New insights on entomopathogenic bacteria isolated from soil of citrus crops to combat the polyphagus aphid pest Hyalopterus pruni (Geoffroy 1762) (Hemiptera, Aphididae)

Abstract Background The employment of entomopathogenic microorganisms is a promising approach for ensuring optimal phytosanitary protection in the framework of biological management of insect crop pests. Among these microbes, entomopathogenic soil-borne bacteria are preferred over pesticides because they help successfully in the natural regulation of arthropod populations, as soil has a favorable ecology for the availability and richness of many beneficial bacterial species. In this study, it was focused on the isolation, identification and characterization of entomopathogenic bacteria isolated from cultivated citrus soils and on the evaluation of their insecticidal potential in the laboratory against the mealy plum aphid, Hyalopterus pruni (Geoffroy 1762) (Hemiptera: Aphididae), a polyphagous and major pest attacking and virus-transmitting of several Prunus crops. Results Isolation results gave 11 bacterial isolates, which were more than 50% effective in selection tests on H. pruni aphids at 1 × 108 CFU/ml. A total of seven isolates generated significant insecticidal potential at different concentrations, and their molecular identification based on 16S rRNA genome sequencing yielded the following results: Lysinibacillus fusiformis (B4), Bacillus thuringiensis (B13), B. thuringiensis (B22), B. thuringiensis (B23), B. thuringiensis (B24), Pseudomonas sp. (P2) and Enterococcus gallinarum (P4). The most pathogenic potential of these strains was for E. gallinarum (P4), which induced 100% H. pruni mortality after 72 h of treatment at the concentration (C3 = 1 × 106 CFU/ml), followed by B. thuringiensis (B23), (B22), (B4), (P2), (B13) and (B24) isolates, which caused 96, 91, 85, 83, 65 and 50% mortality rates, respectively, at the lowest concentration (C4 = 1 × 105 CFU/ml). The LC50 and LT50 values were calculated for the entomopathogenic isolates of (P2), (P4), (B23) and (B24). A lowest LC50 value was 1.08 × 102 CFU/ml for (P4) E. gallinarum after 72 h of treatment, whereas (P2) Pseudomonas sp. presented the shortest LT50 of 33.6 h at concentration (C4 = 1 × 105 CFU/ml). Conclusions The present study's outcomes have confirmed the existence, abundance, and variety of entomopathogenic bacteria at the soil level in citrus groves. Interestingly, these bacteria could be useful for aphids’ population control on a wide scale through the utilization of their toxins and enzymes, even against insect pests of a broad order.