EFFECTS OF ULTRAVIOLET AND VISIBLE RADIATION ON THE INSECTICIDAL ACTIVITY OF THE SPORES AND CRYSTALS OF BACILLUS THURINGIENSIS (PIERIS BRASSICAE)

Available from UMI in association with The British Library.; This study looks at the effects of ultraviolet radiation on the spores and crystals of Bacillus thuringiensis. A xenon arc lamp was used as the radiation source. Long pass filters were used to change the lamp's spectrum by blocking out different regions of the UV and visible radiation.; Viable spore counts tested the effects of the radiation on the spores.; In order to test the effects of the radiation on the crystals, a bioassay had to be developed. The crystals are known to cause feeding inhibition in Pieris brassicae larvae. By feeding different amounts to caterpillars it is possible to find an ED{dollar}sb{lcub}50{rcub}{dollar}, the dose of B.t. that causes 50% reduction in feeding, compared with normal feeding levels.; If the radiation damages the protein crystals, they should be less effective at reducing feeding in caterpillars and therefore the ED{dollar}sb{lcub}50{rcub}{dollar} of damaged crystals should be higher than for undamaged ones. The crystals were painted onto cabbage leaves to feed to the caterpillars.; It was found that the shortest wavelengths (295--345nm) were most effective at killing spores. It took 55 minutes to kill 50% of the spores under these wavelengths.; These short wavelengths were also most effective at damaging crystals. The ED{dollar}sb{lcub}50{rcub}{dollar}'s of crystals irradiated under 295-345nm for 22 hours were significantly higher than the ED{dollar}sb{lcub}50{rcub}{dollar}'s of untreated crystals.; Measurements of the solar UV in the 300nm region were made. It was calculated that the xenon arc system gave 70% of the sun's UV radiation in the region of 300nm.; The effects seen on the crystals took 22-24 hours to happen under the xenon arc lamp and would take 60 hours to happen in the field, in midsummer.; This study suggests that the inactivation caused by ultraviolet radiation would therefore be a significant problem in the field use of B.t.. If B.t. is to be used successfully by farmers and growers it will need to be protected by sunscreens that eliminate wavelengths around 300nm.