Comparative toxicity of two isolates of Bacillus thuringiensis Berliner from Plutella xylostella L. and Papilio demoleus L. to some important lepidopteran pests of horticultural crops R. Asokan1 and Puttaswamy Department of Entomology University of Agricultural Sciences G. K. V. K., Bangalore – 560 065, India E-mail: asokan@iihr.ernet.in ABSTRACT Toxicity of two isolates of Bacillus thuringiensis viz. KPx-1 and IPd-1 isolated from diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae), and citrus butterfly, Papilio demoleus (Lepidoptera: Papilionidae), were tested against cabbage leaf webber, Crocidolomia binotalis, hairy caterpillar, Diacrisia obliqua and tomato fruit borer, Helicoverpa armigera. Among these, C. binotalis was highly susceptible (28.4 and 26.0 ng/cm2, for KPx-1 and IPd-1, respectively), while, H. armigera was the least susceptible (9.5 and 10.0 µg/ml, for KPx-1 and IPd-1, respectively). Key words: Bacillus thuringiensis, isolates, toxicity Short communication 1 Present address: Division of Biotechnology, Indian Institute of Horticultural Research, Hessaraghatta Lake (PO), Bangalore – 560 089, India Bacillus thuringiensis (Bt) has been successfully employed in the management of insect pests in agriculture and in public health. During the survey, two isolates of Bt subsp kurstaki (Btk), namely, KPx-1 and IPd-1 were isolated from diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae), and the citrus butterfly, Papilio demoleus (Lepidoptera:Papilionidae), respectively. In this communication we report the toxicity spectrum of these Bt isolates to some important lepidopteran pests of horticultural crops viz., cabbage leaf webber, Crocidolomia binotalis (Lepidoptera: Pyralidae), hairy caterpillar, Diacrisia obliqua (Lepidoptera:Arctiidae) and tomato fruit borer, Helicoverpa armigera (Lepidoptera:Notuidae). Stock culture of H. armigera was maintained on semisynthetic diet (Nagarkatti and Prakash, 1974), while, that of C. binotalis and D. obliqua maintained on cabbage. The Bt isolates were grown on nutrient agar at 30°C for 72 h and were homogenized in distilled water which formed the spore, crystal preparation (SCP). An aliquot of the SCP was used for protein estimation using Lowry’s method (Lowry et al, 1971). Bioassay was conducted using serial dilutions of the SCP. Two hundred µl of different dilutions were applied on both sides of fresh cabbage leaf discs (62cm2) @ 100µl/side. Single treated leaf disc was placed in individual, sterile plastic Petri plate (10 x10cm) and 20 five-day old larvae of each C. binotalis and D. obliqua were released separately per replication. Similarly, 100 µl of different dilutions were surface - coated on 3 ml of semi synthetic diet and a single, five-day old larva of H. armigera was released per replication. There were five replications per treatment and an untreated control. Mortality of the treated larvae was recorded every 24 h interval for one week and data were subjected to Probit analysis (Abbott, 1925). Bioassay results showed that there was little variation in toxicity of KPx-1 and IPd-1 to all the test insects. Differences in toxicity were 0.1 & 4.0 ng/cm2 and 0.1 µg/ ml for C. binotalis & D. obliqua and H. armigera, respectively. Among the test insects, C. binotalis was found to be the most susceptible while H. armigera was least susceptible to both the isolates (Table 1). Minor differences in toxicity of the above isolates could be attributed to various factors such as protoxin composition, rate of dissolution and activation of protoxins, presence of receptors, etc. in different test insects (Crickmore et al, 1998). Other factors governing toxicity are complex and poorly understood (Jarret, 1985). In the present investigation, as we found no appreciable variation in toxicity of the two isolates, it is likely that composition, rate of dissolution and activation of the protoxin may have been similar in the three test insects used. J. Hort. Sci. Vol. 2 (1): 71-72, 2007 ACKNOWLEDGEMENT The senior author is grateful to the Indian Council of Agricultural Research (ICAR), New Delhi, and Director, Indian Institute of Horticultural Research (IIHR), Bangalore, for encouragement and for granting study leave. REFERENCES Abbott, W. S. 1925. A method of computing the effectiveness of an insecticide. J. Econ. Entomon., 18: 265-267. Crickmore, N., Zeigler, D. R., Feitelson, J., Schnepef, E., van Rie, J., Lereclus, D., Baum, J. and Dean, D. H. (MS Received 8 June 2007, Revised 29 June 2007) Table 1. Comparative toxicity of B. thuringiensis isolates, KPx-1 and IPd-1 to some important lepidopterous pests of horticultural crops Insect species KPx –1 IPd – 1 LC 50 Fiducial limit LC 50 Fiducial limit Lower Upper Lower Upper Crocidolomia binotalis (Lepidoptera: Pyralidae) 28.40 ng/cm2 28.30 28.50 26.00 ng/cm2 25.93 26.07 Diacrisia obliqua(Lepidoptera: Arctiidae) 249.00 ng/cm2 248.89 249.12 245.00 ng/cm2 244.90 245.10 Helicoverpa armigera (Lepidoptera: Noctuidae) 3.30 µg/ml 3.29 3.31 3.40 µg/ml 3.28 3.52 Control 0.00 0.00 0.00 0.00 0.00 0.00 1998. Revision of the nomenclature for the Bacillus thuringiensis pesticidal crystal proteins. Microbiol. Mol. Biol. Rev., 62: 870-813. Jarret, P. 1985. Potency factors in the delta endotoxin of Bacillus thuringiensis var aizawai and the significance of plasmids in their control. J. Appl. Bacteriol., 58: 437-448. Lowry, O. H., Rosenbrough, N. J., Farr, A. L. and Randall, R. J. 1951. Protein measurement with the Folin- Phenol reagents. J. Biol. Chem., 193: 265-275. Nagarkatti, S and Prakash, S. 1974. Rearing Heliothis armigera (Hb.) on an artificial diet. Tech. Bull. No. 17, Commonwealth Institute of Biological Control, p 169. Asokan and Puttaswamy J. Hort. Sci. Vol. 2 (1): 71-72, 2007 72