninoculated insects at vector ratio of 50 %, with high values for the isolate BIITAC6.2.2 followed by MIITAC11.3.4. Environmental factors such as pH, photoperiod, UV radiations, and relative humidity affected significantly the development of Cameroonian entomopathogenic fungi isolates on artificial media. The fungi strain MIITAC11.3.4 and BIITAC6.2.2 tolerated acidic, near neutral, and alkaline conditions. the optimum growth was obtained at pH range 4 – 11 and 4 - 12 respectively for MIITAC11.3.4 and BIITAC6.2.2. BIITAC6.2.2 is less exigent to light and can be suitable for control of insects of all habitat while MIITAC11.3.4 gave optimum development in total dark and 12L:12D. The two isolates tested in this study were found tolerant of UVB-light. The UVB-light delayed conidia survival, and at the same time, stimulated conidia production. The fungi isolate MIITAC11.3.4 was more stable while BIITAC6.2.2 produced a mutant after 30 min exposure to UVB-light. MIITAC11.3.4 and BIITAC6.2.2 could also grow at low to high humidity environments but a moderate humidity environment was preferable for their best expression. Mass-produced conidia of Beauveria bassiana BIITAC6.2.2 showed high quality. Out of the twelve inactive ingredients tested four were found to be the best carriers of BIITAC6.2.2 conidia (kaolin clay, cassava starch, maize flour, and banana flour ), two good stabilizer (charcoal and wood ashes) and four effective UV protectants (kaolin clay, cassava starch, banana flour, and charcoal. The potential of the four formulations developed varied from one condition to another. Under laboratory conditions, formulations conserved good stability at 4 °C, -20 °C, and -50 °C for up to 72 weeks and enhanced insecticidal potential of active ingredient (causing up to 100 % mortality). Results of the greenhouse trial indicated that B. bassiana BIITAC6.2.2 formulations can be used both as prevention (53.33 - 68.67 % mortality) and post-infestation (66 - 84.67 % mortality) methods. When applied alone in the field, mycoinsecticide did not show a clear positive effect for the first crop cycle. However, two other banana pests (Pollytus mellerborgi and unidentified insect) showed signs of mycosis apart from C. sordidus. The preliminary study of the performance of mycoinsecticide combined with pheromone baited traps revealed that the pheromone trap can optimize mycoinsecticide efficacy in the field, causing up to 83.3 and 78.7 % mortality and mycosis rates of adult C. sordidus.

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