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Infectivity of Bacillus Thuringiensis Var.kurstaki and Determination Lc50 Values of the Bacterium for Bombyxmori L.

A Nahar
BSRTI, Rajshahi

Abstract/ Executive Summary:

The level of pathogenicity of Bacillus thuringiensis strain B.T var. Kurstakl was studied for all the instars of a silkworm race. NB18. The Lc50 values were determined for different instars. The Lc50 values for 1.11.1II. IV and V instars after 24 hours were found lo be approximately 0.52 x 106 , 0.72 x 106. 1.94 x 106. 1.01 x 106 and 1.72 x I06. The concentration of the bacterium required for 50% mortality in all the instars after 48 hours were lower as expected and showed linear dose response with increasing age of larvae.

Keywords: Bacillus thuringiensis var. Kurstaki. Pathogenicity, Bombyx mori L.

Location: Bangladesh Sericulture Research and Training Institute

Start Date:

End Date:

Program Area: Pest Management

Commodity: Silkworm

Objectives:

To investigated the pathogencicity o the bacterium B.T. var. Kurstaki and to determined Lc50 values for all the instars of all bivoltine silkworm races NB18.

Methodology:

The bacterium, Bacillus thuringiensis var. Kurstaki was collected from the stock of pathology laboratory, CSRT1, Mysore. Dilutions of the bacterium required for inoculation were prepared from the stock solution of the bacterium. Same stock solution was used for all the instars larvae for preparation of the serial dilutions. The stock solution used for all the instars had a bacterial cell concentration of 2.58 x 106, CFU/ml 0.2 ml of the the solution from each dilution was smeared on to mulberry leaves and fed to each replication having 20 larvae. Each dilution had 3 such replications. The inoculations were done when larvae were just out of moult before resuming feeding. Newly hatched larvae were inoculated by brushing them on to a leaf smeared with the bacterium. The leaves were cut into circular discs used for I and II instar had a diameter of 2 cm and that used for III, IV and V instar had a diameter of 4 cm. One disc was used for each replication of I and II instar larvae while two were used for the other instars. Entire leaf discs were consumed by the larvae of all the instars, except I instar. Mortality was recorded after 24 and 48 hours of inoculation. The analysis of data and calculation Lc50 were carried out according to the prorniiire described by Finncy (1964)

Source of Information: Bulletin of Sericulture Research 1999 vol10:73-76 Published by Bangladesh Sericulture Research and Training Institute.

Article Link (Online Journal):

Funding Source:

1. Government

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The value of Lc50 observer in the present study for the chawki stage are almost in agreement with those reported by Aizawa el al.(1975). However, in the later instars, these values are observed to be higher, indicating better tolerance of the later instars to this bacterium. There is also a clear linear dose response of the different instars from I to V instar to infection of the bacterium. Samson et al (1987) have also observed a similar pattern of dose response in the bivoltine race NB18 against two varieties of Bacillus thwingiensis Viz. B.t.Var thuringiensis and B. t. var dendrolimus.

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