Molecular Characterisation and Toxicity Analysis of Indigenous Bacillus thuringiensis Berliner Isolates against Cucurbit Fruit Fly Maggots, Zeugodacus cucurbitae (Coquilett) (Diptera: Tephritidae)

Indiscriminate use of insecticides against fruit flies (Tephritidae: Diptera) has led to the development of residue and resistance and there is a need to try alternate eco-friendly management practices. Bacillus thuringiensis (Bt) is the most widely used bacterium forming proteins active against insects of different orders. Thus, the present study was framed to characterize and evaluate 50 indigenous Bt isolates against cucurbit fruit fly maggots, Zeugodacus cucurbitae.

Colony morphology was observed visually, while crystal morphology was observed through a microscope. The cry gene content of the Bt isolates was screened by polymerase chain reaction and the proteins harbored were assessed by SDS-PAGE analysis. Full diet contamination method was used to evaluate the toxicity of Bt isolates against cucurbit fruit fly maggots.

Uniform full white, off-white and creamy white colony colours were found except one isolate each, exhibiting full white at centre surrounded by off white and off white at centre surrounded by full white. Spherical shape crystal (45.10%) was predominant followed by cuboidal (29.41%), bipyramidal (17.65%), rectangular (3.92%) and minute crystal attached to spore (3.92%). Insecticidal proteins varied from ~15 to >200kDa in size with one to four and more distinct bands per isolate. PCR screening revealed cry4Aa in 2 isolates, cry4Ba and cry11Aa in one isolate each, cyt1 in 1 isolate. The standard strain Bti 4Q2 (100% mortality of maggots) and the indigenous Bt isolates, T166 (92.59%), T184 (91.11%) and T60 (90.48%) were found to be significantly toxic.