Entomopathogenic fungi for the management of banana rhizome weevil (cosmopolites sordidus germer)

Abstract

The study entitled “entomopathogenic fungi for the management of banana rhizome weevil, Cosmopolites sordidus Germer” was conducted at College of Agriculture, Vellayani during 2015-17. Objective of the study was to evaluate the biocontrol potential of entomopathogenic fungi for the management of banana rhizome weevil. Adults and grubs of the rhizome weevil were collected from Thiruvananthapuram district and were observed for infestation by entomopathogenic fungi. From the infected adult weevil, Fusarium solani (Mart.) Sacc. was isolated and its pathogenicity was proved on adults and grubs of C. sordidus. Pathogenicity studies with entomopathogenic fungi Metarhizium anisopliae (Metchnikoff) Sorokin (Ma4), Beauveria bassiana (Balsamo) Vuillemin (Bb5), Beauveria bassiana (ITCC6063), Purpureocillium lilacinum (Thorn) Samson (ITCC6064) and F. solani were carried out on adults and grubs of C. sordidus. Grubs of C. sordidus treated with M. anisoliae (Ma4) caused mortality at 24 Hours after treatment (HAT) and white mycelial growth initiated two days after death which later changed to green color. In case of adults, mortality occurred 72 HAT. B. bassiana treated on grubs of C. sordidus caused mortality at 24 HAT and developed pink color after three days and later white cottony mycelial growth was observed on body. On adults, death occurred 72 HAT and extensive mycelial growth covered the body except the dorsal elytra after four days of treatment. P. lilacinum when treated on grubs caused mortality at 48 HAT and produced pink color one day after death and white mycelia covered the body. In adults, mortality initiated four days after treatment and extensive mycelial growth was observed on dorsal and ventral side. At 48 HAT, F. solani caused mortality of grubs of C. sordidus and produced brown color on shrunken grubs which later changed to creamy white stiff grubs. All the tested fungi were found to be pathogenic to both adults and grubs of C. sordidus. Under laboratory conditions the effective spore concentrations of the fungi were found out by testing different concentrations on adults and grubs. M. anisopliae (Ma4) @ 1.29×108 spores mL-1recorded 100 per cent mortality of grubs at 168 HAT. On adults M. anisopliae @ 2.1×108 spores mL-1 recorded 35 per cent mortality. B. bassiana (Bb5) @ 1.2×108 spores mL-1 when treated on grubs caused 95 per cent mortality at 216 HAT. Whereas in adults, B. bassiana caused 40 per cent mortality @ 2.5× 108 spores mL-1. P. lilacinum @ 1.9×108 spores mL-1 recorded 85 per cent mortality at 192 HAT when treated on grubs. At a spore concentration of 2.1×108 spores mL-1 P. lilacinum caused 30 per cent mortality. B. bassiana (ITCC6063) and F. solani caused less than 50 per cent mortality of both grubs and adults of C. sordidus. Evaluation of entomopathogenic fungi for the management of C. sordidus under field conditions was done by conducting three pot culture experiments; viz. rhizome dip method, prophylactic soil drenching and curative soil drenching. The study revealed that spore suspension of M. anisopliae @ 108 spores mL-1 when applied as rhizome dip method was more effective with least damage in rhizomes (10.83 per cent), lowest number of tunnels (0.49) and the least number of live immature stages (0). In the prophylactic soil drenching, spore suspension of M. anisopliae @ 108 spores mL-1 was found to be effective with least rhizome damage (14.16 per cent), number of tunnels (1.08) and number of live immature stages (1.25) followed by its talc formulation with a rhizome damage, number of tunnels, and immature stages of 14.16 per cent, 1.16 and 0.66 respectively. In curative soil drenching, spore suspension of M. anisopliae recorded least rhizome damage (10 per cent) and was statistically on par with talc formulation of M. anisopliae and B. bassiana @30 g L-1 (12.49 per cent). Soil drenching given as curative treatment was more effective than the prophylactic method. Talc formulation of Ma4 and Bb5 was equally effective when applied as curative soil drenching. To conclude, isolate F. solani was isolated from the cadaver of C. sordidus. F. solani, NBAIR isolates viz. M. anisopliae (Ma4) and Beauveria bassiana (Bb5) and local isolates available in department of Agricultural Entomology B. bassiana (ITCC6064), P. lilacinum (ITCC6064), were identified to be pathogenic to grubs and adults of C. sordidus. In laboratory conditions, M. anisopliae @ 108 spores mL-1 was found to cause 100 per cent mortality of grubs. Curative application by soil drenching of talc formulation of M. anisopliae and B. bassiana 108 spores mL-1 @ 30g L-1 was found to be effective for the manangement of C. sordidus.