Isolation, molecular characterization and pathogenicity of native Bacillus thuringiensis, from Ethiopia, against the tomato leafminer, Tuta absoluta: Detection of a new high lethal phylogenetic group

Abstract

Tuta absoluta (tomato leafminer) is one of the devastating agricultural pest that attack mainly tomatoes. The continuous use of chemical pesticides is not affordable and poses a collateral damage to human and environmental health. This requires integrated pest management to reduce chemical pesticides. B. thuringiensis is a cosmopolitan, antagonistic soil bacterium used to control agricultural pests. In this study, effective Bt strains were screened from different sample sources based on their lepidopteran specific cry genes and larvicidal efficacy against tomato leafminer, T. absoluta under laboratory conditions. Of the 182 bacterial isolates, 55 (30 %) of isolates harbored parasporal protein crystals. Out of these, 34 (62 %) isolates possess one or more lepidopteran specific cry genes: 20 % of isolates positive for cry2, 18.2 % for cry9, 3.6 % for cry1, 16.4 % for cry2 + cry9, 1.8 % for cry1 + cry9, and 1.8 % for cry1 + cry2 + cry9. However, 21 (38.2 %) isolates did not show any lepidopteran specific cry genes. Isolates positive for cry genes showed 36.7–75 % and 46.7–98.3 % mortality against second and third instar larvae of the T. absoluta at the concentration of 108 colony forming units (CFUs) ml−1. Cry1 and cry1 plus other cry gene positive isolates were relatively more pathogenic against T. absoluta. However, third instar larvae of the T. absoluta was more susceptible than second instar larvae. Two of the isolates, AAUF6 and AAUMF9 were effective and scored LT50 values of 2.3 and 2.7 days and LC50 values of 3.4 × 103 and 4.15 × 103 CFUs ml−1 against the third instar larvae, respectively. The phylogenetic studies showed some congruence of groups with cry gene profiles and lethality level of isolates and very interestingly, we have detected a putative new phylogenetic group of Bt from Ethiopia.