A fast screening method on Bacillus thuringiensis isolates for controlling cotton bollworm
2010
Kheyri, S. | Rezapanah, M. | Salehi-Jozani, G.
Bacillus thuringiensis (Bt) is a common microorganism present in any soil and considered as biological control agent for controlling some pests such as cotton bollworm (Helicovera armigera Hubner). Screening of the Bt isolates is not so easy, making a decision between speed and risk. The studies started with the bioassay performed with the isolate derivated from the Dipel as a control treatment. The rearing of the host was performed on artificial diet. Various experiments were carried out in order to determine the concentration and relevant units. The diet incorporation bioassay showed 38562 CFU/ml as LC50 by probit analysis. In the concentration of 105 CFU/ml (around LC50), the comparative experiment was conducted on with the selected isolates (5 treatments, 3 replications in a CRD format), each replication contained 25 neonate larvae. The mortality of larvae was recorded after 7 days. The ANOVA and the average comparison demonstrated a significant difference between selected isolates (bellow 13%) and the control (58%). Therefore just in the concentration of 105 CFU/ml, the screening and biological variation among isolates and the control were proved. Although in higher concentrations these isolates demonstrated hopeful mortality, they did not have any preference to the control in the fast screening method correctly. It is understood that the selection of the proper concentration (around LC50) and comparison of the average mortality percentages just in this concentration is an appropriate method for the screening of the isolates without detailed bioassay in different doses and causes a time and money to be saved.
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