Characterization of Bacillus thuringiensis isolates toxic to cotton boll weevil (Anthonomus grandis)

The cotton boll weevil (Anthonomus grandis) is the major cotton pest in the Americas. One of the alternatives for its control is the utilization of Bacillus thuringiensis (Bt), an entomopathogenic bacterium characterized by its production of insecticidal crystal proteins. Embrapa Genetic Resources and Biotechnology has a collection of Bacilli in which different isolates of Bt are stored. A method for rearing and maintenance of Anthonomus grandis on artificial diet in the laboratory was developed, and a robust larval bioassay protocol was established for the selection of B. thuringiensis isolates toxic to boll weevil. After preliminary bioassays performed with 215 isolates, 5 were selected that demonstrated a good level of toxicity and these were analyzed in more detail. The most toxic were S601 and S1806 presenting LC50 (lethal concentration to kill 50% if the larvae) of 0.14 mg/ml and 0.30 mg/ml, respectively. S601 showed an LC50 value that was half that of the standard B. thuringiensis subspecies tenebrionis and S1806 demonstrated similar values of LC50 to the standard. S601 contained the cry1B gene and S1806 had the cry4A, cry4B, cry10, cry11, cyt1 and cyt2 genes like B. thuringiensis subspecies israelensis.