Endophytic Strains of Bacillus thuringiensis for the Development of Means to Control Colorado Potato Beetle in Potato Crops

Currently, the search for B. thuringiensis strains to be used as biological control agents is based on their toxicity to insects in laboratory tests. At the same time, a number of strains of these bacteria are able to exist in symbiotic relationships with host plants, including being endophytes. The ability of B. thuringiensis strains to penetrate into the internal tissues of plants was evaluated by counting colony-forming units (CFU) of microorganisms 7 days after inoculation of sterile potato plants in test tubes; insecticidal activity was tested on larvae of the III instar obtained from overwintered adults of the Colorado beetle collected from fields in the Chishminsky and Iglinsky districts of Bashkortostan. It was shown that the strain of B. thuringiensis B-5351, which inhabits the surface (50.01 ± 8.10 CFU × 10 5 /g) and internal tissues (38.92 ± 9.62 CFU × 10 5 /g) of plant shoots, but has less insecticidal activity than the strain B. thuringiensis B-5689, which exhibits high insecticidal activity and colonizes mainly plant roots (25.37 ± 3.82 CFU × 10 5 /g), reduced colonization of potato crops by the Colorado potato beetle, and also increased the yield of tubers in a 2-year experiment (2020–2021). Under the influence of B. thuringiensis B-5351, a decrease in the number of larvae of early age was observed, in contrast to B. thuringiensis B-5689, which caused longer metamorphosis processes. Apparently, the effect of B. thuringiensis B-5351 was the death of insects. It is important that processing B. thuringiensis B-5351 contributed to a significant decrease in the number of late-instar larvae on crops treated with this strain as well as the degree of defoliation of plants by the pest, which was not observed with the action of B. thuringiensis B-5689. On plots treated with B. thuringiensis B-5351, the yield of commercial tubers and the total yield increased. A method is proposed for studying the endophytic potential of strains in relation to the terrestrial part of plants to search for biocontrol agents as a basis for creating algorithms for constructing microbiomes in agrocenoses.