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One of the major obstacles of Bt production as a biopesticide is its expensive bioprocess and fermentation. Therefore, the objective of the present study was to optimize growth condition and develop a low-cost bioprocess for mass production of a native coleopteran-effective Bacillus thuringiensis (Bt) strain (KH4) based on agricultural wastes, at incubator and batch fermenter level. Preliminary experiments showed that the optimum pH and temperature for the strain were 6.5 and 30°C, respectively. The maximum growth and spore/crystal production were observed in the medium containing 2% molasses and 3% corn steep liquor as carbon and nitrogen sources, respectively. Different concentrations of the sea salt were used as a new cheap and available mineral source. Sea salt with final 0.003% w/w concentration showed the highest rate of growth for the strain. The experiments in Batch fermenter showed that volume of 2% bacterial inoculation in total volume of medium culture was the best concentration as preculture. It was shown that pH significantly decreased at the beginning of logarithmic phase, whereas it significantly increased at the end of the logarithmic phase. By increasing fermentation period, the oxygen demand was increased, and by increasing oxygen concentration up to 70%, the bacterial growth and the spores/crystal production was increased. Based on the results, the growth condition of the strain was optimized and a new cheap and available commercial fermentation medium was developed for mass production of the strain in the batch systems.

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The Egyptian cotton leafworm, Spodoptera littoralis (Boisduval) , is known as an important and highly polyphagous pest species worldwide. The objective of the present study was to evaluate synergistic effects of Bacillus thuringiensis subsp. kurstaki and NucleoPolyhedroVirus (SpliMNPV) on the 5-day-old larvae (2^nd instars) of S. littoralis under laboratory conditions. To do this, the larvae of S. littoralis were fed on the treated artificial diet containing only one or combination of Bt (8.31×10⁵, 2.78×10⁷, 9.69×10⁸ spore mL^-1) and SpliMNPV (5.26×10, 7.03×10², 9.39×10³ OB mL^-1). According to the results, the mortality rate for most of the Bt-SpliMNPV combinations (different concentrations) was higher than that in the treatments containing only one of the studied biocontrol agents. The Bt-SpliMNPV combinations showed different types of interactions, including synergistic, additive, or antagonistic effects. The treatment containing 8.31×10⁵ spore mL^-1 of Bt and 5.26×10 OB mL^-1 of the SpliMNPV was interpreted as synergism effect, as the real mortality (72.41±12.43%) was significantly more than the expected (48.28%). In addition, application of the Bt-SpliMNPV combinations could significantly increase larval and pupal periods, and reduce pupation, pupal weight and the adult emergence rate compared to the control and treatments containing only one of Bt or SpliMNPV. Finally, it could be concluded that co-application of Bt and SpliMNPV can enhance economic and efficient control of S. littoralis.