Abstract:
The overall goal was to contribute to crop-productivity by developing sustainable pest management and the objective was to select and identify an efficacious local B. thuringiensis isolate active against the selected stem borers, C. partellus, E.saccharina, B. fusca and S. calamistis.Various isolates of B. thuringiensis were isolated from soils and dead insects collected from different places in Kenya. They were screened against various stem borers including C. partellus, E. saccharina, S. calamistis and B. fi1sca.Based on the relative potencies, seven isolates (ICIPE 001, 012, 023, 027, 040,054 and 061 were screened further and their LT 50 value determined. Three isolates, ICIPE 012, ICIPE 023 and ICIPE 054 were selected from the seven isolates shown above. These three, had LT 50 values ranging from 1.40 to 1. 59 days, tested against C. partellus and were chosen as being most active. The LCso values of these three isolates were from 0.32 to 1.6 x 107spores/ml on C.partel!us, from 0.09 to 0.15 x 107 spores/ml on E. saccharina and from 0.4 to 1.8 x 107 spores/ml on S. calamistis. By the value LC50,isolate ICIPE 023 was then selected for further studies (0.32 X 107 spores/ml).These three isolates were serologically identified as being, B. thuringiensis var.kurstaki. To establish differences among these three isolates, biochemical test using API analytical profile was carried out. The results showed differences in their reaction to citrate utilization, urea production and production of arginine dihydrolase. Only one of the isolates, ICIPE 023, was found to be different from the others in terms of composition of the cell wall proteins.From the LC50 values of the three isolates, ICIPE 023 was found to be more active against C. partellus larvae than ICIPE 012 and ICIPE 054. This isolate was selected for all the subsequent experiments.Using local raw materials, eight different media were formulated and tested for their support for growth, sporulation and 6-endotoxin production of the selected B. thuringiensis isolate, ICIPE 023 . This isolate grew best in one medium composed of cow-dung (3%) and soya (3 against % ) giving LC50 value of 0. 042 x 108 spores/ml, when the first instar of C. partellus larvae were tested.Optimization of growth conditions for this isolate yielded the most potent toxin when cultured in 50 ml volume (in 250 ml Erlenmeyer flasks) and an agitation speed of 300 rpm.Several UV-protectants namely, Congo red, clay soil and molasses were mixed with B. thuringiensis isolate ICIPE 023 grown in the cow-dung/soya medium separately and exposed to sunlight. These were tested at intervals against C.partellus larvae. Results showed that isolate ICIPE 023 grown in the cow- dung/soya medium exposed to the sunlight without UV-protectant retained activity of approximately 60% mortality after 120 hours exposure to natural sunlight.Field trials were conducted to determine the effectiveness of isolate ICIPE 023culture against C. partellus. Results showed no damage to the maize plants sprayed with isolate ICIPE 023 culture- broth but the infested non-B.thuringiensis -treated maize plants showed plant damage as evidenced by stem tunneling of approximately 29%. B. thuringiensis ICIPE 023 culture grown in the medium of cow dung/soya had a shelf-life of approximately one year and gave mortality of approximately 27% using a concentration of0.025 ml/ ml of the insect diet after one year of storage at room temperature (25°C). Samples stored in the cold room (4°C) gave 63.3% mortality against C. partellus after one year. This showed that there was very little loss of activity when kept in the cold room.