VOL. 9, NO. 10, OCTOBER 2014 ISSN 1990-6145 ARPN Journal of Agricultural and Biological Science © 2006-2014 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 333 VIRULENCE OF THREE STRAINS OF Beauveria bassiana AGAINST THE BANANA WEEVIL C. A. Omukoko 1, 2 , K. N.Maniania 2 , J. M. Wesonga 3 , E. M. Kahangi 3 and L. S. Wamocho 4 1 Chuka University, Chuka, Kenya 2 International Centre of Insect Physiology and Ecology (icipe), Nairobi, Kenya 3 Department of Horticulture, Jomo Kenyatta University of Agriculture and Technology, Nairobi, Kenya 4 Masinde Muliro University of Science and Technology, Kakamega, Kenya E-Mail: canaye3@yahoo.com ABSTRACT The banana weevil, Cosmopolites sordidus Germar, is the most important insect pest of banana and plantain. Beauveria bassiana is the most researched and commercialized fungal biopesticide effective against a variety of insects. Laboratory studies have revealed a great potential of this entomopathogen for use against the banana weevil, Cosmopolites sordidus in banana. Use of traps for collecting adults or infecting them with biopesticides will most likely capture only those weevils in the immediate vicinity of the traps. The indiscriminate use of chemicals has resulted in the development of resistance in insect pests, adverse ecological events, affecting beneficial fauna, and accumulation of residues in the environment. There is considerable need therefore to develop safe and cheaper biocontrol alternatives that can be used to complement existing control methods. The potential of utilizing the entomopathogen B. bassiana for control of banana weevil was evaluated by testing the virulence of three isolates of Beauveria bassiana (ICIPE 273, M353 and M207) at three concentration (10 8 , 3x10 8 and 10 9 ). From previous pathogenicity tests these three isolates were the most pathogenic. At higher fungal concentrations of 3x10 8 and 10 9 adult mortality for all the three isolates was between 35%-70%. The highest mortality was achieved using an elevated concentration of 10 9 , causing mortalities varying from 50-70% 40 days after exposure depending on the isolate. ICIPE 273 was the most virulent, killing 70% of adults followed by M353 (65% mortality) and M207 (51% mortality). This was far much greater than when a standard concentration of 10 8 was used (mortalities ranged between 28%-50%). Differences in virulence among the tested isolates were due to their geographical origins. Keywords: Beauveria bassiana, banana, biological control, bioassays, entomopathogens. INTRODUCTION The banana weevil Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae) is a major pest of bananas. Weevil damage is primarily the result of destruction of the corm tissue sometimes accentuated by secondary attacks by other insects and micro organisms leading to an increased risk of toppling (Godonou et al., 2000). Control of the banana weevil relies on costly chemicals which are beyond reach for most small scale farmers (Gold et al., 2000). The indiscriminate use of chemicals has also resulted in the development of resistance in insect pests, adverse ecological events, affecting beneficial fauna and accumulation of residues in the environment (Gold and Messiaen, 2000). There is need therefore to develop safer and cheaper control alternatives that can be used to complement existing control methods (Nankinga et al., 1998). Entomopathogenic fungi have been used successfully to control various agricultural and pasture pests. Beauveria bassiana and Metarhizium anisoplie have gained considerable attention as biological control agents for weevils and other agricultural pests (Kaaya and Hassan, 2000). In pathogenicity tests conducted previously B. bassiana isolates ICIPE 273, M353 and M207 were identified as possible biocontrol agents based on their virulence against the banana weevil. This paper describes the experiment that was undertaken to determine the virulence of these three isolates against the banana weevil under laboratory conditions in Kenya. The objective of our research was to evaluate further the virulence of three isolates of B. bassiana to the banana weevil. The LC 50’S and LT 50’S of the three isolates were determined and compared. MATERIALS AND METHODS Insects Adults of banana weevils were obtained from naturally infested banana plants at Kenya Agricultural Research Institute Mwea. The weevils were maintained in the laboratory in plastic containers at room temperature for one week before being used in the experiments. The covers of the containers were perforated for ventilation. Banana suckers of susceptible variety were pared and corm tissue placed in the containers. Fungal isolates and cultures Isolates of Beauveria bassiana ICIPE 273, M313 and M207 were obtained from the ICIPE’s Arthropod Germplasm Centre, Duduville, Nairobi, Kenya. The isolates were cultured on Sabouraud Dextrose Agar (SDA) medium for three weeks for complete sporulation in an incubator (27°C). Antibiotic chloramphenical was added to the medium to keep off any bacterial contamination. 65 g SDA was weighed and dissolved in 1000 ml boiled distilled water in a conical flask a magnetic stirrer was