جداسازی و کشنده گی کریستال Bacillus thuringiensis بر روی لارو بید آرد Ephestia kuehniella (Lep.: Pyralidae) در شرایط آزمایشگاهی

The Mediterranean Ephestia kuehniella (Lep.: Pyralidae) is an important warehouse pest, especially on grains, flour, dried fruits, legumes and other stored food products. Hence, controlling this pest bears high importance. Nowadays, due to the importance of removing the residues of chemical pesticides and the subsequent risks of them, the use of biological compounds such as Bacillus thuringiensis, which causes disease by spores and crystals, has been taken into consideration. The isolated crystal of B. thuringiensis bacteria was used in this study. To isolate the bacteria, the culture medium containing the bacteria was placed on a shaker for four days; then, the medium was centrifuged at 8000 rpm for 15 minutes.
The sediment was washed three times with 1 M NaCl and then dissolved in carbonate buffer (10 ml of buffer per gram of sediment). This mixture was placed at 37 degrees for two hours and vortexed every half an hour. In the final step, the solution was centrifuged; the resulting supernatant containing bacterial crystals was determined by the Bradford method. This experiment was conducted in the form of a completely randomized design with four treatments and five replications, each replication containing five instar larvae of the same age. Three concentrations of bacterial crystals (6x103, 6x104, and 6x105), and sterile distilled water as the control, were used for bioassay. Due to the digestive nature of the bacteria, the bioassay was done by contaminating the food material; the insect larvae were kept without food for 24 hours, then 10 microliters of each concentration was removed and mixed with 100 mg of the food of the target pest, became available to the larvae. Then, the amount of LC50 resulting from changes in the concentration of bacterial crystals on the last instar larvae of this pest was determined during 12, 24, 36, and 48 hours, which were 660,000, 30,000, 10,000, and 6,000 ppm, respectively. The results of the investigation showed that LC50 decrease and mortality increase have occurred with increasing time and concentration. This test showed that the bacterial crystal alone has a good fatality power and is effective in pest control, but due to the absence of spores, there is no epidemic.